defun Special Form
interactive
let
if Special Form
save-excursion
car, cdr,
cons: Fundamental Functions
defun
count-words-in-defun
Function
defuns
Within a File
lengths-list-file in
Detail
defuns in
Different Files
line-to-top-of-window
the-the Function
Most of the GNU Emacs text editor is written in the programming language called Emacs Lisp. The code written in this programming language is the software--the sets of instructions--that tell the computer what to do when you give it commands. Emacs is designed so that you can write new code in Emacs Lisp and easily install it as an extension to the editor. This is why Emacs is called the "extensible editor".
(Indeed, since Emacs does so much more than provide editing capabilities, it should perhaps be called an "extensible computing environment", but that phrase is quite a mouthful. Also, everything you do in Emacs--find the Mayan date and phases of the moon, simplify polynomials, debug code, manage files, read letters, write books--all these activities are kinds of editing in the most general sense of the word.)
Although Emacs Lisp is usually thought of in association with the text editor, it is a full computer programming language. You can use it as you would any other programming language.
Perhaps you want to understand programming; perhaps you want to extend Emacs; or perhaps you want to become a programmer. This introduction to Emacs Lisp is designed to get you started: to guide you in learning the fundamentals of programming, and more importantly, to show you how you can teach yourself to go further.
All through this document, you will see little sample programs you can run inside of Emacs. If you read this document in Info inside of GNU Emacs, you can run the programs as they appear. (This is easy to do and is explained when the examples are presented.) Alternatively, you can read this introduction as a printed book while sitting beside a computer running Emacs. (This is what I like to do; I like printed books.) If you don't have a running Emacs beside you, you can still read this book, but in this case, it is best to treat it as a novel or as a travel guide to a country not yet visited: interesting, but not the same as being there.
Much of this introduction is dedicated to walk-throughs or guided tours of code used in GNU Emacs. These tours are designed for two purposes: first, to give you familiarity with real, working code (code you use every day); and, second, to give you familiarity with the way Emacs works. It is interesting to see how an editor is implemented. Also, I hope that you will pick up the habit of browsing through source code. You can learn from it and mine it for ideas. Having GNU Emacs is like having a dragon's cave of treasures.
In addition to learning about Emacs as an editor and Emacs Lisp
as a programming language, the examples and guided tours will
give you an opportunity to get acquainted with Emacs as a Lisp
programming environment. GNU Emacs supports programming and
provides tools that you will want to become comfortable using,
such as M-. (the key which invokes the
find-tag command). You will also learn about buffers
and other objects that are part of the editing environment.
Learning about these features of Emacs is like learning new
routes around your home town.
Finally, I hope to convey some of the skills for using Emacs to learn aspects of programming that you don't know. You can often use Emacs to help you understand what puzzles you or to find out how to do something new. This self-reliance is not only a pleasure, but an advantage.
This text is written as an elementary introduction for people who are not programmers. If you are a programmer, you may not be satisfied with this manual. The reason is that you may have become expert at reading reference manuals and be put off by the way this text is organized.
An expert programmer who reviewed this text said to me:
I prefer to learn from reference manuals. I "dive into" each paragraph, and "come up for air" between paragraphs.
When I get to the end of a paragraph, I assume that that subject is done, finished, that I know everything I need (with the possible exception of the case when the next paragraph starts talking about it in more detail). I expect that a well written reference manual will not have a lot of redundancy, and that it will have excellent pointers to the (one) place where the information I want is.
This introduction is not written for this person!
Firstly, I try to say everything at least three times: first, to introduce it; second, to show it in context; and third, to show it in a different context, or to review it.
Secondly, I hardly ever put all the information about a subject in one place, much less in one paragraph. To my way of reading, that imposes too heavy a burden on the reader. Instead I try to explain only what you need to know at the time. (Sometimes I include a little extra information so you won't be surprised later when the additional information is formally introduced.)
When you read this text, you are not expected to learn everything the first time. Frequently, you need only make, as it were, a `nodding acquaintance' with some of the items mentioned. My hope is that I have structured the text and given you enough hints that you will be alert to what is important, and concentrate on it.
You will need to "dive into" some paragraphs; there is no other way to read them. But I have tried to keep down the number of such paragraphs. This book is intended as an approachable hill, rather than as a daunting mountain.
This introduction to Programming in Emacs Lisp has a companion document, The GNU Emacs Lisp Reference Manual. The reference manual has more detail than this introduction. In the reference manual, all the information about one topic is concentrated in one place. You should turn to it if you are like the programmer quoted above. And, of course, after you have read this Introduction, you will find the Reference Manual useful when you are writing your own programs.
Lisp was first developed in the late 1950s at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language makes it superior for other purposes as well, such as writing editor commands.
GNU Emacs Lisp is largely inspired by Maclisp, which was written at MIT in the 1960's. It is somewhat inspired by Common Lisp, which became a standard in the 1980s. However, Emacs Lisp is much simpler than Common Lisp. (The standard Emacs distribution contains an optional extensions file, `cl.el', that adds many Common Lisp features to Emacs Lisp.)
If you don't know GNU Emacs, you can still read this document profitably. However, I recommend you learn Emacs, if only to learn to move around your computer screen. You can teach yourself how to use Emacs with the on-line tutorial. To use it, type C-h t. (This means you press and release the CTRL key and the h at the same time, and then press and release t.)
Also, I often refer to one of Emacs's standard commands by
listing the keys which you press to invoke the command and then
giving the name of the command in parentheses, like this:
M-C-\ (indent-region). What this means is
that the indent-region command is customarily
invoked by typing M-C-\. (You can, if you wish, change
the keys that are typed to invoke the command; this is called
rebinding. See section 16.11 Keymaps.) The
abbreviation M-C-\ means that you type your
META key, CTRL key and \ key all
at the same time. Sometimes a combination like this is called a
keychord, since it is similar to the way you play a chord on a
piano. If your keyboard does not have a META key, the
ESC key prefix is used in place of it. In this case,
M-C-\ means that you press and release your
ESC key and then type the CTRL key and the
\ key at the same time.
If you are reading this in Info using GNU Emacs, you may find it convenient to use the Info command g * RET. This way, you can read through this whole document in sequence, without having to jump from node to node. (To learn about Info, type C-h i and then select Info.)
A note on terminology: when I use the word Lisp alone, I am usually referring to the various dialects of Lisp in general, but when I speak of Emacs Lisp, I am referring to GNU Emacs Lisp in particular.
My thanks to all who helped me with this book. My especial thanks to Jim Blandy, Noah Friedman, Jim Kingdon, Roland McGrath, Randy Smith, Richard M. Stallman, and Melissa Weisshaus. My thanks also go to both Philip Johnson and David Stampe for their patient encouragement. My mistakes are my own.
@pageno = 1
To the untutored eye, Lisp is a strange programming language. In Lisp code there are parentheses everywhere. Some people even claim that the name stands for `Lots of Isolated Silly Parentheses'. But the claim is unwarranted. Lisp stands for LISt Processing and the programming language handles lists (and lists of lists) by putting them between parentheses. The parentheses mark the boundaries of the list. Sometimes a list is preceded by a single apostrophe or quotation mark, `''. Lists are the basis of Lisp.
In Lisp, a list looks like this: '(rose violet daisy
buttercup). This list is preceded by a single apostrophe.
It could just as well be written as follows, which looks more
like the kind of list you are likely to be familiar with:
'(rose violet daisy buttercup)
The elements of this list are the names of the four different flowers, separated from each other by whitespace and surrounded by parentheses, like flowers in a field with a stone wall around it.
Lists can also have numbers in them, as in this list: (+ 2
2). This list has a plus-sign, `+', followed
by two `2's, each separated by whitespace.
In Lisp, both data and programs are represented the same way; that is, they are both lists of words, numbers, or other lists, separated by whitespace and surrounded by parentheses. (Since a program looks like data, one program may easily serve as data for another; this is a very powerful feature of Lisp.) (Incidentally, these two parenthetical remarks are not Lisp lists, because they contain `;' and `.' as punctuation marks.)
Here is another list, this time with a list inside of it:
'(this list has (this list inside of it))
The components of this list are the words `this', `list', `has', and the list `(this list inside of it)'. The interior list is made up of the words `this', `list', `inside', `of', `it'.
In Lisp, what we have been calling words are called
atoms. This term comes from the historical
meaning of the word atom, which means `indivisible'. As far as
Lisp is concerned, the words we have been using in the lists
cannot be divided into any smaller parts and still mean the same
thing as part of a program; likewise with numbers and single
character symbols like `+'. On the other hand,
unlike an atom, a list can be split into parts. (See section
7 car,
cdr, cons: Fundamental Functions.)
In a list, atoms are separated from each other by whitespace. They can be right next to a parenthesis.
Technically speaking, a list in Lisp consists
of parentheses surrounding atoms separated by whitespace or
surrounding other lists or surrounding both atoms and other
lists. A list can have just one atom in it or have nothing in
it at all. A list with nothing in it looks like this:
(), and is called the empty
list. Unlike anything else, an empty list is
considered both an atom and a list at the same time.
The printed representation of both atoms and lists are called symbolic expressions or, more concisely, s-expressions. The word expression by itself can refer to either the printed representation, or to the atom or list as it is held internally in the computer. Often, people use the term expression indiscriminately. (Also, in many texts, the word form is used as a synonym for expression.)
Incidentally, the atoms that make up our universe were named such when they were thought to be indivisible; but it has been found that physical atoms are not indivisible. Parts can split off an atom or it can fission into two parts of roughly equal size. Physical atoms were named prematurely, before their truer nature was found. In Lisp, certain kinds of atom, such as an array, can be separated into parts; but the mechanism for doing this is different from the mechanism for splitting a list. As far as list operations are concerned, the atoms of a list are unsplittable.
As in English, the meanings of the component letters of a Lisp atom are different from the meaning the letters make as a word. For example, the word for the South American sloth, the `ai', is completely different from the two words, `a', and `i'.
There are many kinds of atom in nature but only a few in Lisp: for example, numbers, such as 37, 511, or 1729, and symbols, such as `+', `foo', or `forward-line'. The words we have listed in the examples above are all symbols. In everyday Lisp conversation, the word "atom" is not often used, because programmers usually try to be more specific about what kind of atom they are dealing with. Lisp programming is mostly about symbols (and sometimes numbers) within lists. (Incidentally, the preceding three word parenthetical remark is a proper list in Lisp, since it consists of atoms, which in this case are symbols, separated by whitespace and enclosed by parentheses, without any non-Lisp punctuation.)
In addition, text between double quotation marks--even sentences or paragraphs--is an atom. Here is an example:
'(this list includes "text between quotation marks.")
In Lisp, all of the quoted text including the punctuation mark and the blank spaces is a single atom. This kind of atom is called a string (for `string of characters') and is the sort of thing that is used for messages that a computer can print for a human to read. Strings are a different kind of atom than numbers or symbols and are used differently.
The amount of whitespace in a list does not matter. From the point of view of the Lisp language,
'(this list looks like this)
is exactly the same as this:
'(this list looks like this)
Both examples show what to Lisp is the same list, the list made up of the symbols `this', `list', `looks', `like', and `this' in that order.
Extra whitespace and newlines are designed to make a list more readable by humans. When Lisp reads the expression, it gets rid of all the extra whitespace (but it needs to have at least one space between atoms in order to tell them apart.)
Odd as it seems, the examples we have seen cover almost all of what Lisp lists look like! Every other list in Lisp looks more or less like one of these examples, except that the list may be longer and more complex. In brief, a list is between parentheses, a string is between quotation marks, a symbol looks like a word, and a number looks like a number. (For certain situations, square brackets, dots and a few other special characters may be used; however, we will go quite far without them.)
If you type a Lisp expression in GNU Emacs using either Lisp Interaction mode or Emacs Lisp mode, you will have available to you several commands to format the Lisp expression so it is easy to read. For example, pressing the TAB key automatically indents the line the cursor is on by the right amount. A command to properly indent the code in a region is customarily bound to M-C-\. Indentation is designed so that you can see which elements of a list belongs to which list--elements of a sub-list are indented more than the elements of the enclosing list.
In addition, when you type a closing parenthesis, Emacs momentarily jumps the cursor back to the matching opening parenthesis, so you can see which one it is. This is very useful, since every list you type in Lisp must have its closing parenthesis match its opening parenthesis. (See section `Major Modes' in The GNU Emacs Manual, for more information about Emacs' modes.)
A list in Lisp--any list--is a program ready to run. If you run it (for which the Lisp jargon is evaluate), the computer will do one of three things: do nothing, except return to you the list itself; send you an error message; or, treat the first symbol in the list as a command to do something. (Usually, of course, it is the last of these three things that you really want!)
The single apostrophe, ', that I put in front of
some of the example lists in preceding sections is called a
quote; when it precedes a list, it tells Lisp to
do nothing with the list, other than take it as it is written.
But if there is no quote preceding a list, the first item of the
list is special: it is a command for the computer to obey. (In
Lisp, these commands are called functions.) The list
(+ 2 2) shown above did not have a quote in front of
it, so Lisp understands that the + is an instruction
to do something with the rest of the list; in this case, to add
the numbers that follow.
If you are reading this inside of GNU Emacs in Info, here is how you can evaluate such a list: place your cursor immediately after the right hand parenthesis of the following list and then type C-x C-e:
(+ 2 2)
You will see the number 4 appear in the echo area.
(In the jargon, what you have just done is "evaluate the list."
The echo area is the line at the bottom of the screen that
displays or "echoes" text.) Now try the same thing with a quoted
list: place the cursor right after the following list and type
C-x C-e:
'(this is a quoted list)
In this case, you will see (this is a quoted list)
appear in the echo area.
In both cases, what you are doing is giving a command to the program inside of GNU Emacs called the Lisp interpreter---giving the interpreter a command to evaluate the expression. The name of the Lisp interpreter comes from the word for the task done by a human who comes up with the meaning of an expression--who "interprets" it.
You can also evaluate an atom that is not part of a list--one that is not surrounded by parentheses; again, the Lisp interpreter translates from the humanly readable expression to the language of the computer. But before discussing this (see section 1.7 Variables), we will discuss what the Lisp interpreter does when you make an error.
Partly so you won't worry if you do it accidentally, we will now give a command to the Lisp interpreter that generates an error message. This is a harmless activity; and indeed, we will often try to generate error messages intentionally. Once you understand the jargon, error messages can be informative. Instead of being called "error" messages, they should be called "help" messages. They are like signposts to a traveller in a strange country; decyphering them can be hard, but once understood, they can point the way.
What we will do is evaluate a list that is not quoted and does not have a meaningful command as its first element. Here is a list almost exactly the same as the one we just used, but without the single-quote in front of it. Position the cursor right after it and type C-x C-e:
(this is an unquoted list)
This time, you will see the following appear in the echo area:
Symbol's function definition is void: this
(Also, your terminal may beep at you--some do, some don't; and others blink. This is just a device to get your attention.) The message goes away as soon as you type another key, even just to move the cursor.
Based on what we already know, we can almost read this error message. We know the meaning of the word `Symbol'. In this case, it refers to the first atom of the list, the word `this'. The word `function' was mentioned once before. It is a very important word. For our purposes, we can define it by saying that a function is a set of instructions to the computer that tell the computer to do something. (Technically, the symbol tells the computer where to find the instructions, but this is a complication we can ignore for the moment.)
Now we can begin to understand the error message: `Symbol's function definition is void: this'. The symbol (that is, the word `this') does not have a definition of any set of instructions for the computer to carry out.
The slightly odd wording of the message, `function definition is void', is designed to cover the way Emacs Lisp is implemented, which is that when the symbol does not have a function definition attached to it, the place that should contains the instructions is `void'.
On the other hand, since we were able to add 2 plus 2
successfully, by evaluating (+ 2 2), we can infer
that the symbol + must have a set of instructions
for the computer to obey and those instructions must be to add
the numbers that follow the +.
We can articulate another characteristic of Lisp based on what we
have discussed so far--an important characteristic: a symbol,
like +, is not itself the set of instructions for
the computer to carry out. Instead, the symbol is used, perhaps
temporarily, as a way of locating the definition or set of
instructions. What we see is the name through which the
instructions can be found. Names of people work the same way. I
can be referred to as `Bob'; however, I am not the
letters `B', `o', `b' but
am the consciousness consistently associated with a particular
life-form. The name is not me, but it can be used to refer to me.
In Lisp, one set of instructions can be attached to several
names. For example, the computer instructions for adding numbers
can be linked to the symbol plus as well as to the
symbol + (and are in some dialects of Lisp). Among
humans, I can be referred to as `Robert' as well as
`Bob' and by other words as well.
On the other hand, a symbol can have only one function definition attached to it at a time. Otherwise, the computer would be confused as to which definition to use. If this were the case among people, only one person in the world could be named `Bob'. However, the function definition to which the name refers can be changed readily. (See section 3.2 Install a Function Definition.)
Since Emacs Lisp is large, it is customary to name symbols in a way that identifies the part of Emacs to which the function belongs. Thus, all the names for functions that deal with Texinfo start with `texinfo-' and those for functions that deal with reading mail start with `rmail-'.
Based on what we have seen, we can now start to figure out what the Lisp interpreter does when we command it to evaluate a list. First, it looks to see whether there is a quote before the list; if there is, the interpreter just gives us the list. On the other hand, if there is no quote, the interpreter looks at the first element in the list and sees whether it has a function definition. If it does, the interpreter carries out the instructions in the function definition. Otherwise, the interpreter prints an error message.
This is how Lisp works. Simple. There are added complications which we will get to in a minute, but these are the fundamentals. Of course, to write Lisp programs, you need to know how to write function definitions and attach them to names, and how to do this without confusing either yourself or the computer.
Now, for the first complication. In addition to lists, the Lisp interpreter can evaluate a symbol that is not quoted and does not have parentheses around it. In this case, the Lisp interpreter will attempt to determine the symbol's value as a variable. This situation is described in the section on variables. (See section 1.7 Variables.)
The second complication occurs because some functions are unusual and do not work in the usual manner. Those that don't are called special forms. They are used for special jobs, like defining a function, and there are not many of them. In the next few chapters, you will be introduced to several of the more important special forms.
The third and final complication is this: if the function that the Lisp interpreter is looking at is not a special form, and if it is part of a list, the Lisp interpreter looks to see whether the list has a list inside of it. If there is an inner list, the Lisp interpreter first figures out what it should do with the inside list, and then it works on the outside list. If there is yet another list embedded inside the inner list, it works on that one first, and so on. It always works on the innermost list first. The interpreter works on the innermost list first in order to find out the result of doing that. The result may be used by the enclosing expression.
Otherwise, the interpreter works left to right, from one expression to the next.
One other aspect of interpreting: the Lisp interpreter is able to interpret two kinds of entity: humanly readable code, on which we will focus exclusively, and specially processed code, called byte compiled code, which is not humanly readable. Byte compiled code runs faster than humanly readable code.
You can transform humanly readable code into byte compiled code
by running one of the compile commands such as
byte-compile-file. Byte compiled code is usually
stored in a file that ends with a `.elc' extension
rather than a `.el' extension. You will see both kinds
of file in the `emacs/lisp' directory; the files to read
are those with `.el' extensions.
As a practical matter, for most things you might do to customize or extend Emacs, you do not need to byte compile; and I will not discuss the topic here. See section `Byte Compilation' in The GNU Emacs Lisp Reference Manual, for a full description of byte compilation.
When the Lisp interpreter works on an expression, the term for the activity is called evaluation. We say that the interpreter `evaluates the expression'. I've used this term several times before. The word comes from its use in everyday language, `to ascertain the value or amount of; to appraise', according to Webster's New Collegiate Dictionary.
After evaluating an expression, the Lisp interpreter will most likely return the value that the computer produces by carrying out the instructions it found in the function definition, or perhaps it will give up on that function and produce an error message. (The interpreter may also find itself tossed, so to speak, to a different function or it may attempt to repeat continually what it is doing for ever and ever in what is called an `infinite loop'. These actions are less common; and we can ignore them.) Most frequently, the interpreter returns a value.
At the same time the interpreter returns a value, it may do something else as well, such as move a cursor or copy a file; this other kind of action is called a side effect. Actions that we humans think are important, such as printing results, are often "side effects" to the Lisp interpreter. The jargon can sound peculiar, but it turns out that it is fairly easy to learn to use side effects.
In summary, evaluating a symbolic expression most commonly causes the Lisp interpreter to return a value and perhaps carry out a side effect; or else produce an error.
If evaluation applies to a list that is inside another list, the outer list may use the value returned by the first evaluation as information when the outer list is evaluated. This explains why inner expressions are evaluated first: the values they return are used by the outer expressions.
We can investigate this process by evaluating another addition example. Place your cursor after the following expression and type C-x C-e:
(+ 2 (+ 3 3))
The number 8 will appear in the echo area.
What happens is that the Lisp interpreter first evaluates the
inner expression, (+ 3 3), for which the value 6 is
returned; then it evaluates the outer expression as if it were
written (+ 2 6), which returns the value 8. Since
there are no more enclosing expressions to evaluate, the
interpreter prints that value in the echo area.
Now it is easy to understand the name of the command invoked by
the keystrokes C-x C-e: the name is
eval-last-sexp. The letters sexp are an
abbreviation for `symbolic expression', and eval is
an abbreviation for `evaluate'. The command means `evaluate last
symbolic expression'.
As an experiment, you can try evaluating the expression by putting the cursor at the beginning of the next line immediately following the expression, or inside the expression.
Here is another copy of the expression:
(+ 2 (+ 3 3))
If you place the cursor at the beginning of the blank line that
immediately follows the expression and type C-x C-e,
you will still get the value 8 printed in the echo area. Now try
putting the cursor inside the expression. If you put it right
after the next to last parenthesis (so it appears to sit on top
of the last parenthesis), you will get a 6 printed in the echo
area! This is because the command evaluates the expression
(+ 3 3).
Now put the cursor immediately after a number. Type C-x
C-e and you will get the number itself. In Lisp, if you
evaluate a number, you get the number itself--this is how numbers
differ from symbols. If you evaluate a list starting with a
symbol like +, you will get a value returned that is
the result of the computer carrying out the instructions in the
function definition attached to that name. If a symbol by itself
is evaluated, something different happens, as we will see in the
next section.
In Lisp, a symbol can have a value attached to it just as it can have a function definition attached to it. The two are different. The function definition is a set of instructions that a computer will obey. A value, on the other hand, is something, such as number or a name, that can vary (which is why such a symbol is called a variable). The value of a symbol can be any expression in Lisp, such as a symbol, number, list, or string. A symbol that has a value is often called a variable.
A symbol can have both a function definition and a value attached to it at the same time. The two are separate. This is somewhat similar to the way the name Cambridge can refer to the city in Massachusetts and have some information attached to the name as well, such as "great programming center".
Another way of thinking of this is to imagine a symbol as being a chest of drawers. The function definition is put in one drawer, the value in another, and so on. What is put in the drawer holding the value can be changed without affecting the contents of the drawer holding the function definition, and vice-versa.
The variable fill-column illustrates a symbol with a
value attached to it: in every GNU Emacs buffer, this symbol is
set to some value, usually 72 or 70, but sometimes to some other
value. To find the value of this symbol, evaluate it by itself.
If you are reading this in Info inside of GNU Emacs, you can do
this by putting the cursor after the symbol and typing C-x
C-e:
fill-column
After I typed C-x C-e, Emacs printed the number 72 in
my echo area. This is the value for which
fill-column is set for me as I write this. It may be
different for you in your Info buffer. Notice that the value
returned as a variable is printed in exactly the same way as the
value returned by a function carrying out its instructions. From
the point of view of the Lisp interpreter, a value returned is a
value returned. What kind of expression it came from ceases to
matter once the value is known.
A symbol can have any value attached to it or, to use the jargon,
we can bind the variable to a value: to a
number, such as 72; to a string, "such as this"; to
a list, such as (spruce pine oak); we can even bind
a variable to a function definition.
A symbol can be bound to a value in several ways. See section 1.9 Setting the Value of a Variable, for information about one way to do this.
Notice that there were no parentheses around the word
fill-column when we evaluated it to find its value.
This is because we did not intend to use it as a function name.
If fill-column were the first or only element of a
list, the Lisp interpreter would attempt to find the function
definition attached to it. But fill-column has no
function definition. Try evaluating this:
(fill-column)
You will produce an error message that says: Symbol's function definition is void: fill-column
If you attempt to evaluate a symbol that does not have a value
bound to it, you will receive an error message. You can see this
by experimenting with our 2 plus 2 addition. In the following
expression, put your cursor right after the +,
before the first number 2, type C-x C-e:
(+ 2 2)
You will get an error message that says:
Symbol's value as variable is void: +
This is different from the first error message we saw, which said, `Symbol's function definition is void: this'. In this case, the symbol does not have a value as a variable; in the other case, the symbol (which was the word `this') did not have a function definition.
In this experiment with the +, what we did was cause
the Lisp interpreter to evaluate the + and look for
the value of the variable instead of the function definition. We
did this by placing the cursor right after the symbol rather than
after the parenthesis of the enclosing list as we did before. As
a consequence, the Lisp interpreter evaluated the preceding
s-expression, which in this case was the + by
itself.
Since + does not have a value bound to it, just the
function definition, the error message reported that the symbol's
value as a variable was void.
To see how information is passed to functions, let's look again at our old standby, the addition of two plus two. In Lisp, this is written as follows:
(+ 2 2)
If you evaluate this expression, the number 4 will appear in your
echo area. What the Lisp interpreter does is add the numbers that
follow the +.
The numbers added by + are called
the arguments of the function +.
These numbers are the information that is given to or
passed to the function.
The word `argument' comes from the way it is used in mathematics
and does not refer to a disputation between two people; instead
it refers to the information presented to the function, in this
case, to the +. In Lisp, the arguments to a function
are the atoms or lists that follow the function. The values
returned by the evaluation of these atoms or lists are passed to
the function. Different functions require different numbers of
arguments; some functions require none at all.(1)
The type of data that should be passed to a function depends on
what kind of information it uses. The arguments to a function
such as + must have values that are numbers, since
+ adds numbers. Other functions use different kinds
of data for their arguments.
For example, the concat function
links together or unites two or more strings of text to
produce a string. The arguments are strings. Concatinating the
two character strings abc, def
produces the single string abcdef. This can be
seen by evaluating the following:
(concat "abc" "def")
The value produced by evaluating this expression is
"abcdef".
A function such as substring uses both a string and
numbers as arguments. The function returns a part of the string,
a substring of the first argument. This function takes three
arguments. Its first argument is the string of characters, the
second and third arguments are numbers that indicate the
beginning and end of the substring. The numbers are a count of
the number of characters (including spaces and punctuations) from
the beginning of the string.
For example, if you evaluate the following:
(substring "The quick brown fox jumped." 16 19)
you will see "fox" appear in the echo area. The
arguments are the string and the two numbers.
Note that the string passed to substring is a single
atom even though it is made up of several words separated by
spaces. Lisp counts everything between the two quotation marks as
part of the string, including the spaces. You can think of the
substring function as a kind of `atom smasher' since
it takes an otherwise indivisible atom and extracts a part.
However, substring is only able to extract a
substring from an argument that is a string, not from another
type of atom such as a number or symbol.
An argument can be a symbol that returns a value when it is
evaluated. For example, when the symbol fill-column
by itself is evaluated, it returns a number. This number can be
used in an addition. Position the cursor after the following
expression and type C-x C-e:
(+ 2 fill-column)
The value will be a number two more than what you get by
evaluating fill-column alone. For me, this is 74,
because the value of fill-column is 72.
As we have just seen, an argument can be a symbol that returns a
value when evaluated. In addition, an argument can be a list that
returns a value when it is evaluated. For example, in the
following expression, the arguments to the function
concat are the strings "The " and
" red foxes." and the list (+ 2
fill-column).
(concat "The " (+ 2 fill-column) " red foxes.")
If you evaluate this expression, "The 74 red foxes."
will appear in the echo area. (Note that you must put spaces
after the word `The' and before the word
`red' so they will appear in the final string.)
Some functions, such as concat, + or
*, take any number of arguments. (The *
is the symbol for multiplication.) This can be seen by evaluating
each of the following expressions in the usual way. What you will
see in the echo area is printed in this text after
`=>', which you may read as `evaluates to'.
In the first set, the functions have no arguments:
(+) => 0 (*) => 1
In this set, the functions have one argument each:
(+ 3) => 3 (* 3) => 3
In this set, the functions have three arguments each:
(+ 3 4 5) => 12 (* 3 4 5) => 60
When a function is passed an argument of the wrong type, the Lisp
interpreter produces an error message. For example, the
+ function expects the values of its arguments to be
numbers. As an experiment we can pass it the quoted symbol
hello instead of a number. Position the cursor after
the following expression and type C-x C-e:
(+ 2 'hello)
When you do this you will generate an error message. What has
happened is that + has tried to add the 2 to the
value returned by 'hello, but the value returned by
'hello is the symbol hello, not a
number. Only numbers can be added. So + could not
carry out its addition.
As usual, the error message tries to be helpful and makes sense after you learn how to read it. What it says is this:
Wrong type argument: integer-or-marker-p, hello
The first part of the error message is straightforward; it says
`Wrong type argument'. Next comes the mysterious
jargon word `integer-or-marker-p'. This word is
trying to tell you what kind of argument the +
expected.
The symbol integer-or-marker-p says that the Lisp
interpreter is trying to determine whether the information
presented it (the value of the argument) is an integer (that is,
a whole number) or a marker (a special object representing a
buffer position). What it does is test to see whether the
+ is being given whole numbers to add. It also tests
to see whether the argument is something called a marker, which
is a specific feature of Emacs Lisp. (In Emacs, locations in a
buffer are recorded as markers. When the mark is set with the
C-@ or C-SPC command, its position is kept
as a marker. The mark can be considered a number--the number of
characters the location is from the beginning of the buffer.) In
Emacs Lisp, + can be used to add the numeric value
of marker positions as numbers.
The `p' of integer-or-marker-p is the
embodiment of a practice started in the early days of Lisp
programming. The `p' stands for `predicate'. In the
jargon used by the early Lisp researchers, a predicate refers to
a function to determine whether some property is true or false.
So the `p' tells us that
integer-or-marker-p is the name of a function that
determines whether it is true or false that the argument supplied
is an integer or a marker. Other Lisp symbols that end in
`p' include zerop, a function that
tests whether its argument has the value of zero, and
listp, a function that tests whether its argument is
a list.
Finally, the last part of the error message is the symbol
hello. This is the value of the argument that was
passed to +. If the addition had been passed the
correct type of object, the value passed would have been a
number, such as 37, rather than a symbol like hello.
But then you would not have got the error message.
message Function
Like +, the message function takes a
variable number of arguments. It is used to send messages to the
user and is so useful that we will describe it here.
A message is printed in the echo area. For example, you can print a message in your echo area by evaluating the following list:
(message "This message appears in the echo area!")
The whole string between double quotation marks is a single
argument and is printed in toto. (Note that in this example, the
message itself will appear in the echo area within double quotes;
that is because you see the value returned by the
message function. In most uses of
message in programs that you write, the text will be
printed in the echo area as a side-effect, without the quotes.
See section 3.3.1 An
Interactive multiply-by-seven., for an example
of this.)
However, if there is a `%s' in the quoted string of
characters, the message function does not print the
`%s' as such, but looks to the argument that follows
the string. It evaluates the second argument and prints the value
in the location in the string where the `%s' is.
You can see this by positioning the cursor after the following expression and typing C-x C-e:
(message "The name of this buffer is: %s." (buffer-name))
In Info, "The name of this buffer is: *info*." will
appear in the echo area. The function buffer-name
returns the name of the buffer as a string, which the
message function inserts in place of
%s.
To print a value as a decimal number, use `%d' in
the same way as `%s'. For example, to print a
message in the echo area that states the value of the
fill-column, evaluate the following:
(message "The value of fill-column is %d." fill-column)
On my system, when I evaluate this list, "The value of
fill-column is 72." appears in my echo area.
If there is more than one `%s' in the quoted string, the value of the first argument following the quoted string is printed at the location of the first `%s' and the value of the second argument is printed at the location of the second `%s', and so on. For example, if you evaluate the following,
(message "There are %d %s in the office!"
(- fill-column 14) "pink elephants")
a rather whimsical message will appear in your echo area. On my
system it says, "There are 58 pink elephants in the
office!".
The expression (- fill-column 14) is evaluated and
the resulting number is inserted in place of the
`%d'; and the string in double quotes, "pink
elephants", is treated as a single argument and inserted
in place of the `%s'. (That is to say, a string
between double quotes evaluates to itself, like a number.)
Finally, here is a somewhat complex example that not only illustrates the computation of a number, but also shows how you can use an expression within an expression to generate the text that is substituted for `%s':
(message "He saw %d %s"
(- fill-column 34)
(concat "red "
(substring
"The quick brown foxes jumped." 16 21)
" leaping."))
In this example, message has three arguments: the
string, "He saw %d %s", the expression, (-
fill-column 32), and the expression beginning with the
function concat. The value resulting from the
evaluation of (- fill-column 32) is inserted in
place of the `%d'; and the value returned by the
expression beginning with concat is inserted in
place of the `%s'.
When I evaluate the expression, the message, "He saw 38 red
foxes leaping.", appears in my echo area.
There are several ways by which a variable can
be given a value. One of the ways is to use either the
function set or the function setq.
Another way is to use let (see section 3.6 let). (The
jargon for this process is to bind a variable
to a value.)
The following sections not only describe how set and
setq work but also illustrate how arguments are
passed.
set
To set the value of the symbol flowers to the list
'(rose violet daisy buttercup), evaluate the
following expression by positioning the cursor after the
expression and typing C-x C-e.
(set 'flowers '(rose violet daisy buttercup))
The list (rose violet daisy buttercup) will appear
in the echo area. This is what is returned by the
set function. As a side effect, the symbol
flowers is bound to the list ; that is, the symbol
flowers, which can be viewed as a variable, is given
the list as its value. (This process, by the way, illustrates how
a side effect to the Lisp interpreter, setting the value, can be
the primary effect that we humans are interested in. This is
because every Lisp function must return a value if it does not
get an error, but it will only have a side effect if it is
designed to have one.)
After evaluating the set expression, you can
evaluate the symbol flowers and it will return the
value you just set. Here is the symbol. Place your cursor after
it and type C-x C-e.
flowers
When you evaluate flowers, the list (rose
violet daisy buttercup) appears in the echo area.
Incidentally, if you evaluate 'flowers, the variable
with a quote in front of it, what you will see in the echo area
is the symbol itself, flowers. Here is the quoted
symbol, so you can try this:
'flowers
Note also, that when you use set, you need to quote
both arguments to set, unless you want them
evaluated. In this case, we do not want either argument
evaluated, neither the variable flowers nor the list
(rose violet daisy buttercup), so both are quoted.
(When you use set without quoting its first
argument, the first argument is evaluated before anything else is
done. If you did this and flowers did not have a
value already, you would get an error message that the
`Symbol's value as variable is void'; on the other
hand, if flowers did return a value after it was
evaluated, the set would attempt to set the value
that was returned. There are situations where this is the right
thing for the function to do; but such situations are rare.)
setq
As a practical matter, you almost always quote the first argument
to set. The combination of set and a
quoted first argument is so common that it has its own name: the
special form setq. This special form is just like
set except that the first argument is quoted
automatically, so you don't need to type the quote mark yourself.
Also, as an added convenience, setq permits you to
set several different variables to different values, all in one
expression.
To set the value of the variable carnivores to the
list '(lion tiger leopard) using setq,
the following expression is used:
(setq carnivores '(lion tiger leopard))
This is exactly the same as using set except the
first argument is automatically quoted by setq. (The
`q' in setq means quote.)
With set, the expression would look like this:
(set 'carnivores '(lion tiger leopard))
Also, setq can be used to assign different values to
different variables. The first argument is bound to the value of
the second argument, the third argument is bound to the value of
the fourth argument, and so on. For example, you could use the
following to assign a list of trees to the symbol
trees and a list of herbivores to the symbol
herbivores:
(setq trees '(pine fir oak maple)
herbivores '(gazelle antelope zebra))
(The expression could just as well have been on one line, but it might not have fit on a page; and humans find it easier to read nicely formatted lists.)
Although I have been using the term `assign', there is another
way of thinking about the workings of set and
setq; and that to say that set and
setq make the symbol point to the list.
This latter way of thinking is very common and in forthcoming
chapters we shall come upon at least one symbol that has
`pointer' as part of its name. The name is chosen because the
symbol has a value, specifically a list, attached to it; or,
expressed in this other way, the symbol is set to "point" to the
list.
Here is an example that shows how to use setq in a
counter. You might use this to count how many times a part of
your program repeats itself. First set a variable to zero; then
add one to the number each time the program repeats itself. To do
this, you need a variable that serves as a counter, and two
expressions: an initial setq expression that sets
the counter variable to zero; and a second setq
expression that increments the counter each time it is evaluated.
(setq counter 0) ; Let's call this the initializer. (setq counter (+ counter 1)) ; This is the incrementer. counter ; This is the counter.
(The text following the `;' are comments. See section 3.2.1 Change a Function Definition.)
If you evaluate the first of these expressions, the initializer,
(setq counter 0), and then evaluate the third
expression, counter, the number 0 will
appear in the echo area. If you then evaluate the second
expression, the incrementer, (setq counter (+ counter
1)), the counter will get the value 1. So if you again
evaluate counter, the number 1 will
appear in the echo area. Each time you evaluate the second
expression, the value of the counter will be incremented.
When you evaluate the incrementer, (setq counter (+ counter
1)), the Lisp interpreter first evaluates the innermost
list; this is the addition. In order to evaluate this list, it
must evaluate the variable counter and the number
1. When it evaluates the variable
counter, it receives its current value. It passes
this value and the number 1 to the +
which adds them together. The sum is then returned as the value
of the inner list and passed to the setq which sets
the variable counter to this new value. Thus, the
value of the variable, counter, is changed.
Learning Lisp is like climbing a hill in which the first part is the steepest. You have now climbed the most difficult part; what remains becomes easier as you progress onwards.
In summary,
forward-paragraph, single character symbols like
+, strings of characters between double quotation
marks, or numbers.
', tells the Lisp interpreter
that it should return the following expression as written, and
not evaluate it as it would if the quote were not there.
A few simple exercises:
Before learning how to write a function definition in Emacs Lisp, it is useful to spend a little time evaluating various expressions that have already been written. These expressions will be lists with the functions as their first (and often only) element. Since some of the functions associated with buffers are both simple and interesting, we will start with those. In this section, we will evaluate a few of these. In another section, we will study the code of several other buffer-related functions, to see how they were written.
Whenever you give an editing command to Emacs Lisp, such as the command to move the cursor or to scroll the screen, you are evaluating an expression, the first element of which is the function. This is how Emacs works.
When you type keys, you cause the Lisp
interpreter to evaluate an expression and that is how you get
your results. Even typing plain text involves evaluating an
Emacs Lisp function, in this case, one that uses
self-insert-command, which simply inserts the
character you typed. The functions you evaluate by typing
keystrokes are called interactive functions,
or commands; how you make a function
interactive will be illustrated in the chapter on how to write
function definitions. See section 3.3 Make a Function
Interactive.
In addition to typing keyboard commands, we have seen a second way to evaluate an expression: by positioning the cursor after a list and typing C-x C-e. This is what we will do in the rest of this section. There are other ways to evaluate an expression as well; these ways will be described in other sections as we come to them.
Besides being used for practicing evaluation, the functions shown in the next few sections are important in their own right. A study of these functions makes clear the distinction between buffers and files, how to switch to a buffer, and how to determine a location within it.
The two functions, buffer-name and
buffer-file-name, show the difference between a file
and a buffer. When you evaluate the following expression,
(buffer-name), the name of the buffer appears in the
echo area. When you evaluate (buffer-file-name), the
name of the file to which the buffer refers appears in the echo
area. Usually, the name returned by (buffer-name) is
the same as the name of the file to which it refers, and the name
returned by (buffer-file-name) is the full path-name
of the file.
A file and a buffer are two different entities. A file is information recorded permanently in the computer (unless you delete it). A buffer, on the other hand, is information inside of Emacs that will vanish at the end of the editing session (or when you kill the buffer). Usually, a buffer contains information that you have copied from a file; we say the buffer is visiting that file. This copy is what you work on and modify. Changes to the buffer do not change the file, until you save the buffer. When you save the buffer, the buffer is copied to the file and is thus saved permanently.
If you are reading this in Info inside of GNU Emacs, you can evaluate each of the following expressions by positioning the cursor after it and typing C-x C-e.
(buffer-name) (buffer-file-name)
When I do this, `"introduction.texinfo"' is the value
returned by evaluating (buffer-name), and
`"/gnu/work/intro/introduction.texinfo"' is the value
returned by evaluating (buffer-file-name). The
former is the name of the buffer and the latter is the name of
the file. (In the expressions, the parentheses tell the Lisp
interpreter to treat buffer-name and
buffer-file-name as functions; without the
parentheses, the interpreter would attempt to evaluate the
symbols as variables. See section 1.7 Variables.)
In spite of the distinction between files and buffers, you will often find that people refer to a file when they mean a buffer and vice-versa. Indeed, most people say, "I am editing a file," rather than saying, "I am editing a buffer which I will soon save to a file." It is almost always clear from context what people mean. When dealing with computer programs, however, it is important to keep the distinction in mind, since the computer is not as smart as a person.
The word `buffer', by the way, comes from the meaning of the word as a cushion that deadens the force of a collision. In early computers, a buffer cushioned the interaction between files and the computer's central processing unit. The drums or tapes that held a file and the central processing unit were pieces of equipment that were very different from each other, working at their own speeds, in spurts. The buffer made it possible for them to work together effectively. Eventually, the buffer grew from being an intermediary, a temporary holding place, to being the place where work is done. This transformation is rather like that of a small seaport that grew into a great city: once it was merely the place where cargo was warehoused temporarily before being loaded onto ships; then it became a business and cultural center in its own right.
Not all buffers are associated with files. For example, when you
start an Emacs session by typing the command emacs
alone, without naming any files, Emacs will start with the
`*scratch*' buffer on the screen. This buffer is not
visiting any file. Similarly, a `*Help*' buffer is not
associated with any file.
If you switch to the `*scratch*'
buffer, type (buffer-name), position the cursor
after it, and type C-x C-e to evaluate the
expression, the name "*scratch*" is returned and
will appear in the echo area. "*scratch*" is the
name of the buffer. However, if you type
(buffer-file-name) in the `*scratch*'
buffer and evaluate that, nil will appear in the
echo area. nil is from the Latin word for
`nothing'; in this case, it means that the
`*scratch*' buffer is not associated with any file.
(In Lisp, nil is also used to mean `false' and is
a synonym for the empty list, ().)
Incidentally, if you are in the `*scratch*' buffer and want the value returned by an expression to appear in the `*scratch*' buffer itself rather than in the echo area, type C-u C-x C-e instead of C-x C-e. This causes the value returned to appear after the expression. The buffer will look like this:
(buffer-name)"*scratch*"
You cannot do this in Info since Info is read-only and it will not allow you to change the contents of the buffer. But you can do this in any buffer you can edit; and when you write code or documentation (such as this manual), this feature is very useful.
The buffer-name function returns the name
of the buffer; to get the buffer itself, a different
function is needed: the current-buffer function. If
you use this function in code, what you get is the buffer itself.
A name and the object or entity to which the name refers are
different from each other. You are not your name. You are a
person to whom others refer by name. If you ask to speak to
George and someone hands you a card with the letters
`G', `e', `o',
`r', `g', and `e' written
on it, you might be amused, but you would not be satisfied. You
do not want to speak to the name, but to the person to whom the
name refers. A buffer is similar: the name of the scratch buffer
is `*scratch*', but the name is not the buffer. To get a
buffer itself, you need to use a function such as
current-buffer.
However, there is a slight complication: if you evaluate
current-buffer in an expression on its own, as we
will do here, what you see is a printed representation of the
name of the buffer without the contents of the buffer. Emacs
works this way for two reasons: the buffer may be thousands of
lines long--too long to be conveniently displayed; and, another
buffer may have the same contents but a different name, and it is
important to distinguish between them.
Here is an expression containing the function:
(current-buffer)
If you evaluate the expression in the usual way, `#<buffer *info*>' appears in the echo area. The special format indicates that the buffer itself is being returned, rather than just its name.
Incidentally, while you can type a number or symbol into a
program, you cannot do that with the printed representation of a
buffer: the only way to get a buffer itself is with a function
such as current-buffer.
A related function is other-buffer. This returns the
most recently selected buffer other than the one you are in
currently. If you have recently switched back and forth from the
`*scratch*' buffer, other-buffer will
return that buffer.
You can see this by evaluating the expression:
(other-buffer)
You should see `#<buffer *scratch*>' appear in the echo area, or the name of whatever other buffer you switched back from most recently.
The other-buffer function actually provides a buffer
when it is used as an argument to a function that requires one.
We can see this by using other-buffer and
switch-to-buffer to switch to a different buffer.
But first, a brief introduction to the
switch-to-buffer function. When you switched back
and forth from Info to the `*scratch*' buffer to
evaluate (buffer-name), you most likely typed
C-x b and then typed `*scratch*' when
prompted in the minibuffer for the name of the buffer to which
you wanted to switch. The keystrokes, C-x b, cause the
Lisp interpreter to evaluate the interactive Emacs Lisp function
switch-to-buffer. As we said before, this is how
Emacs works: different keystrokes call or run different
functions. For example, C-f calls
forward-char, M-e calls
forward-sentence, and so on.
By writing switch-to-buffer in an expression, and
giving it a buffer to switch to, we can switch buffers just the
way C-x b does.
Here is the Lisp expression:
(switch-to-buffer (other-buffer))
The symbol switch-to-buffer is the first element of
the list, so the Lisp interpreter will treat it as a function and
carry out the instructions that are attached to it. But before
doing that, the interpreter will note that
other-buffer is inside parentheses and work on that
symbol first. other-buffer is the first (and in this
case, the only) element of this list, so the Lisp interpreter
calls or runs the function. It returns another buffer. Next, the
interpreter runs switch-to-buffer, passing to it, as
an argument, the other buffer, which is what Emacs will switch
to. If you are reading this in Info, try this now. Evaluate the
expression. (To get back, type C-x b RET.)
In the programming examples in later sections of this document,
you will see the function set-buffer more often than
switch-to-buffer. This is because of a difference
between computer programs and humans: humans have eyes and expect
to see the buffer on which they are working on their computer
terminals. This is so obvious, it almost goes without saying.
However, programs do not have eyes. When a computer program works
on a buffer, that buffer does not need to be visible on the
screen.
switch-to-buffer is designed for humans and does two
different things: it switches the buffer to which Emacs attention
is directed; and it switches the buffer displayed in the window
to the new buffer. set-buffer, on the other hand,
does only one thing: it switches the attention of the computer
program to a different buffer. The buffer on the screen remains
unchanged (of course, normally nothing happens there until the
command finishes running).
Also, we have just introduced another jargon term, the word call. When you evaluate a list in which the first symbol is a function, you are calling that function. The use of the term comes from the notion of the function as an entity that can do something for you if you `call' it--just as a plumber is an entity who can fix a leak if you call him or her.
Finally, let's look at several rather simple functions,
buffer-size, point,
point-min, and point-max. These give
information about the size of a buffer and the location of point
within it.
The function buffer-size tells you the size of the
current buffer; that is, the function returns a count of the
number of characters in the buffer.
(buffer-size)
You can evaluate this in the usual way, by positioning the cursor after the expression and typing C-x C-e.
In Emacs, the current position of the cursor
is called point. The expression
(point) returns a number that tells you where the
cursor is located as a count of the number of characters from
the beginning of the buffer up to point. You can see the
character count for point in this buffer by evaluating the
following expression in the usual way:
(point)
As I write this, the value of point is 65724. The
point function is frequently used in some of the
examples later in this manual.
The value of point depends, of course, on its location within the buffer. If you evaluate point in this spot, the number will be larger:
(point)
For me, the value of point in this location is 66043, which means that there are 319 characters (including spaces) between the two expressions.
The function point-min is
somewhat similar to point, but it returns the
value of the minimum permissible value of point in the current
buffer. This is the number 1 unless narrowing
is in effect. (Narrowing is a mechanism whereby you can
restrict yourself, or a program, to operations on just a part
of a buffer. See section 6 Narrowing and Widening.)
Likewise, the function point-max returns the
value of the maximum permissible value of point in the current
buffer.
Find a file with which you are working and move towards its middle. Find its buffer name, file name, length, and your position in the file.
When the Lisp interpreter evaluates a list, it looks to see whether the first symbol on the list has a function definition attached to it; or, put another way, whether the symbol points to a function definition. If it does, the computer carries out the instructions in the definition. A symbol that has a function definition is called, simply, a function (although, properly speaking, the definition is the function and the symbol refers to it.)
All functions are defined in terms of other functions, except for a few primitive functions that are written in the C programming language. When you write functions' definitions, you will write them in Emacs Lisp and use other functions as your building blocks. Some of the functions you will use will themselves be written in Emacs Lisp (perhaps by you) and some will be primitives written in C. The primitive functions are used exactly like those written in Emacs Lisp and behave like them. They are written in C so we can easily run GNU Emacs on any computer that has sufficient power and can run C.
Let me re-emphasize this: when you write code in Emacs Lisp, you do not distinguish between the use of functions written in C and the use of functions written in Emacs Lisp. The difference is irrelevant. I mention the distinction only because it is interesting to know. Indeed, unless you investigate, you won't know whether an already-written function is written in Emacs Lisp or C.
defun Special Form
In Lisp, a symbol such as
mark-whole-buffer has code attached to it that
tells the computer what to do when the function is called.
This code is called the function definition
and is created by evaluating a Lisp expression that starts
with the symbol defun (which is an abbreviation
for define function). Because defun does
not evaluate its arguments in the usual way, it is called a
special form.
In subsequent sections, we will look at function definitions from
the Emacs source code, such as mark-whole-buffer. In
this section, we will describe a simple function definition so
you can see how it looks. This function definition uses
arithmetic because it makes for a simple example. Some people
dislike examples using arithmetic; however, if you are such a
person, do not despair. Hardly any of the code we will study in
the remainder of this introduction involves arithmetic or
mathematics. The examples mostly involve text in one way or
another.
A function definition has up to five parts following the word
defun:
().
It is helpful to think of the five parts of a function definition as being organized in a template, with slots for each part:
(defun function-name (arguments...) "optional-documentation..." (interactive argument-passing-info) ; optional body...)
As an example, here is the code for a function that multiplies its argument by 7. (This example is not interactive. See section 3.3 Make a Function Interactive, for that information.)
(defun multiply-by-seven (number) "Multiply NUMBER by seven." (* 7 number))
This definition begins with a parenthesis and the symbol
defun, followed by the name of the function.
The name of the function is followed by a list
that contains the arguments that will be passed to the
function. This list is called the argument
list. In this case, the list has only one element,
the symbol, number. When the function is used,
the symbol will be bound to the value that is used as the
argument to the function.
Instead of choosing the word number for the name of
the argument, I could have picked any other name. For example, I
could have chosen the word multiplicand. I picked
the word `number' because it tells what kind of value is intended
for this slot; but I could just as well have chosen the word
`multiplicand' to indicate the role that the value placed in this
slot will play in the workings of the function. I could have
called it foogle, but that would have been a bad
choice because it would not tell humans what it means. The choice
of name is up to the programmer and should be chosen to make the
meaning of the function clear.
Indeed, you can choose any name you wish for a symbol in an
argument list, even the name of a symbol used in some other
function: the name you use in an argument list is private to that
particular definition. In that definition, the name refers to a
different entity than any use of the same name outside the
function definition. Suppose you have a nick-name `Shorty' in
your family; when your family members refer to `Shorty', they
mean you. But outside your family, in a movie, for example, the
name `Shorty' refers to someone else. Because a name in an
argument list is private to the function definition, you can
change the value of such a symbol inside the body of a function
without changing its value outside the function. The effect is
similar to that produced by a let expression. (See
section 3.6
let.)
The argument list is followed by the documentation string that
describes the function. This is what you see when you type
C-h f and the name of a function. Incidentally, when
you write a documentation string like this, you should make the
first line a complete sentence since some commands, such as
apropos, print only the first line of a multi-line
documentation string. Also, you should not indent the second line
of a documentation string, if you have one, because that looks
odd when you use C-h f. The documentation string is
optional, but it is so useful, it should be included in almost
every function you write.
The third line of the example consists of the
body of the function definition. (Most functions' definitions,
of course, are longer than this.) In this case, the body is
the list, (* 7 number), which says to multiply
the value of number by 7. (In Emacs Lisp,
* is the function for multiplication, just as
+ is the function for addition.)
When you use the multiply-by-seven function, the
argument number evaluates to the actual number you
want used. Here is an example that shows how
multiply-by-seven is used; but don't try to evaluate
this yet!
(multiply-by-seven 3)
The symbol number, specified in the function
definition in the next section, is given or "bound to" the value
3 in the actual use of the function. Note that although
number was inside parentheses in the function
definition, the argument passed to the
multiply-by-seven function is not in parentheses.
The parentheses are written in the function definition so the
computer can figure out where the argument list ends and the rest
of the function definition begins.
If you evaluate this example, you are likely to get an error message. (Go ahead, try it!) This is because we have written the function definition, but not yet told the computer about the definition--we have not yet installed (or `loaded') the function definition in Emacs. Installing a function is the process that tells the Lisp interpreter the definition of the function. Installation is described in the next section.
If you are reading this inside of Info in Emacs, you can try out
the multiply-by-seven function by first evaluating
the function definition and then evaluating
(multiply-by-seven 3). A copy of the function
definition follows. Place the cursor after the last parenthesis
of the function definition and type C-x C-e. When you
do this, multiply-by-seven will appear in the echo
area. (What this means is that when a function definition is
evaluated, the value it returns is the name of the defined
function.) At the same time, this action installs the function
definition.
(defun multiply-by-seven (number) "Multiply NUMBER by seven." (* 7 number))
By evaluating this defun, you have just installed
multiply-by-seven in Emacs. The function is now just
as much a part of Emacs as forward-word or any other
editing function you use. (multiply-by-seven will
stay installed until you quit Emacs. To reload code automatically
whenever you start Emacs, see section 3.5 Install Code Permanently.)
You can see the effect of installing
multiply-by-seven by evaluating the following
sample. Place the cursor after the following expression and type
C-x C-e. The number 21 will appear in the echo area.
(multiply-by-seven 3)
If you wish, you can read the documentation for the function by
typing C-h f (describe-function) and then
the name of the function, multiply-by-seven. When
you do this, a `*Help*' window will appear on your
screen that says:
multiply-by-seven: Multiply NUMBER by seven.
(To return to a single window on your screen, type C-x 1.)
If you want to change the code in multiply-by-seven,
just rewrite it. To install the new version in place of the old
one, evaluate the function definition again. This is how you
modify code in Emacs. It is very simple.
As an example, you can change the multiply-by-seven
function to add the number to itself seven times instead of
multiplying the number by seven. The produces the same answer,
but by a different path. At the same time, we will add a comment
to the code; a comment is text that the Lisp interpreter ignores,
but that a human reader may find useful or enlightening. In this
case the comment is that this is the "second version".
(defun multiply-by-seven (number) ; Second version. "Multiply NUMBER by seven." (+ number number number number number number number))
The comment follows a semi-colon, `;'. In Lisp, everything on a line that follows a semi-colon is a comment. The end of the line is the end of the comment. To stretch a comment over two or more lines, begin each line with a semi-colon.
See section 16.3 Beginning a `.emacs' File, and section `Comments' in The GNU Emacs Lisp Reference Manual, for more about comments.
You can install this version of the
multiply-by-seven function by evaluating it in the
same way you evaluated the first function: place the cursor after
the last parenthesis and type C-x C-e.
In summary, this is how you write code in Emacs Lisp: you write a function; install it; test it; and then make fixes or enhancements and install it again.
You make a function interactive by placing a list that begins
with the special form interactive immediately after
the documentation. A user can invoke an interactive function by
typing M-x and then the name of the function; or by
typing the keys to which it is bound, for example, by typing
C-n for next-line or C-x h for
mark-whole-buffer.
Interestingly, when you call an interactive function interactively, the value returned is not automatically displayed in the echo area. This is because you often call an interactive function for its side effects, such as moving forward by a word or line, and not for the value returned. If the returned value were displayed in the echo area each time you typed a key, it would be very distracting.
Both the use of the special form interactive and one
way to display a value in the echo area can be illustrated by
creating an interactive version of
multiply-by-seven.
Here is the code:
(defun multiply-by-seven (number) ; Interactive version. "Multiply NUMBER by seven." (interactive "p") (message "The result is %d" (* 7 number)))
You can install this code by placing your cursor after it and typing C-x C-e. The name of the function will appear in your echo area. Then, you can use this code by typing C-u and a number and then typing M-x multiply-by-seven and pressing RET. The phrase `The result is ...' followed by the product will appear in the echo area.
Speaking more generally, you invoke a function like this in either of two ways:
Both the examples just mentioned work identically to move point
forward three sentences. (Since multiply-by-seven is
not bound to a key, it could not be used as an example of key
binding.)
(See section 16.7 Some Keybindings, to learn how to bind a command to a key.)
A prefix argument is passed to an interactive function by typing the META key followed by a number, for example, M-3 M-e, or by typing C-u and then a number, for example, C-u 3 M-e (if you type C-u without a number, it defaults to 4).
multiply-by-seven.
Let's look at the use of the special form
interactive and then at the function
message in the interactive version of
multiply-by-seven. You will recall that the function
definition looks like this:
(defun multiply-by-seven (number) ; Interactive version. "Multiply NUMBER by seven." (interactive "p") (message "The result is %d" (* 7 number)))
In this function, the expression, (interactive "p"),
is a list of two elements. The "p" tells Emacs to
pass the prefix argument to the function and use its value for
the argument of the function.
The argument will be a number. This is means that the symbol
number will be bound to a number in the line:
(message "The result is %d" (* 7 number))
For example, if your prefix argument is 5, the Lisp interpreter will evaluate the line as if it were:
(message "The result is %d" (* 7 5))
(If you are reading this in GNU Emacs, you can evaluate this
expression yourself.) First, the interpreter will evaluate the
inner list, which is (* 7 5). This returns a value
of 35. Next, it will evaluate the outer list, passing the values
of the second and subsequent elements of the list to the function
message.
As we have seen, message is an Emacs Lisp function
especially designed for sending a one line message to a user.
(See section 1.8.5 The
message Function.) In summary, the
message function prints its first argument in the
echo area as is, except for occurrences of `%d',
`%s', or `%c'. When it sees one of
these control sequences, the function looks to the second and
subsequent arguments and prints the value of the argument in the
location in the string where the control sequence is located.
In the interactive multiply-by-seven function, the
control string is `%d', which requires a number, and
the value returned by evaluating (* 7 5) is the
number 35. Consequently, the number 35 is printed in place of the
`%d' and the message is `The result is
35'.
(Note that when you call the function
multiply-by-seven, the message is printed without
quotes, but when you call message, the text is
printed in double quotes. This is because the value returned by
message is what appears in the echo area when you
evaluate an expression whose first element is
message; but when embedded in a function,
message prints the text as a side effect without
quotes.)
interactive
In the example, multiply-by-seven used
"p" as the argument to interactive.
This argument told Emacs to interpret your typing either
C-u followed by a number or META followed
by a number as a command to pass that number to the function as
its argument. Emacs has more than twenty characters predefined
for use with interactive. In almost every case, one
or other of these options will enable you to pass the right
information interactively to a function. (See section `Code
Characters for interactive' in The GNU Emacs
Lisp Reference Manual.)
For example, the character `r' causes Emacs to pass the beginning and end of the region (the current values of point and mark) to the function as two separate arguments. It is used as follows:
(interactive "r")
On the other hand, a `B' tells Emacs to ask for the
name of a buffer that will be passed to the function. In this
case, Emacs will ask for the name by prompting the user in the
minibuffer, using a string that follows the `B', as
in "BAppend to buffer: ". Not only will Emacs prompt
for the name, but Emacs will complete the name if you type enough
of it and press TAB.
A function with two or more arguments can have information passed
to each argument by adding parts to the string that follows
interactive. When you do this, the information is
passed to each argument in the same order it is specified in the
interactive list. In the string, each part is
separated from the next part by a `\n', which is a
newline. For example, you could follow "BAppend to buffer:
" with a `\n') and an `r'. This
would cause Emacs to pass the values of point and mark to the
function as well as prompt you for the buffer--three arguments in
all.
In this case, the function definition would look like the
following, where buffer, start, and
end are the symbols to which
interactive binds the buffer and the current values
of the beginning and ending of the region:
(defun name-of-function (buffer start end) "documentation..." (interactive "BAppend to buffer: \nr") body-of-function...)
(The space after the colon in the prompt makes it look better
when you are prompted. The append-to-buffer function
looks exactly like this. See section 4.4 The Definition of
append-to-buffer.)
If a function does not have arguments, then
interactive does not require any. Such a function
contains the simple expression (interactive). The
mark-whole-buffer function is like this.
Alternatively, if the special letter-codes are not right for your
application, you can pass your own arguments to
interactive as a list. See section `Using
Interactive' in The GNU Emacs Lisp Reference
Manual, for more information about this advanced
technique.
When you install a function definition by evaluating it, it will stay installed until you quit Emacs. The next time you start a new session of Emacs, the function will not be installed unless you evaluate the function definition again.
At some point, you may want to have code installed automatically whenever you start a new session of Emacs. There are several ways of doing this:
load function to cause Emacs to evaluate and thereby
install each of the functions in the files. See section
16.8 Loading Files.
Finally, if you have code that everyone who uses Emacs may want, you can post it on a computer network or send a copy to the Free Software Foundation. (When you do this, please put a copyleft notice on the code before posting it.) If you send a copy of your code to the Free Software Foundation, it may be included in the next release of Emacs. In large part, this is how Emacs has grown over the past years, by donations.
let
The let expression is a special form in Lisp that
you will need to use in most function definitions. Because it is
so common, let will be described in this section.
let is used to attach or bind a symbol to a value in
such a way that the Lisp interpreter will not confuse the
variable with a variable of the same name that is not part of the
function. To understand why this special form is necessary,
consider the situation in which you own a home that you generally
refer to as `the house', as in the sentence, "The house needs
painting." If you are visiting a friend and your host refers to
`the house', he is likely to be referring to his house,
not yours, that is, to a different house. If he is referring to
his house and you think he is referring to your house, you may be
in for some confusion. The same thing could happen in Lisp if a
variable that is used inside of one function has the same name as
a variable that is used inside of another function, and the two
are not intended to refer to the same value.
The let special form prevents
this kind of confusion. let creates a name for a
local variable that overshadows any use of
the same name outside the let expression. This is
like understanding that whenever your host refers to `the
house', he means his house, not yours. (Symbols used in
argument lists work the same way. See section 3.1 The defun
Special Form.)
Local variables created by a let expression retain
their value only within the let expression
itself (and within expressions called within the let
expression); the local variables have no effect outside the
let expression.
let can create more than one variable at once. Also,
let gives each variable it creates an initial value,
either a value specified by you, or nil. (In the
jargon, this is called `binding the variable to the value'.)
After let has created and bound the variables, it
executes the code in the body of the let, and
returns the value of the last expression in the body, as the
value of the whole let expression. (`Execute' is a
jargon term that means to evaluate a list; it comes from the use
of the word meaning `to give practical effect to' (Oxford
English Dictionary). Since you evaluate an expression to
perform an action, `execute' has evolved as a synonym to
`evaluate'.)
let
Expression
A let expression is a list of
three parts. The first part is the symbol let.
The second part is a list, called a varlist,
each element of which is either a symbol by itself or a
two-element list, the first element of which is a symbol. The
third part of the let expression is the body of
the let. The body usually consists of one or more
lists.
A template for a let expression looks like this:
(let varlist body...)
The symbols in the varlist are the variables that are given
initial values by the let special form. Symbols by
themselves are given the initial value of nil; and
each symbol that is the first element of a two-element list is
bound to the value that is returned when the Lisp interpreter
evaluates the second element.
Thus, a varlist might look like this: (thread (needles
3)). In this case, in a let expression, Emacs
binds the symbol thread to an initial value of
nil, and binds the symbol needles to an
initial value of 3.
When you write a let expression, what you do is put
the appropriate expressions in the slots of the let
expression template.
If the varlist is composed of two-element lists, as is often the
case, the template for the let expression looks like
this:
(let ((variable value)
(variable value)
...)
body...)
let Expression
The following expression creates and gives initial values to the
two variables zebra and tiger. The body
of the let expression is a list which calls the
message function.
(let ((zebra 'stripes)
(tiger 'fierce))
(message "One kind of animal has %s and another is %s."
zebra tiger))
Here, the varlist is ((zebra 'stripes) (tiger
'fierce)).
The two variables are zebra and tiger.
Each variable is the first element of a two-element list and each
value is the second element of its two-element list. In the
varlist, Emacs binds the variable zebra to the value
stripes, and binds the variable tiger
to the value fierce. In this case, both values are
symbols preceded by a quote. The values could just as well have
been another list or a string. The body of the let
follows after the list holding the variables. In this case, the
body is a list that uses the message function to
print a string in the echo area.
You may evaluate the example in the usual fashion, by placing the cursor after the last parenthesis and typing C-x C-e. When you do this, the following will appear in the echo area:
"One kind of animal has stripes and another is fierce."
As we have seen before, the message function prints
its first argument, except for `%s'. In this case,
the value of the variable zebra is printed at the
location of the first `%s' and the value of the
variable tiger is printed at the location of the
second `%s'.
let Statement
If you do not bind the variables in a let statement
to specific initial values, they will automatically be bound to
an initial value of nil, as in the following
expression:
(let ((birch 3)
pine
fir
(oak 'some))
(message
"Here are %d variables with %s, %s, and %s value."
birch pine fir oak))
Here, the varlist is ((birch 3) pine fir (oak
'some)).
If you evaluate this expression in the usual way, the following will appear in your echo area:
"Here are 3 variables with nil, nil, and some value."
In this case, Emacs binds the symbol birch to the
number 3, binds the symbols pine and
fir to nil, and binds the symbol
oak to the value some.
Note that in the first part of the let, the
variables pine and fir stand alone as
atoms that are not surrounded by parentheses; this is because
they are being bound to nil, the empty list. But
oak is bound to some and so is a part
of the list (oak 'some). Similarly,
birch is bound to the number 3 and so is in a list
with that number. (Since a number evaluates to itself, the number
does not need to be quoted. Also, the number is printed in the
message using a `%d' rather than a
`%s'.) The four variables as a group are put into a
list to delimit them from the body of the let.
if Special Form
A third special form, in addition to defun and
let, is the conditional if. This form
is used to instruct the computer to make decisions. You can write
function definitions without using if, but it is
used often enough, and is important enough, to be included in
this chapter. It is used, for example, in the code for the
function beginning-of-buffer.
The basic idea behind an if, is that "if a
test is true, then an expression is evaluated." If the
test is not true, the expression is not evaluated. For example,
you might make a decision such as, "if it is warm and sunny, then
go to the beach!"
An if expression written in Lisp
does not use the word `then'; the test and the action are the
second and third elements of the list whose first element is
if. Nonetheless, the test part of an
if expression is often called the
if-part and the second argument is often
called the then-part.
Also, when an if expression is written, the
true-or-false-test is usually written on the same line as the
symbol if, but the action to carry out if the test
is true, the "then-part", is written on the second and subsequent
lines. This makes the if expression easier to read.
(if true-or-false-test
action-to-carry-out-if-test-is-true)
The true-or-false-test will be an expression that is evaluated by the Lisp interpreter.
Here is an example that you can evaluate in the usual manner. The test is whether the number 5 is greater than the number 4. Since it is, the message `5 is greater than 4!' will be printed.
(if (> 5 4) ; if-part
(message "5 is greater than 4!")) ; then-part
(The function > tests whether its first argument
is greater than its second argument and returns true if it is.)
Of course, in actual use, the test in an if
expression will not be fixed for all time as it is by the
expression (> 5 4). Instead, at least one of the
variables used in the test will be bound to a value that is not
known ahead of time. (If the value were known ahead of time, we
would not need to run the test!)
For example, the value may be bound to an argument of a function
definition. In the following function definition, the character
of the animal is a value that is passed to the function. If the
value bound to characteristic is
fierce, then the message, `It's a
tiger!' will be printed; otherwise, nil will
be returned.
(defun type-of-animal (characteristic)
"Print message in echo area depending on CHARACTERISTIC.
If the CHARACTERISTIC is the symbol `fierce',
then warn of a tiger."
(if (equal characteristic 'fierce)
(message "It's a tiger!")))
If you are reading this inside of GNU Emacs, you can evaluate the function definition in the usual way to install it in Emacs, and then you can evaluate the following two expressions to see the results:
(type-of-animal 'fierce) (type-of-animal 'zebra)
When you evaluate (type-of-animal 'fierce), you will
see the following message printed in the echo area: "It's a
tiger!"; and when you evaluate (type-of-animal
'zebra) you will see nil printed in the echo
area.
type-of-animal Function in
Detail
Let's look at the type-of-animal function in detail.
The function definition for type-of-animal was
written by filling the slots of two templates, one for a function
definition as a whole, and a second for an if
expression.
The template for every function that is not interactive is:
(defun name-of-function (argument-list) "documentation..." body...)
The parts of the function that match this template look like this:
(defun type-of-animal (characteristic)
"Print message in echo area depending on CHARACTERISTIC.
If the CHARACTERISTIC is the symbol `fierce',
then warn of a tiger."
body: the if expression)
In this case, the name of function is
type-of-animal; it is passed the value of one
argument. The argument list is followed by a multi-line
documentation string. The documentation string is included in the
example because it is a good habit to write documentation string
for every function definition. The body of the function
definition consists of the if expression.
The template for an if expression looks like this:
(if true-or-false-test
action-to-carry-out-if-the-test-returns-true)
In the type-of-animal function, the actual code for
the if looks like this:
(if (equal characteristic 'fierce)
(message "It's a tiger!")))
Here, the true-or-false-test is the expression:
(equal characteristic 'fierce)
In Lisp, equal is a function that determines whether
its first argument is equal to its second argument. The second
argument is the quoted symbol 'fierce and the first
argument is the value of the symbol
characteristic---in other words, the argument passed
to this function.
In the first exercise of type-of-animal, the
argument fierce is passed to
type-of-animal. Since fierce is equal
to fierce, the expression, (equal
characteristic 'fierce), returns a value of true. When
this happens, the if evaluates the second argument
or then-part of the if: (message "It's
tiger!").
On the other hand, in the second exercise of
type-of-animal, the argument zebra is
passed to type-of-animal. zebra is not
equal to fierce, so the then-part is not evaluated
and nil is returned by the if
expression.
An if expression may have an optional third
argument, called the else-part, for the case
when the true-or-false-test returns false. When this happens, the
second argument or then-part of the overall if
expression is not evaluated, but the third or else-part
is evaluated. You might think of this as the cloudy day
alternative for the decision `if it is warm and sunny, then go to
the beach, else read a book!".
The word "else" is not written in the Lisp code; the else-part of
an if expression comes after the then-part. In the
written Lisp, the else-part is usually written to start on a line
of its own and is indented less than the then-part:
(if true-or-false-test
action-to-carry-out-if-the-test-returns-true)
action-to-carry-out-if-the-test-returns-false)
For example, the following if expression prints the
message `4 is not greater than 5!' when you evaluate
it in the usual way:
(if (> 4 5) ; if-part
(message "5 is greater than 4!") ; then-part
(message "4 is not greater than 5!")) ; else-part
Note that the different levels of indentation make it easy to
distinguish the then-part from the else-part. (GNU Emacs has
several commands that automatically indent if
expressions correctly. See section 1.1.3 GNU Emacs Helps You Type
Lists.)
We can extend the type-of-animal function to include
an else-part by simply incorporating an additional part to the
if expression.
You can see the consequences of doing this if you evaluate the
following version of the type-of-animal function
definition to install it and then evaluate the two subsequent
expressions to pass different arguments to the function.
(defun type-of-animal (characteristic) ; Second version.
"Print message in echo area depending on CHARACTERISTIC.
If the CHARACTERISTIC is the symbol `fierce',
then warn of a tiger;
else say it's not fierce."
(if (equal characteristic 'fierce)
(message "It's a tiger!")
(message "It's not fierce!")))
(type-of-animal 'fierce) (type-of-animal 'zebra)
When you evaluate (type-of-animal 'fierce), you will
see the following message printed in the echo area: "It's a
tiger!"; but when you evaluate (type-of-animal
'zebra), you will see "It's not fierce!".
(Of course, if the characteristic were
ferocious, the message "It's not
fierce!" would be printed; and it would be misleading!
When you write code, you need to take into account the
possibility that some such argument will be tested by the
if and write your program accordingly.)
There is an important aspect to the truth test in an
if expression. So far, we have spoken of `true' and
`false' as values of predicates as if they were new kinds of Lisp
objects. In fact, `false' is just our old friend
nil. Anything else--anything at all--is `true'.
The expression that tests for truth is interpreted as
true if the result of evaluating it is a value
that is not nil. In other words, the result of the
test is considered true if the value returned is a number such as
47, a string such as "hello", or a symbol (other
than nil) such as flowers, or a list,
or even a buffer!
Before illustrating this, we need an explanation of
nil.
In Lisp, the symbol nil has two meanings. First, it
means the empty list. Second, it means false and is the value
returned when a true-or-false-test tests false. nil
can be written as an empty list, (), or as
nil. As far as the Lisp interpreter is concerned,
() and nil are the same. Humans,
however, tend to use nil for false and
() for the empty list.
In Lisp, any value that is not nil---is not the
empty list--is considered true. This means that if an evaluation
returns something that is not an empty list, an if
expression will test true. For example, if a number is put in the
slot for the test, it will be evaluated and will return itself,
since that is what numbers do when evaluated. In this case, the
if expression will test true. The expression tests
false only when nil, an empty list, is returned by
evaluating the expression.
You can see this by evaluating the two expressions in the following examples.
In the first example, the number 4 is evaluated as the test in
the if expression and returns itself; consequently,
the then-part of the expression is evaluated and returned:
`true' appears in the echo area. In the second
example, the nil indicates false; consequently, the
else-part of the expression is evaluated and returned:
`false' appears in the echo area.
(if 4
'true
'false)
(if nil
'true
'false)
Incidentally, if some other useful value is not available for a
test that returns true, then the Lisp interpreter will return the
symbol t for true. For example, the expression
(> 5 4) returns t when evaluated, as
you can see by evaluating it in the usual way:
(> 5 4)
On the other hand, this function returns nil if the
test is false.
(> 4 5)
save-excursion
The save-excursion function is the fourth and final
special form that we will discuss in this chapter.
In Emacs Lisp programs used for editing, the
save-excursion function is very common. It saves the
location of point and mark, executes the body of the function,
and then restores point and mark to their previous positions if
their locations were changed. Its primary purpose is to keep the
user from being surprised and disturbed by unexpected movement of
point or mark.
Before discussing save-excursion, however, it may be
useful first to review what point and mark are in GNU Emacs.
Point is the current location of the cursor.
Wherever the cursor is, that is point. More precisely, on
terminals where the cursor appears to be on top of a character,
point is immediately before the character. In Emacs Lisp, point
is an integer. The first character in a buffer is number one, the
second is number two, and so on. The function point
returns the current position of the cursor as a number. Each
buffer has its own value for point.
The mark is another position in the buffer; its
value can be set with a command such as C-SPC
(set-mark-command). If a mark has been set, you can
use the command C-x C-x
(exchange-point-and-mark) to cause the cursor to
jump to the mark and set the mark to be the previous position of
point. In addition, if you set another mark, the position of the
previous mark is saved in the mark ring. Many mark positions can
be saved this way. You can jump the cursor to a saved mark by
typing C-u C-SPC one or more times.
The part of the buffer between point and mark is called
the region. Numerous commands work on the
region, including center-region,
count-lines-region, kill-region, and
print-region.
The save-excursion special form saves the locations
of point and mark and restores those positions after the code
within the body of the special form is evaluated by the Lisp
interpreter. Thus, if point were in the beginning of a piece of
text and some code moved point to the end of the buffer, the
save-excursion would put point back to where it was
before, after the expressions in the body of the function were
evaluated.
In Emacs, a function frequently moves point as part of its
internal workings even though a user would not expect this. For
example, count-lines-region moves point. To prevent
the user from being bothered by jumps that are both unexpected
and (from the user's point of view) unnecessary,
save-excursion is often used to keep point and mark
in the location expected by the user. The use of
save-excursion is good housekeeping.
To make sure the house stays clean, save-excursion
restores the values of point and mark even if something goes
wrong in the code inside of it (or, to be more precise and to use
the proper jargon, "in case of abnormal exit"). This feature is
very helpful.
In addition to recording the values of point and mark,
save-excursion keeps track of the current buffer,
and restores it, too. This means you can write code that will
change the buffer and have save-excursion switch you
back to the original buffer. This is how
save-excursion is used in
append-to-buffer. (See section 4.4 The Definition of
append-to-buffer.)
save-excursion
Expression
The template for code using save-excursion is
simple:
(save-excursion body...)
The body of the function is one or more expressions that will be
evaluated in sequence by the Lisp interpreter. If there is more
than one expression in the body, the value of the last one will
be returned as the value of the save-excursion
function. The other expressions in the body are evaluated only
for their side effects; and save-excursion itself is
used only for its side effect (which is restoring the positions
of point and mark).
In more detail, the template for a save-excursion
expression looks like this:
(save-excursion first-expression-in-body second-expression-in-body third-expression-in-body ... last-expression-in-body)
An expression, of course, may be a symbol on its own or a list.
In Emacs Lisp code, a save-excursion expression
often occurs within the body of a let expression. It
looks like this:
(let varlist
(save-excursion
body...))
In the last few chapters we have introduced a fair number of functions and special forms. Here they are described in brief, along with a few similar functions that have not been mentioned yet.
eval-last-sexp
defun
(defun back-to-indentation () "Point to first visible character on line." (interactive) (beginning-of-line 1) (skip-chars-forward " \t"))
interactive
let
let and give them an initial value, either
nil or a specified value; then evaluate the rest
of the expressions in the body of the let and
return the value of the last one. Inside the body of the
let, the Lisp interpreter does not see the values
of the variables of the same names that are bound outside of
the let. For example,
(let ((foo (buffer-name))
(bar (buffer-size)))
(message
"This buffer is %s and has %d characters."
foo bar))
save-excursion
(message "We are %d characters into this buffer."
(- (point)
(save-excursion
(goto-char (point-min)) (point))))
if
if special form is called a
conditional. There are other conditionals in
Emacs Lisp, but if is perhaps the most commonly
used. For example,
(if (string= (int-to-string 19)
(substring (emacs-version) 10 12))
(message "This is version 19 Emacs")
(message "This is not version 19 Emacs"))
equal
eq
equal
returns true if the two objects have a similar structure and
contents. Another function, eq, returns true if
both arguments are actually the same object.
<
>
<=
>=
< function tests whether the first argument
is smaller than the second argument. A corresponding function,
>, tests whether the first argument is greater
than the second. Likewise, <= tests whether the
first argument is less than or equal to the second and
>= tests whether the first argument is greater
than or equal to the second. In all cases, both arguments must
be numbers.
message
setq
set
setq function sets the value of its first
argument to the value of the second argument. The first
argument is automatically quoted by setq. It does
the same for succeeding pairs of arguments. Another function,
set, takes only two arguments and evaluates both
of them before setting the value returned by its first argument
to the value returned by its second argument.
buffer-name
buffer-file-name
current-buffer
other-buffer
other-buffer as an argument and other
than the current buffer).
switch-to-buffer
set-buffer
buffer-size
point
point-min
point-max
Write a non-interactive function that doubles the value of its argument, a number. Make that function interactive.
Write a function that tests whether the current value of
fill-column is greater than the argument passed the
function, and if so, prints an appropriate message.
In this chapter we study in detail several of the functions used in GNU Emacs. This is called a "walk-through". These functions are used as examples of Lisp code, but are not imaginary examples; with the exception of the first, simplified function definition, these functions show the actual code used in GNU Emacs. You can learn a great deal from these definitions. The functions described here are all related to buffers. Later, we will study other functions.
In this walk-through, I will describe each new function as we come to it, sometimes in detail and sometimes briefly. If you are interested, you can get the full documentation of any Emacs Lisp function at any time by typing C-h f and then the name of the function (and then RET). Similarly, you can get the full documentation for a variable by typing C-h v and then the name of the variable (and then RET).
Also, if you want to see a function in its
original source file, you can use the find-tags
function to jump to it. Type M-. (i.e., type the
META and the period key at the same time, or else
type the ESC key and then type the period key), and
then, at the prompt, type in the name of the function whose
source code you want to see, such as
mark-whole-buffer, and then type RET.
Emacs will switch buffers and display the source code for the
function on your screen. To switch back to this buffer, type
C-x b RET.
Depending on how the initial default values
of your copy of Emacs are set, you may also need to specify a
`tags table', which is a file called `TAGS'. The one
you will most likely want to specify is in the
`emacs/src' directory; thus you would use the
M-x visit-tags-table command and specify a
pathname such as `/usr/local/emacs/src/TAGS'. See
section `Tag Tables' in The GNU Emacs Manual.
Also, see section 12.5
Create Your Own `TAGS' File, for how to create
your own.
After you become more familiar with Emacs Lisp, you will find
that you will frequently use find-tags to navigate
your way around source code; and you will create your own
`TAGS' tables.
Incidentally, the files that contain Lisp code are conventionally called libraries. The metaphor is derived from that of a specialized library, such as a law library or an engineering library, rather than a general library. Each library, or file, contains functions that relate to a particular topic or activity, such as `abbrev.el' for handling abbreviations and other typing shortcuts, and `help.el' for on-line help. (Sometimes several libraries provide code for a single activity, as the various `rmail...' files provide code for reading electronic mail.) In The GNU Emacs Manual, you will see sentences such as "The C-h p command lets you search the standard Emacs Lisp libraries by topic keywords."
beginning-of-buffer
Definition
The beginning-of-buffer command is a good function
to start with since you are likely to be familiar with it and it
is easy to understand. Used as an interactive command,
beginning-of-buffer moves the cursor to the
beginning of the buffer, leaving the mark at the previous
position. It is generally bound to M-<.
In this section, we will discuss a shortened version of the
function that shows how it is most frequently used. This
shortened function works as written, but it does not contain the
code for a complex option. In another section, we will describe
the entire function. (See section 5.3 Complete Definition of
beginning-of-buffer.)
Before looking at the code, let's consider what the function definition has to contain: it must include an expression that makes the function interactive so it can be called by typing M-x beginning-of-buffer or by typing a keychord such as C-<; it must include code to leave a mark at the original position in the buffer; and it must include code to move the cursor to the beginning of the buffer.
Here is the complete text of the shortened version of the function:
(defun simplified-beginning-of-buffer () "Move point to the beginning of the buffer; leave mark at previous position." (interactive) (push-mark) (goto-char (point-min)))
Like all function definitions, this definition has five parts
following the special form defun:
simplified-beginning-of-buffer.
(),
In this function definition, the argument list is empty; this means that this function does not require any arguments. (When we look at the definition for the complete function, we will see that it may be passed an optional argument.)
The interactive expression tells Emacs that the function is
intended to be used interactively. In this case,
interactive does not have an argument because
simplified-beginning-of-buffer does not require one.
The body of the function consists of the two lines:
(push-mark) (goto-char (point-min))
The first of these lines is the expression,
(push-mark). When this expression is evaluated by
the Lisp interpreter, it sets a mark at the current position of
the cursor, wherever that may be. The position of this mark is
saved in the mark ring.
The next line is (goto-char (point-min)). This
expression jumps the cursor to the minimum point in the buffer,
that is, to the beginning of the buffer (or to the beginning of
the accessible portion of the buffer if it is narrowed. See
section 6 Narrowing and
Widening.)
The push-mark command sets a mark at the place where
the cursor was located before it was moved to the beginning of
the buffer by the (goto-char (point-min))
expression. Consequently, you can, if you wish, go back to where
you were originally by typing C-x C-x.
That is all there is to the function definition!
When you are reading code such as this and
come upon an unfamiliar function, such as
goto-char, you can find out what it does by using
the describe-function command. To use this
command, type C-h f and then type in the name of
the function and press RET. The
describe-function command will print the
function's documentation string in a `*Help*' window.
For example, the documentation for goto-char is:
One arg, a number. Set point to that number. Beginning of buffer is position (point-min), end is (point-max).
(The prompt for describe-function will offer you the
symbol preceding the cursor, so you can save typing by
positioning the cursor right after the function and then typing
C-h f RET.)
The end-of-buffer function definition is written in
the same way as the beginning-of-buffer definition
except that the body of the function contains the expression
(goto-char (point-max)) in place of (goto-char
(point-min)).
mark-whole-buffer
The mark-whole-buffer function is no harder to
understand than the simplified-beginning-of-buffer
function. In this case, however, we will look at the complete
function, not a shortened version.
The mark-whole-buffer function is not as commonly
used as the beginning-of-buffer function, but is
useful nonetheless: it marks a whole buffer as a region by
putting point at the beginning and a mark at the end of the
buffer. It is generally bound to C-x h.
The code for the complete function looks like this:
(defun mark-whole-buffer () "Put point at beginning and mark at end of buffer." (interactive) (push-mark (point)) (push-mark (point-max)) (goto-char (point-min)))
Like all other functions, the mark-whole-buffer
function fits into the template for a function definition. The
template looks like this:
(defun name-of-function (argument-list) "documentation..." (interactive-expression...) body...)
Here is how the function works: the name of the function is
mark-whole-buffer; it is followed by an empty
argument list, `()', which means that the function
does not require arguments. The documentation comes next.
The next line is an (interactive) expression that
tells Emacs that the function will be used interactively. These
details are similar to the
simplified-beginning-of-buffer function described in
the previous section.
mark-whole-buffer
The body of the mark-whole-buffer function consists
of three lines of code:
(push-mark (point)) (push-mark (point-max)) (goto-char (point-min))
The first of these lines is the expression, (push-mark
(point)).
This line does exactly the same job as the first line of the body
of the simplified-beginning-of-buffer function,
which is written (push-mark). In both cases, the
Lisp interpreter sets a mark at the current position of the
cursor.
I don't know why the expression in mark-whole-buffer
is written (push-mark (point)) and the expression in
beginning-of-buffer is written
(push-mark). Perhaps whoever wrote the code did not
know that the argument for push-mark is optional and
that if push-mark is not passed an argument, the
function automatically sets mark at the location of point by
default. Or perhaps the expression was written so as to parallel
the structure of the next line. In any case, the line causes
Emacs to determine the position of point and set a mark there.
The next line of mark-whole-buffer is
(push-mark (point-max)). This expression sets a mark
at the point in the buffer that has the highest number. This will
be the end of the buffer (or, if the buffer is narrowed, the end
of the accessible portion of the buffer. See section 6 Narrowing and Widening, for
more about narrowing.) After this mark has been set, the previous
mark, the one set at point, is no longer set, but Emacs remembers
its position, just as all other recent marks are always
remembered. This means that you can, if you wish, go back to that
position by typing C-u C-SPC twice.
Finally, the last line of the function is (goto-char
(point-min))). This is written exactly the same way as it
is written in beginning-of-buffer. The expression
moves the cursor to the minimum point in the buffer, that is, to
the beginning of the buffer (or to the beginning of the
accessible portion of the buffer). As a result of this, point is
placed at the beginning of the buffer and mark is set at the end
of the buffer. The whole buffer is, therefore, the region.
append-to-buffer
The append-to-buffer command is very nearly as
simple as the mark-whole-buffer command. What it
does is copy the region (that is, the part of the buffer between
point and mark) from the current buffer to a specified buffer.
The append-to-buffer command
uses the insert-buffer-substring function to copy
the region. insert-buffer-substring is described
by its name: it takes a string of characters from part of a
buffer, a "substring", and inserts them into another buffer.
Most of append-to-buffer is concerned with
setting up the conditions for
insert-buffer-substring to work: the code must
specify both the buffer to which the text will go and the
region that will be copied. Here is the complete text of the
function:
(defun append-to-buffer (buffer start end)
"Append to specified buffer the text of the region.
It is inserted into that buffer before its point.
When calling from a program, give three arguments:
a buffer or the name of one, and two character numbers
specifying the portion of the current buffer to be copied."
(interactive "BAppend to buffer: \nr")
(let ((oldbuf (current-buffer)))
(save-excursion
(set-buffer (get-buffer-create buffer))
(insert-buffer-substring oldbuf start end))))
The function can be understood by looking at it as a series of filled-in templates.
The outermost template is for the function definition. In this case, it looks like this (with several slots filled in):
(defun append-to-buffer (buffer start end) "documentation..." (interactive "BAppend to buffer: \nr") body...)
The first line of the function includes its name and three
arguments. The arguments are the buffer to which the
text will be copied, and the start and
end of the region in the current buffer that will be
copied.
The next part of the function is the documentation, which is clear and complete.
append-to-buffer
Interactive Expression
Since the append-to-buffer function will be used
interactively, the function must have an interactive
expression. (For a review of interactive, see
section 3.3 Make a Function
Interactive.) The expression reads as follows:
(interactive "BAppend to buffer: \nr")
This expression has an argument inside of quotation marks and that argument has two parts, separated by `\n'.
The first part is `BAppend to buffer: '. Here, the
`B' tells Emacs to ask for the name of the buffer
that will be passed to the function. Emacs will ask for the name
by prompting the user in the minibuffer, using the string
following the `B', which is the string `Append
to buffer: '. Emacs then binds the variable
buffer in the function's argument list to the
specified buffer.
The newline, `\n', separates the first part of the
argument from the second part. It is followed by an
`r' that tells Emacs to bind the two arguments that
follow the symbol buffer in the function's argument
list (that is, start and end) to the
values of point and mark.
append-to-buffer
The body of the append-to-buffer function begins
with let.
As we have seen before (see section 3.6 let), the
purpose of a let expression is to create and give
initial values to one or more variables that will only be used
within the body of the let. This means that such a
variable will not be confused with any variable of the same name
outside the let expression.
We can see how the let expression fits into the
function as a whole by showing a template for
append-to-buffer with the let
expression in outline:
(defun append-to-buffer (buffer start end)
"documentation..."
(interactive "BAppend to buffer: \nr")
(let ((variable value))
body...)
The let expression has three elements:
let;
(variable value);
let expression.
In the append-to-buffer function, the varlist looks
like this:
(oldbuf (current-buffer))
In this part of the let expression, the one
variable, oldbuf, is bound to the value returned by
the (current-buffer) expression. The variable,
oldbuf, is used to keep track of the buffer in which
you are working.
The element or elements of a varlist are surrounded by a set of
parentheses so the Lisp interpreter can distinguish the varlist
from the body of the let. As a consequence, the
two-element list within the varlist is surrounded by
circumscribing set of parentheses. The line looks like this:
(let ((oldbuf (current-buffer))) ... )
The two parentheses before oldbuf might surprise you
if you did not realize that the first parenthesis before
oldbuf marks the boundary of the varlist and the
second parenthesis marks the beginning of the two-element list,
(oldbuf (current-buffer)).
save-excursion in
append-to-buffer
The body of the let expression in
append-to-buffer consists of a
save-excursion expression.
The save-excursion function saves the locations of
point and mark, and restores them to those positions after the
expressions in the body of the save-excursion
complete execution. In addition, save-excursion
keeps track of the original buffer, and restores it. This is how
save-excursion is used in
append-to-buffer.
Incidentally, it is worth noting here that a
Lisp function is normally formatted so that everything that is
enclosed in a multi-line spread is indented more to the right
than the first symbol. In this function definition, the
let is indented more than the defun,
and the save-excursion is indented more than the
let, like this:
(defun ...
...
...
(let...
(save-excursion
...
This formatting convention makes it easy to see that the two
lines in the body of the save-excursion are enclosed
by the parentheses associated with save-excursion,
just as the save-excursion itself is enclosed by the
parentheses associated with the let:
(let ((oldbuf (current-buffer)))
(save-excursion
(set-buffer (get-buffer-create buffer))
(insert-buffer-substring oldbuf start end))))
The use of the save-excursion function can be viewed
as a process of filling in the slots of a template:
(save-excursion first-expression-in-body second-expression-in-body ... last-expression-in-body)
In this function, the body of the save-excursion
contains only two expressions. The body looks like this:
(set-buffer (get-buffer-create buffer)) (insert-buffer-substring oldbuf start end)
When the append-to-buffer function is evaluated, the
two expressions in the body of the save-excursion
are evaluated in sequence. The value of the last expression is
returned as the value of the save-excursion
function; the other expression is evaluated only for its side
effects.
The first line in the body of the save-excursion
uses the set-buffer function to change the current
buffer to the one specified in the first argument to
append-to-buffer. (Changing the buffer is the side
effect; as we have said before, in Lisp, a side effect is often
the primary thing we want.) The second line does the primary work
of the function.
The set-buffer function changes Emacs' attention to
the buffer to which the text will be copied and from which
save-excursion will return. The line looks like
this:
(set-buffer (get-buffer-create buffer))
The innermost expression of this list is (get-buffer-create
buffer). This expression uses the
get-buffer-create function, which either gets the
named buffer, or if it does not exist, creates one with the given
name. This means you can use append-to-buffer to put
text into a buffer that did not previously exist.
get-buffer-create also keeps set-buffer
from getting an unnecessary error: set-buffer needs
a buffer to go to; if you were to specify a buffer that does not
exist, Emacs would baulk. Since get-buffer-create
will create a buffer if none exists, set-buffer is
always provided with a buffer.
The last line of append-to-buffer does the work of
appending the text:
(insert-buffer-substring oldbuf start end)
The insert-buffer-substring function copies a string
from the buffer specified as its first argument and
inserts the string into the present buffer. In this case, the
argument to insert-buffer-substring is the value of
the variable created and bound by the let, namely
the value of oldbuf, which was the current buffer
when you gave the append-to-buffer command.
After insert-buffer-substring has done its work,
save-excursion will restore the action to the
original buffer and append-to-buffer will have done
its job.
Written in skeletal form, the workings of the body look like this:
(let (bind-oldbuf-to-value-of-current-buffer) (save-excursion ; Keep track of buffer. change-buffer insert-substring-from-oldbuf-into-buffer) change-back-to-original-buffer-when-finished let-the-local-meaning-of-oldbuf-disappear-when-finished
In summary, append-to-buffer works as follows: it
saves the value of the current buffer in the variable called
oldbuf. It gets the new buffer, creating one if need
be, and switches Emacs to it. Using the value of
oldbuf, it inserts the region of text from the old
buffer into the new buffer; and then using
save-excursion, it brings you back to your original
buffer.
In looking at append-to-buffer, you have explored a
fairly complex function. It shows how to use let and
save-excursion, and how to change to and come back
from another buffer. Many functions definitions use
let, save-excursion, and
set-buffer this way.
Here is a brief summary of the various functions discussed in this chapter.
describe-function
describe-variable
find-tag
save-excursion
save-excursion have been
evaluated. Also, remember the current buffer and return to it.
push-mark
goto-char
(point-min).
insert-buffer-substring
mark-whole-buffer
set-buffer
get-buffer-create
get-buffer
get-buffer function returns
nil if the named buffer does not exist.
Write your own simplified-end-of-buffer function
definition; then test it to see whether it works.
Use if and get-buffer to write a
function that prints a message telling you whether a buffer
exists.
Using find-tag, find the source for the
copy-to-buffer function.
In this chapter, we build on what we have learned in previous
chapters by looking at more complex functions. The
copy-to-buffer function illustrates use of two
save-excursion expressions in one definition, while
the insert-buffer function illustrates use of
* in an interactive expression, use of
or, and the important distinction between a name and
the object to which the name refers.
copy-to-buffer
After understanding how append-to-buffer works, it
is easy to understand copy-to-buffer. This function
copies text into a buffer, but instead of adding to the second
buffer, it replaces the previous text in the second buffer. The
code for the copy-to-buffer function is almost the
same as the code for append-to-buffer, except that
erase-buffer and a second
save-excursion are used. (See section 4.4 The Definition of
append-to-buffer, for the description of
append-to-buffer.)
The body of copy-to-buffer looks like this
...
(interactive "BCopy to buffer: \nr")
(let ((oldbuf (current-buffer)))
(save-excursion
(set-buffer (get-buffer-create buffer))
(erase-buffer)
(save-excursion
(insert-buffer-substring oldbuf start end)))))
This code is similar to the code in
append-to-buffer: it is only after changing to the
buffer to which the text will be copied that the definition for
this function diverges from the definition for
append-to-buffer: the copy-to-buffer
function erases the buffer's former contents. (This is what is
meant by `replacement'; to replace text, Emacs erases the
previous text and then inserts new text.) After erasing the
previous contents of the buffer, save-excursion is
used for a second time and the new text is inserted.
Why is save-excursion used twice? Consider again
what the function does.
In outline, the body of copy-to-buffer looks like
this:
(let (bind-oldbuf-to-value-of-current-buffer) (save-excursion ; First use ofsave-excursion. change-buffer (erase-buffer) (save-excursion ; Second use ofsave-excursion. insert-substring-from-oldbuf-into-buffer)))
The first use of save-excursion returns Emacs to the
buffer from which the text is being copied. That is clear, and is
just like its use in append-to-buffer. Why the
second use? The reason is that
insert-buffer-substring always leaves point at the
end of the region being inserted. The second
save-excursion causes Emacs to leave point at the
beginning of the text being inserted. In most circumstances,
users prefer to find point at the beginning of inserted text. (Of
course, the copy-to-buffer function returns the user
to the original buffer when done--but if the user then
switches to the copied-to buffer, point will go to the beginning
of the text. Thus, this use of a second
save-excursion is a little nicety.)
insert-buffer
insert-buffer is yet another buffer-related
function. This command copies another buffer into the
current buffer. It is the reverse of
append-to-buffer or copy-to-buffer,
since they copy a region of text from the current buffer
to another buffer.
In addition, this code illustrates the use of
interactive with a buffer that might be
read-only and the important distinction between
the name of an object and the object actually referred to. Here
is the code:
(defun insert-buffer (buffer)
"Insert after point the contents of BUFFER.
Puts mark after the inserted text.
BUFFER may be a buffer or a buffer name."
(interactive "*bInsert buffer: ")
(or (bufferp buffer)
(setq buffer (get-buffer buffer)))
(let (start end newmark)
(save-excursion
(save-excursion
(set-buffer buffer)
(setq start (point-min) end (point-max)))
(insert-buffer-substring buffer start end)
(setq newmark (point)))
(push-mark newmark)))
As with other function definitions, you can use a template to see an outline of the function:
(defun insert-buffer (buffer) "documentation..." (interactive "*bInsert buffer: ") body...)
insert-buffer
In insert-buffer, the argument to the
interactive declaration has two parts, an asterisk,
`*', and `bInsert buffer: '.
The asterisk is for the situation when the buffer is a read-only
buffer--a buffer that cannot be modified. If
insert-buffer is called on a buffer that is
read-only, a message to this effect is printed in the echo area
and the terminal may beep or blink at you; you will not be
permitted to insert anything into current buffer. The asterisk
does not need to be followed by a newline to separate it from the
next argument.
The next argument starts with a lower case `b'.
(This is different from the code for
append-to-buffer, which uses an upper-case
`B'. See section 4.4 The Definition of
append-to-buffer.) The lower-case
`b' tells the Lisp interpreter that the argument for
insert-buffer should be an existing buffer or else
its name. (The upper-case `B' option provides for
the possibility that the buffer does not exist.) Emacs will
prompt you for the name of the buffer, offering you a default
buffer, with name completion enabled. If the buffer does not
exist, you receive a message that says "No match"; your terminal
may beep at you as well.
insert-buffer
Function
The body of the insert-buffer function has two major
parts: an or expression and a let
expression. The purpose of the or expression is to
ensure that the argument buffer is bound to a buffer
and not just the name of a buffer. The body of the
let expression contains the code which copies the
other buffer into the current buffer.
In outline, the two expressions fit into the
insert-buffer function like this:
(defun insert-buffer (buffer)
"documentation..."
(interactive "*bInsert buffer: ")
(or ...
...
(let (varlist)
body-of-let... )
To understand how the or expression ensures that the
argument buffer is bound to a buffer and not to the
name of a buffer, it is first necessary to understand the
or function.
Before doing this, let me rewrite this part of the function using
if so that you can see what is done in a manner that
will be familiar.
insert-buffer With an
if Instead of an or
The job to be done is to make sure the value of
buffer is a buffer itself and not the name of a
buffer. If the value is the name, then the buffer itself must be
got.
You can imagine yourself at a conference where an usher is wandering around holding a list with your name on it and looking for you: the usher is "bound" to your name, not to you; but when the usher finds you and takes your arm, the usher becomes "bound" to you.
In Lisp, you might describe this situation like this:
(if (not (holding-on-to-guest))
(find-and-take-arm-of-guest))
We want to do the same thing with a buffer--if we do not have the buffer itself, we want to get it.
Using a predicate called bufferp that tells us
whether we have a buffer (rather than its name), we can write the
code like this:
(if (not (bufferp buffer)) ; if-part
(setq buffer (get-buffer buffer))) ; then-part
Here, the true-or-false-test of the if expression is
(not (bufferp buffer)); and the then-part is the
expression (setq buffer (get-buffer buffer)).
In the test, the function bufferp returns true if
its argument is a buffer--but false if its argument is the name
of the buffer. (The last character of the function name
bufferp is the character `p'; as we saw
earlier, such use of `p' is a convention that
indicates that the function is a predicate, which is a term that
means that the function will determine whether some property is
true or false. See section 1.8.4 Using the Wrong Type Object
as an Argument.)
The function not precedes the expression
(bufferp buffer), so the true-or-false-test looks
like this:
(not (bufferp buffer))
not is a function the returns true if its argument
is false and false if its argument is true. So if (bufferp
buffer) returns true, the not expression
returns false and vice-versa: what is "not true" is false and
what is "not false" is true.
Using this test, the if expression works as follows:
when the value of the variable buffer is actually a
buffer rather then its name, the true-or-false-test returns false
and the if expression does not evaluate the
then-part. This is fine, since we do not need to do anything to
the variable buffer if it really is a buffer.
On the other hand, when the value of buffer is not a
buffer itself, but the name of a buffer, the true-or-false-test
returns true and the then-part of the expression is evaluated. In
this case, the then-part is (setq buffer (get-buffer
buffer)). This expression uses the get-buffer
function to return an actual buffer itself, given its name. The
setq then sets the variable buffer to
the value of the buffer itself, replacing its previous value
(which was the name of the buffer).
or in the Body
The purpose of the or expression in the
insert-buffer function is to ensure that the
argument buffer is bound to a buffer and not just
the name of a buffer. The previous section shows how the job
could have been done using an if expression.
However, the insert-buffer function actually uses
or. To understand this, it is necessary to
understand how or works.
An or function can have any
number of arguments. It evaluates each argument in turn and
returns the value of the first of its arguments that is not
nil. Also, and this is a crucial feature of
or, it does not evaluate any subsequent arguments
after returning the first non-nil value.
The or expression looks like this:
(or (bufferp buffer)
(setq buffer (get-buffer buffer)))
The first argument to or is the expression
(bufferp buffer). This expression returns true (a
non-nil value) if the buffer is actually a buffer,
and not just the name of a buffer. In the or
expression, if this is the case, the or expression
returns this true value and does not evaluate the next
expression--and this is fine with us, since we do not want to do
anything to the value of buffer if it really is a
buffer.
On the other hand, if the value of (bufferp buffer)
is nil, which it will be if the value of
buffer is the name of a buffer, the Lisp interpreter
evaluates the next element of the or expression.
This is the expression (setq buffer (get-buffer
buffer)). This expression returns a non-nil
value, which is the value to which it sets the variable
buffer---and this value is a buffer itself, not the
name of a buffer.
The result of all this is that the symbol buffer is
always bound to a buffer itself rather than the name of a buffer.
All this is necessary because the set-buffer
function in a following line only works with a buffer itself, not
with the name to a buffer.
Incidentally, using or, the situation with the usher
would be written like this:
(or (holding-on-to-guest) (find-and-take-arm-of-guest))
let Expression in
insert-buffer
After ensuring that the variable buffer refers to a
buffer itself and not just to the name of a buffer, the
insert-buffer function continues with a
let expression. This specifies three local
variables, start, end, and
newmark and binds them to the initial value
nil. These variables are used inside the remainder
of the let and temporarily hide any other occurrence
of variables of the same name in Emacs until the end of the
let.
The body of the let contains two
save-excursion expressions. First, we will look at
the inner save-excursion expression in detail. The
expression looks like this:
(save-excursion (set-buffer buffer) (setq start (point-min) end (point-max)))
The expression (set-buffer buffer) changes Emacs's
attention from the current buffer to the one from which the text
will copied. In that buffer, the variables start and
end are set to the beginning and end of the buffer,
using the commands point-min and
point-max. Note that we have here an illustration of
how setq is able to set two variables in the same
expression. setq's first argument is set to the
value of its second and its third argument is set to the value of
its fourth.
After the body of the inner save-excursion is
evaluated, the save-excursion restores the original
buffer, but start and end remain set to
the values of the beginning and end of the buffer from which the
text will be copied.
The outer save-excursion expression looks like this:
(save-excursion (inner-save-excursion-expression (go-to-new-buffer-and-set-start-and-end) (insert-buffer-substring buffer start end) (setq newmark (point)))
The insert-buffer-substring function copies the text
into the current buffer from the region
indicated by start and end in
buffer. Since the whole of the second buffer lies
between start and end, the whole of the
second buffer is copied into the buffer you are editing. Next,
the value of point, which will be at the end of the inserted
text, is recorded in the variable newmark.
After the body of the outer save-excursion is
evaluated, point and mark are relocated to their original places.
However, it is convenient to locate a mark at the end of the
newly inserted text and locate point at its beginning. The
newmark variable records the end of the inserted
text. In the last line of the let expression, the
(push-mark newmark) expression function sets a mark
to this location. (The previous location of the mark is still
accessible; it is recorded on the mark ring and you can go back
to it with C-u C-SPC.) Meanwhile, point is located at
the beginning of the inserted text, which is where it was before
you called the insert function.
The whole let expression looks like this:
(let (start end newmark)
(save-excursion
(save-excursion
(set-buffer buffer)
(setq start (point-min) end (point-max)))
(insert-buffer-substring buffer start end)
(setq newmark (point)))
(push-mark newmark))
Like the append-to-buffer function, the
insert-buffer function uses let,
save-excursion, and set-buffer. In
addition, the function illustrates one way to use
or. All these functions are building blocks that we
will find and use again and again.
beginning-of-buffer
The basic structure of the beginning-of-buffer
function has already been discussed. (See section 4.2 A Simplified
beginning-of-buffer Definition.) This section
describes the complex part of the definition.
As previously described, when invoked without an argument,
beginning-of-buffer moves the cursor to the
beginning of the buffer, leaving the mark at the previous
position. However, when the command is invoked with a number
between one and ten, the function considers that number to be a
fraction of the length of the buffer, measured in tenths, and
Emacs moves the cursor that fraction of the way from the
beginning of the buffer. Thus, you can either call this function
with the key command M-<, which will move the
cursor to the beginning of the buffer, or with a key command such
as C-u 7 M-< which will move the cursor to a point
70% of the way through the buffer. If a number bigger than ten is
used for the argument, it moves to the end of the buffer.
The beginning-of-buffer function can be called with
or without an argument. The use of the argument is optional.
Unless told otherwise, Lisp expects that a function with an argument in its function definition will be called with a value for that argument. If that does not happen, you get an error and a message that says `Wrong number of arguments'.
However, optional arguments are a feature of
Lisp: a keyword may be used to tell the Lisp
interpreter that an argument is optional. The keyword is
&optional. (The `&' in front
of `optional' is part of the keyword.) In a
function definition, if an argument follows the keyword
&optional, a value does not need to be passed
to that argument when the function is called.
The first line of the function definition of
beginning-of-buffer therefore looks like this:
(defun beginning-of-buffer (&optional arg)
In outline, the whole function looks like this:
(defun beginning-of-buffer (&optional arg)
"documentation..."
(interactive "P")
(push-mark)
(goto-char
(if-there-is-an-argument
figure-out-where-to-go
else-go-to
(point-min))))
The function is similar to
simplified-beginning-of-buffer except that the
interactive expression has "P" as an
argument and the goto-char function is followed by
an if-then-else expression that figures out where to put the
cursor if there is an argument.
The "P" in the interactive expression
tells Emacs to pass a prefix argument, if there is one, to the
function. A prefix argument is made by typing the META
key followed by a number, or by typing C-u and then a
number (if you don't type a number, C-u defaults to
4).
The true-or-false-test of the if expression is
simple: it is simply the argument arg. If
arg has a value that is not nil, which
will be the case if beginning-of-buffer is called
with an argument, then this true-or-false-test will return true
and the then-part of the if expression will be
evaluated. On the other hand, if beginning-of-buffer
is not called with an argument, the value of arg
will be nil and the else-part of the if
expression will be evaluated. The else-part is simply
point-min, and when this is the outcome, the whole
goto-char expression is (goto-char
(point-min)), which is how we saw the
beginning-of-buffer function in its simplified form.
beginning-of-buffer with an
Argument
When beginning-of-buffer is called with an argument,
an expression is evaluated which calculates what value to pass to
goto-char. This expression is rather complicated at
first sight. It includes an inner if expression and
much arithmetic. It looks like this:
(if (> (buffer-size) 10000)
;; Avoid overflow for large buffer sizes!
(* (prefix-numeric-value arg) (/ (buffer-size) 10))
(/
(+ 10
(*
(buffer-size) (prefix-numeric-value arg))) 10))
Like other complex-looking expressions, this one can be distangled by looking at it as parts of a template, in this case, the template for an if-then-else expression. When in skeletal form, the expression looks like this:
(if (buffer-is-large
divide-buffer-size-by-10-and-multiply-by-arg
else-use-alternate-calculation
The true-or-false-test of this inner if expression
checks the size of the buffer. The reason for this is that
version 18 Emacs Lisp uses numbers that are no bigger than eight
million or so (bigger numbers are not needed) and in the
computation that follows, Emacs might try to use over-large
numbers if the buffer were large. The term `overflow', mentioned
in the comment, means numbers that are over large.
There are two cases: if the buffer is large and if it is not.
In beginning-of-buffer, the inner if
expression tests whether the size of the buffer is greater than
10,000 characters. To do this, it uses the >
function and the buffer-size function. The line
looks like this:
(if (> (buffer-size) 10000)
When the buffer is large, the then-part of the if
expression is evaluated. It reads like this (after formatting for
easy reading):
(* (prefix-numeric-value arg) (/ (buffer-size) 10))
This expression is a multiplication, with two arguments to the
function *.
The first argument is (prefix-numeric-value arg).
When "P" is used as the argument for
interactive, the value passed to the function as its
argument is passed a "raw prefix argument", and not a number. (It
is a number in a list.) To perform the arithmetic, a conversion
is necessary, and prefix-numeric-value does the job.
The second argument is (/ (buffer-size)
10). This expression divides the numeric value of the
buffer by ten. This produces a number that tells how many
characters make up one tenth of the buffer size. (In Lisp,
/ is used for division, just as * is
used for multiplication.)
In the multiplication expression as a whole, this amount is multiplied by the value of the prefix argument--the multiplication looks like this:
(* numeric-value-of-prefix-arg number-of-characters-in-one-tenth-of-the-buffer)
If, for example, the prefix argument is `7', the one-tenth value will be multiplied by 7 to give a position 70% of the way through the buffer.
The result of all this is that if the buffer is large, the
goto-char expression reads like this:
(goto-char (* (prefix-numeric-value arg)
(/ (buffer-size) 10)))
This puts the cursor where we want it.
If the buffer contains fewer than 10,000 characters, a slightly different computation is performed. You might think this is not necessary, since the first computation could do the job. However, in a small buffer, the first method may not put the cursor on exactly the desired line; the second method does a better job.
The code looks like this:
(/ (+ 10 (* (buffer-size) (prefix-numeric-value arg))) 10))
This is code in which you figure out what happens by discovering how the functions are embedded in parentheses. It is easier to read if you reformat it with each expression indented more deeply than its enclosing expression:
(/
(+ 10
(*
(buffer-size)
(prefix-numeric-value arg)))
10))
Looking at parentheses, we see that the innermost operation is
(prefix-numeric-value arg), which converts the raw
argument to a number. This number is multiplied by the buffer
size in the following expression:
(* (buffer-size) (prefix-numeric-value arg)
This multiplication creates a number that may be larger than the size of the buffer--seven times larger if the argument is 7, for example. Ten is then added to this number and finally the large number is divided by ten to provide a value that is one character larger than the percentage position in the buffer.
The number that results from all this is passed to
goto-char and the cursor is moved to that point.
beginning-of-buffer
Here is the complete text of the beginning-of-buffer
function:
(defun beginning-of-buffer (&optional arg)
"Move point to the beginning of the buffer;
leave mark at previous position.
With arg N, put point N/10 of the way
from the true beginning.
Don't use this in Lisp programs!
\(goto-char (point-min)) is faster
and does not set the mark."
(interactive "P")
(push-mark)
(goto-char
(if arg
(if (> (buffer-size) 10000)
;; Avoid overflow for large buffer sizes!
(* (prefix-numeric-value arg)
(/ (buffer-size) 10))
(/ (+ 10 (* (buffer-size)
(prefix-numeric-value arg)))
10))
(point-min)))
(if arg (forward-line 1)))
Except for two small points, the previous discussion shows how this function works. The first point deals with a detail in the documentation string, and the second point concerns the last line of the function.
In the documentation string, there is reference to an expression:
\(goto-char (point-min))
A `\' is used before the first parenthesis of this expression. This `\' tells the Lisp interpreter that the expression should be printed as shown in the documentation rather than evaluated as an symbolic expression, which is what it looks like.
Finally, the last line of the beginning-of-buffer
command says to move point to the beginning of the next line if
the command is invoked with an argument:
(if arg (forward-line 1)))
This puts the cursor at the beginning of the first line after the appropriate tenths position in the buffer. This is a flourish that means that the cursor is always located at least the requested tenths of the way through the buffer, which is a nicety that is, perhaps, not necessary, but which, if it did not occur, would be sure to draw complaints.
Here is a brief summary of some of the topics covered in this chapter.
or
nil; if none return a
value that is not nil, return nil. In
brief, return the first true value of the arguments; return a
true value if one or any of the other are true.
and
nil, return nil; if none are
nil, return the value of the last argument. In
brief, return a true value only if all the arguments are true;
return a true value if one and each of the others is
true.
&optional
prefix-numeric-value
(interactive "P") to a numeric value.
forward-line
forward-line goes forward as far as it can and
then returns a count of the number of additional lines it was
supposed to move but couldn't.
erase-buffer
bufferp
t if its argument is a buffer; otherwise
return nil.
&optional Argument
Exercise
Write an interactive function with an optional argument that
tests whether its argument, a number, is greater or less than the
value of fill-column, and tells you which, in a
message. However, if you do not pass an argument to the function,
use 56 as a default value.
Narrowing is a feature of Emacs that makes it possible for you to focus on a specific part of a buffer, and work without accidentally changing other parts. Narrowing is normally disabled since it can confuse novices.
With narrowing, the rest of a buffer is made invisible, as if it weren't there. This is an advantage if, for example, you want to replace a word in one part of a buffer but not in another: you narrow to the part you want and the replacement is carried out only in that section, not in the rest of the buffer. Searches will only work within a narrowed region, not outside of one, so if you are fixing a part of a document, you can keep yourself from accidentally finding parts you do not need to fix by narrowing just to the region you want.
However, narrowing does make the rest of the buffer invisible,
which can scare people who inadvertently invoke narrowing and
think they have deleted a part of their file. Moreover, the
undo command (which is usually bound to C-x
u) does not turn off narrowing (nor should it), so people
can become quite desperate if they do not know that they can
return the rest of a buffer to visibility with the
widen command. (In Emacs version 18, the key binding
for widen is C-x w; in version 19, it is
C-x n w.)
Narrowing is just as useful to the Lisp interpreter as to a
human. Often, an Emacs Lisp function is designed to work on just
part of a buffer; or conversely, an Emacs Lisp function needs to
work on all of a buffer that has been narrowed. The
what-line function, for example, removes the
narrowing from a buffer, if it has any narrowing and when it has
finished its job, restores the narrowing to what it was. On the
other hand, the count-lines function, which is
called by what-line, uses narrowing to restrict
itself to just that portion of the buffer in which it is
interested and then restores the previous situation.
save-restriction Special
Form
In Emacs Lisp, you can use the save-restriction
special form to keep track of whatever narrowing is in effect, if
any. When the Lisp interpreter meets with
save-restriction, it executes the code in the body
of the save-restriction expression, and then undoes
any changes to narrowing that the code caused. If, for example,
the buffer is narrowed and the code that follows
save-restriction gets rid of the narrowing,
save-restriction returns the buffer to its narrowed
region afterwards. In the what-line command, any
narrowing the buffer may have is undone by the widen
command that immediately follows the
save-restriction command. Any original narrowing is
restored just before the completion of the function.
The template for a save-restriction expression is
simple:
(save-restriction body... )
The body of the save-restriction is one or more
expressions that will be evaluated in sequence by the Lisp
interpreter.
Finally, a point to note: when you use both
save-excursion and save-restriction,
one right after the other, you should use
save-excursion outermost. If you write them in
reverse order, you may fail to record narrowing in the buffer to
which Emacs switches after calling save-excursion.
Thus, when written together, save-excursion and
save-restriction should be written like this:
(save-excursion
(save-restriction
body...))
what-line
The what-line command tells you the number of the
line in which the cursor is located. The function illustrates the
use of the save-restriction and
save-excursion commands. Here is the text of the
function in full:
(defun what-line ()
"Print the current line number (in the buffer) of point."
(interactive)
(save-restriction
(widen)
(save-excursion
(beginning-of-line)
(message "Line %d"
(1+ (count-lines 1 (point)))))))
The function has a documentation line and is interactive, as you
would expect. The next two lines use the functions
save-restriction and widen.
The save-restriction special form notes whatever
narrowing is in effect, if any, in the current buffer and
restores that narrowing after the code in the body of the
save-restriction has been evaluated.
The save-restriction special form is followed by
widen. This function undoes any narrowing the
current buffer may have had when what-line was
called. (The narrowing that was there is the narrowing that
save-restriction remembers.) This widening makes it
possible for the line counting commands to count from the
beginning of the buffer. Otherwise, they would have been limited
to counting within the accessible region. Any original narrowing
is restored just before the completion of the function by the
save-restriction special form.
The call to widen is followed by
save-excursion, which saves the location of the
cursor (i.e., of point) and of the mark, and restores them after
the code in the body of the save-excursion uses the
beginning-of-line function to move point.
(Note that the (widen) expression comes between
save-restriction and save-excursion.
When you write the two save- ... expressions in
sequence, write save-excursion outermost.)
The last two lines of the what-line function are
functions to count the number of lines in the buffer and then
print the number in the echo area.
(message "Line %d"
(1+ (count-lines 1 (point)))))))
The message function prints a one-line message at
the bottom of the Emacs screen. The first argument is inside of
quotation marks and is printed as a string of characters.
However, it may contain `%d', `%s', or
`%c' to print arguments that follow the string.
`%d' prints the argument as a decimal, so the
message will say something such as `Line 243'.
The number that is printed in place of the `%d' is computed by the last line of the function:
(1+ (count-lines 1 (point)))
What this does is count the lines from the first position of the
buffer, indicated by the 1, up to
(point), and then add one to that number. (The
1+ function adds one to its argument.) We add one to
it because line 2 has only one line before it, and
count-lines counts only the lines before
the current line.
After count-lines has done it job, and the message
has been printed in the echo area, the
save-excursion restores point and mark to their
original positions; and save-restriction restores
the original narrowing, if any.
Write a function that will display the first 60 characters of the
current buffer, even if you have narrowed the buffer to its
latter half so that the first line is inaccessible. Restore
point, mark, and narrowing. For this exercise, you need to use
save-restriction, widen,
goto-char, point-min,
buffer-substring, message, and other
functions, a whole potpourri.
car, cdr,
cons: Fundamental Functions
In Lisp, car, cdr, and
cons are fundamental functions. The
cons function is used to construct lists, and the
car and cdr functions are used to take
them apart.
In the walk through of the copy-region-as-kill
function, we will see cons as well as two variants
on cdr, namely, setcdr and
nthcdr. (See section 8.5
copy-region-as-kill.)
The name of the cons function is not unreasonable:
it is an abbreviation of the word `construct'. The origins of the
names for car and cdr, on the other
hand, are esoteric: car is an acronym from the
phrase `Contents of the Address part of the Register'; and
cdr (pronounced `could-er') is an acronym from the
phrase `Contents of the Decrement part of the Register'. These
phrases refer to specific pieces of hardware on the very early
computer on which the original Lisp was developed. Besides being
obsolete, the phrases have been completely irrelevant for more
than 25 years to anyone thinking about Lisp. Nonetheless,
although a few brave scholars have begun to use more reasonable
names for these functions, the old terms are still in use. In
particular, since the terms are used in the Emacs Lisp source
code, we will use them in this introduction.
car and cdr
The car of a list is, quite simply, the first item
in the list. Thus the car of the list (rose
violet daisy buttercup) is rose.
If you are reading this in Info in GNU Emacs, you can see this by evaluating the following:
(car '(rose violet daisy buttercup))
After evaluating the expression, rose will appear in
the echo area.
Clearly, a more reasonable name for the car function
would be first and this is often suggested.
car does not remove the first item from the list; it
only reports what it is. After car has been applied
to a list, the list is still the same as it was. In the jargon,
car is `non-destructive'. This feature turns out to
be important.
The cdr of a list is the rest of the list, that is,
the cdr function returns the part of the list that
follows the first item. Thus, while the car of the
list '(rose violet daisy buttercup) is
rose, the rest of the list, the value returned by
cdr, is (violet daisy buttercup).
You can see this by evaluating the following in the usual way:
(cdr '(rose violet daisy buttercup))
When you evaluate this, (violet daisy buttercup)
will appear in the echo area.
Like car, cdr does not remove any
elements form the list--it just returns a report of what the
second and subsequent elements are.
Incidentally, in the example, the list of flowers is quoted. If
it were not, the Lisp interpreter would try to evaluate the list
by calling rose as a function. In this example, we
do not want to do that.
Clearly, a more reasonable name for cdr would be
rest.
(There is a lesson here: when you name new functions, consider very carefully about what you are doing, since you may be stuck with the names for far longer than you expect. The reason this document perpetuates these names is that the Emacs Lisp source code uses them, and if I did not use them, you would have a hard time reading the code; but do please try to avoid using these terms yourself. The people who come after you will be grateful to you.)
When car and cdr are applied to a list
made up of symbols, such as the list (pine fir oak
maple), the element of the list returned by the function
car is the symbol pine without any
parentheses around it. pine is the first element in
the list. However, the cdr of the list is a list
itself, (fir oak maple), as you can see by
evaluating the following expressions in the usual way:
(car '(pine fir oak maple)) (cdr '(pine fir oak maple))
On the other hand, in a list of lists, the first element is
itself a list. car returns this first element as a
list. For example, the following list contains three sub-lists, a
list of carnivores, a list of herbivores and a list of sea
mammals:
(car '((lion tiger cheetah)
(gazelle antelope zebra)
(whale dolphin seal)))
In this case, the first element or car of the list
is the list of carnivores, (lion tiger cheetah), and
the rest of the list is ((gazelle antelope zebra) (whale
dolphin seal)).
(cdr '((lion tiger cheetah)
(gazelle antelope zebra)
(whale dolphin seal)))
It is worth saying again that car and
cdr are non-destructive--that is, they do not modify
or change lists to which they are applied. This is very important
for how they are used.
Also, in the first chapter, in the discussion about atoms, I said
that in Lisp, "certain kinds of atom, such as an array, can be
separated into parts; but the mechanism for doing this is
different from the mechanism for splitting a list. As far as Lisp
is concerned, the atoms of a list are unsplittable." (See section
1.1.1 Lisp Atoms.) The
car and cdr functions are used for
splitting lists and are considered fundamental to Lisp. Since
they cannot split or gain access to the parts of an array, an
array is considered an atom. Conversely, the other fundamental
function, cons, can put together or construct a
list, but not an array. (Arrays are handled by array-specific
functions. See section `Arrays' in The GNU Emacs Lisp
Reference Manual.)
cons
The cons function constructs lists; it is the
inverse of car and cdr. For example,
cons can be used to make a four element list from
the three element list, (fir oak maple):
(cons 'pine '(fir oak maple))
After evaluating this list, you will see
(pine fir oak maple)
appear in the echo area. cons puts a new element at
the beginning of a list; it attaches or pushes elements onto the
list.
cons must have a list to attach to.(2) You cannot start from absolutely nothing.
If you are building a list, you need to provide at least an
empty list at the beginning. Here is a series of
cons's that build up a list of flowers. If you
are reading this in Info in GNU Emacs, you can evaluate each
of the expressions in the usual way; the value is printed in
this text after `=>', which you may read as
`evaluates to'.
(cons 'buttercup ())
=> (buttercup)
(cons 'daisy '(buttercup))
=> (daisy buttercup)
(cons 'violet '(daisy buttercup))
=> (violet daisy buttercup)
(cons 'rose '(violet daisy buttercup))
=> (rose violet daisy buttercup)
In the first example, the empty list is shown as ()
and a list made up of buttercup followed by the
empty list is constructed. As you can see, the empty list is not
shown in the list that was constructed. All that you see is
(buttercup). The empty list is not counted as an
element of a list because there is nothing in an empty list.
Generally speaking, an empty list is invisible.
The second example, (cons 'daisy '(buttercup))
constructs a new, two element list by putting daisy
in front of buttercup; and the third example
constructs a three element list by putting violet in
front of daisy and buttercup.
length
You can find out how many elements there are in a list by using
the Lisp function length, as in the following
examples:
(length '(buttercup))
=> 1
(length '(daisy buttercup))
=> 2
(length (cons 'violet '(daisy buttercup)))
=> 3
In the third example, the cons function is used to
construct a three element list which is then passed to the
length function as its argument.
We can also use length to count the number of
elements in an empty list:
(length ())
=> 0
As you would expect, the number of elements in an empty list is zero.
An interesting experiment is to find out what happens if you try
to find the length of no list at all; that is, if you try to call
length without giving it an argument, not even an
empty list:
(length )
What you see, if you evaluate this, is the error message
Wrong number of arguments: #<subr length>, 0
This means is that the function receives the wrong number of arguments, zero, when it expects some other number of arguments. In this case, one argument is expected, the argument being a list whose length the function is measuring. (Note that one list is one argument, even if the list has many elements inside it.)
The part of the error message that says `#<subr
length>' is the name of the function. This is written
with a special notation, `#<subr', that indicates
that the function length is one of the primitive
functions written in C rather than in Emacs Lisp.
(`subr' is an abbreviation for `subroutine'.) See
section `What Is a Function?' in The GNU Emacs Lisp
Reference Manual, for more about subroutines.
nthcdr
The nthcdr function is associated with the
cdr function. What it does is take the
cdr of a list repeatedly.
If you take the cdr of the list (pine fir oak
maple), you will be returned the list (fir oak
maple). If you repeat this on what was returned, you will
be returned the list (oak maple). (Of course,
repeated cdring on the original list will just give
you the original cdr since the function does not
change the list. You need to evaluate the cdr of the
cdr and so on.) If you continue this, eventually you
will be returned an empty list, which in this case, instead of
being shown as () is shown as nil.
For review, here is a series of repeated cdrs, the
text following the `=>' shows what is returned.
(cdr '(pine fir oak maple))
=>(fir oak maple)
(cdr '(fir oak maple))
=> (oak maple)
(cdr '(oak maple))
=>(maple)
(cdr '(maple))
=> nil
(cdr 'nil)
=> nil
(cdr ())
=> nil
You can also do several cdrs without printing the
values in between, like this:
(cdr (cdr '(pine fir oak maple)))
=> (oak maple)
In this case, the Lisp interpreter evaluates the innermost list
first. The innermost list is quoted, so it just passes the list
as it is to the innermost cdr. This cdr
passes a list made up of the second and subsequent elements of
the list to the outermost cdr, which produces a list
composed of the third and subsequent elements of the original
list. In this example, the cdr function is repeated
twice and returns a list that consists of the original list
without its first two elements.
The nthcdr function does the same as repeating the
call to cdr. In the following example, the argument
2 is passed to the function nthcdr, along with the
list, and the value returned is the list without its first two
items, which is exactly the same as repeating cdr
twice on the list:
(nthcdr 2 '(pine fir oak maple))
=> (oak maple)
Using the original four element list, we can see what happens
when various numeric arguments are passed to nthcdr,
including 0, 1, and 5:
;; Leave the list as it was.
(nthcdr 0 '(pine fir oak maple))
=> (pine fir oak maple)
;; Return a copy without the first element.
(nthcdr 1 '(pine fir oak maple))
=> (fir oak maple)
;; Return a copy of the list without three elements.
(nthcdr 3 '(pine fir oak maple))
=> (maple)
;; Return a copy lacking all four elements.
(nthcdr 4 '(pine fir oak maple))
=> nil
;; Return a copy lacking all elements.
(nthcdr 5 '(pine fir oak maple))
=> nil
It is worth mentioning that nthcdr, like
cdr, does not change the original list--the function
is non-destructive. This is in sharp contrast to the
setcar and setcdr functions.
setcar
As you might guess from their names, the setcar and
setcdr functions set the car or the
cdr of a list to a new value. They actually change
the original list, unlike car and cdr
which leave the original list as it was. One way to find out how
this works is to experiment. We will start with the
setcar function.
First, we can make a list and then set the value of a variable to
the list, using the setq function. Here is a list of
animals:
(setq animals '(giraffe antelope tiger lion))
If you are reading this in Info inside of GNU Emacs, you can evaluate this expression in the usual fashion, by positioning the cursor after the expression and typing C-x C-e. (I'm doing this right here as I write this. This is one of the advantages of having the interpreter built into the computing environment.)
When we evaluate the variable animals, we see that
it is bound to the list (giraffe antelope tiger
lion):
animals
=> (giraffe antelope tiger lion)
Put another way, the variable animals points to the
list (giraffe antelope tiger lion).
Next, evaluate the function setcar while passing it
two arguments, the variable animals and the quoted
symbol hippopotamus; this is done by writing the
three element list (setcar animals 'hippopotamus)
and then evaluating it in the usual fashion:
(setcar animals 'hippopotamus)
After evaluating this expression, evaluate the variable
animals again. You will see that the list of animals
has changed:
animals
=> (hippopotamus antelope tiger lion)
The first element on the list, giraffe is changed to
hippopotamus.
So we can see that setcar did not add a new element
to the list as cons would have; it replaced
giraffe with hippopotamus; it
changed the first element of the list.
setcdr
The setcdr function is similar to the
setcar function, except that the function changes
the second and subsequent elements of a list rather than the
first element.
To see how this works, set the value of the variable to a list of domesticated animals by evaluating the following expression:
(setq domesticated-animals '(horse cow sheep goat))
If you now evaluate the list, you will be returned the list
(horse cow sheep goat):
domesticated-animals
=> (horse cow sheep goat)
Next, evaluate setcdr with two arguments, the name
of the variable which has a list as its value, and the list to
which the cdr of the first list will be set;
(setcdr domesticated-animals '(cat dog))
If you evaluate this expression, the list (cat dog)
will appear in the echo area. This is the value returned by the
function. The result we are interested in is the "side effect",
which we can see by evaluating the variable
domesticated-animals:
domesticated-animals
=> (horse cat dog)
Indeed, the list is changed from (horse cow sheep
goat) to (horse cat dog). The
cdr of the list is changed from (cow sheep
goat) to (cat dog).
Construct a list of four birds by evaluating several expressions
with cons. Find out what happens when you
cons a list onto itself. Replace the first element
of the list of four birds with a fish. Replace the rest of that
list with a list of other fish.
Whenever you cut or clip text out of a buffer with a `kill' command in GNU Emacs, it is stored in a list and you can bring it back with a `yank' command.
(The use of the word `kill' in Emacs for processes which specifically do not destroy the values of the entities is an unfortunate historical accident. A much more appropriate word would be `clip' since that is what the kill commands do; they clip text out of a buffer and put it into storage from which it can be brought back. I have often been tempted to replace globally all occurrences of `kill' in the Emacs sources with `clip' and all occurrences of `killed' with `clipped'.)
When text is cut out of a buffer, it is stored on a list. Successive pieces of text are stored on the list successively, so the list might look like this:
("a piece of text" "last piece")
The function cons can be used to add a piece of text
to the list, like this:
(cons "another piece"
'("a piece of text" "last piece"))
If you evaluate this expression, a list of three elements will appear in the echo area:
("another piece" "a piece of text" "last piece")
With the car and nthcdr functions, you
can retrieve whichever piece of text you want. For example, in
the following code, nthcdr 1 ... returns the list
with the first item removed; and the car returns the
first element of that remainder--the second element of the
original list:
(car (nthcdr 1 '("another piece"
"a piece of text"
"last piece")))
=> "a piece of text"
Functions in Emacs are more complex than this, of course. The code for cutting and retrieving text has to be written so that Emacs can figure out which element in the list you want--the first, second, third, or whatever. In addition, when you get to the end of the list, Emacs should give you the first element of the list, rather than nothing at all.
The list that holds the pieces of text is called the kill
ring. This chapter leads up to a description of the kill
ring and how it is used by first tracing how the
zap-to-char function works. This function uses (or
`calls') a function that invokes a function that manipulates the
kill ring. Thus, before reaching the mountains, we climb the
foothills.
A subsequent chapter describes how text that is cut from the buffer is retrieved. See section 10 Yanking Text Back.
zap-to-char
The zap-to-char function is written differently in
GNU Emacs version 18 and version 19. The version 19
implementation is simpler, and works slightly differently. We
will first show the function as it is written for version 19 and
then for version 18.
The Emacs version 19 implementation of the interactive
zap-to-char function removes the text in the region
between the location of the cursor (i.e., of point) up to and
including the next occurrence of a specified character. The text
that zap-to-char removes is put in the kill ring;
and it can be retrieved from the kill ring by typing
C-y (yank). If the command is given an
argument, it removes text through that number of occurrences.
Thus, if the cursor were at the beginning of this sentence and
the character were `s', `Thus' would be
removed. If the argument were two, `Thus, if the
curs' would be removed, up to and including the
`s' in `cursor'.
The Emacs version 18 implementation removes the text from point up to but not including the specified character. Thus, in the example shown in the previous paragraph, the `s' would not be removed.
In addition, the version 18 implementation will go to the end of the buffer if the specified character is not found; but the version 19 implementation will simply generate an error (and not remove any text).
In order to determine how much text to remove, both versions of
zap-to-char use a search function. Searches are used
extensively in code that manipulates text, and it is worth
focusing attention on the search function as well as on the
deletion command.
Here is the complete text of the version 19 implementation of the function:
(defun zap-to-char (arg char) ; version 19 implementation
"Kill up to and including ARG'th occurrence of CHAR.
Goes backward if ARG is negative; error if CHAR not found."
(interactive "*p\ncZap to char: ")
(kill-region (point)
(progn
(search-forward
(char-to-string char) nil nil arg)
(point))))
interactive Expression
The interactive expression in the zap-to-char
command looks like this:
(interactive "*p\ncZap to char: ")
The part within quotation marks, "*p\ncZap to char:
", specifies three different things. First, and most
simply, the asterisk, `*', causes an error to be
signalled if the buffer is read-only. This means that if you try
zap-to-char in a read-only buffer you will not be
able to remove text, and you will receive a message that says
"Buffer is read-only"; your terminal may beep at you as well.
The second part of "*p\ncZap to char: " is the
`p'. This part is ended by a newline,
`\n'. The `p' means that the first
argument to the function will be passed the value of a `processed
prefix'. The prefix argument is passed by typing C-u
and a number, or M- and a number. If the function is
called interactively without a prefix, 1 is passed to this
argument.
The third part of "*p\ncZap to char: " is
`cZap to char: '. In this part, the lower case
`c' indicates that interactive expects
a prompt and that the argument will be a character. The prompt
follows the `c' and is the string `Zap to
char: ' (with a space after the colon to make it look
good).
What all this does is prepare the arguments to
zap-to-char so they are of the right type, and give
the user a prompt.
zap-to-char
The body of the zap-to-char function contains the
code that kills (that is, removes) the text in the region from
the current position of the cursor up to and including the
specified character. The first part of the code looks like this:
(kill-region (point) ...
(point) is the current position of the cursor.
The next part of the code is an expression using
progn. The body of the progn consists
of calls to search-forward and point.
It is easier to understand how progn works after
learning about search-forward, so we will look at
search-forward and then at progn.
search-forward
Function
The search-forward function is used to locate the
zapped-for-character in zap-to-char. If the search
is successful, search-forward leaves point
immediately after the last character in the target string. (In
this case the target string is just one character long.) If the
search is backwards, search-forward leaves point
just before the first character in the target. Also,
search-forward returns t for true.
(Moving point is therefore a `side effect'.)
In zap-to-char, the search-forward
function looks like this:
(search-forward (char-to-string char) nil nil arg)
The search-forward function takes four arguments:
zap-to-char is a single
character. Because of the way computers are built, the Lisp
interpreter treats a single character as being different from a
string of characters. Inside the computer, a single character has
a different electronic format than a string of one character. (A
single character can often be recorded in the computer using
exactly one byte; but a string may be longer or shorter, and the
computer needs to be ready for this.) Since the
search-forward function searches for a string, the
character that the zap-to-char function receives as
its argument must be converted inside the computer from one
format to the other; otherwise the search-forward
function will fail. The char-to-string function is
used to make this conversion.
nil.
nil. A nil as the third
argument causes the function to signal an error when the search
fails.
search-forward is the
repeat count--how many occurrences of the string to look for.
This argument is optional and if the function is called without a
repeat count, this argument is passed the value 1. If this
argument is negative, the search goes backwards.
In template form, a search-forward expression looks
like this:
(search-forward "target-string"
limit-of-search
what-to-do-if-search-fails
repeat-count)
We will look at progn next.
progn Function
progn is a function that causes each of its
arguments to be evaluated in sequence and then returns the value
of the last one. The preceding expressions are evaluated only for
the side effects they perform. The values produced by them are
discarded.
The template for a progn expression is very simple:
(progn body...)
In zap-to-char, the progn expression
has to do two things: put point in exactly the right position;
and return the location of point so that kill-region
will know how far to kill to.
The first argument to the progn is
search-forward. When search-forward
finds the string, the function leaves point immediately after the
last character in the target string. (In this case the target
string is just one character long.) If the search is backwards,
search-forward leaves point just before the first
character in the target. The movement of point is a side effect.
The second and last argument to progn is the
expression (point). This expression returns the
value of point, which in this case will be the location to which
it has been moved by search-forward. This value is
returned by the progn expression and is passed to
kill-region as kill-region's second
argument.
zap-to-char
Now that we have seen how search-forward and progn
work, we can see how the zap-to-char function works
as a whole.
The first argument to kill-region is the position of
the cursor when the zap-to-char command is
given--the value of point at that time. Within the
progn, the search function then moves point to just
after the zapped-to-character and point returns the
value of this location. The kill-region function
puts together these two values of point, the first one as the
beginning of the region and the second one as the end of the
region, and removes the region.
The progn function is necessary because the
kill-region command takes two arguments; and it
would fail if search-forward and point
expressions were written in sequence as two additional arguments.
The progn expression is a single argument to
kill-region and returns the one value that
kill-region needs for its second argument.
The version 18 implementation of zap-to-char is
slightly different from the version 19 implementation: it zaps
the text up to but not including the zapped-to-character; and
zaps to the end of the buffer if the specified character is not
found.
The difference is in the second argument to the
kill-region command. Where the version 19
implementation looks like this:
(progn (search-forward (char-to-string char) nil nil arg) (point))
The version 18 implementation looks like this:
(if (search-forward (char-to-string char) nil t arg)
(progn (goto-char
(if (> arg 0) (1- (point)) (1+ (point))))
(point))
(if (> arg 0)
(point-max)
(point-min)))
This looks considerably more complicated, but the code can be readily understood if it is looked at part by part.
The first part is:
(if (search-forward (char-to-string char) nil t arg)
This fits into an if expression that does the
following job, as we shall see:
(if able-to-locate-zapped-for-character-and-move-point-to-it
then-move-point-to-the-exact-spot-and-return-this-location
else-move-to-end-of-buffer-and-return-that-location)
Evaluation of the if expression specifies the second
argument to kill-region. Since the first argument is
point, this process makes it possible for
kill-region to remove the text between point and the
zapped-to location.
We have already described how search-forward moves
point as a side effect. The value that
search-forward returns is t if the
search is successful and either nil or an error
message depending on the value of the third argument to
search-forward. In this case, t is the
third argument and it causes the function to return
nil when the search fails. As we will see, it is
easy to write the code for handling the case when the search
returns nil.
In the version 18 implementation of zap-to-char, the
search takes place because the if causes the search
expression to be evaluated as its true-or-false-test. If the
search is successful, Emacs evaluates the then-part of the
if expression. On the other hand, if the search
fails, Emacs evaluates the else-part of the if
expression.
In the if expression, when the search succeeds, a
progn expression is executed--which is to say, it is
run as a program.
As we said earlier, progn is a function that causes
each of its arguments to be evaluated in sequence and then
returns the value of the last one. The preceding expressions are
evaluated only for the side effects they perform. The values
produced by them are discarded.
In this version of zap-to-char, the
progn expression is executed when the
search-forward function finds the character for
which it is searching. The progn expression has to
do two things: put point in exactly the right position; and
return the location of point so that kill-region
will know how far to kill to.
The reason for all the code in the progn is that
when search-forward finds the string it is looking
for, it leaves point immediately after the last character in the
target string. (In this case the target string is just one
character long.) If the search is backwards,
search-forward leaves point just before the first
character in the target.
However, this version of the zap-to-char function is
designed so that it does not remove the character being zapped
to. For example, if zap-to-char is to remove all the
text up to a `z', this version will not remove the
`z' as well. So point has to be moved just enough
that the zapped-to character is not removed.
progn expression
The body of the progn consists of two expressions.
Spread out to delineate the different parts more clearly, and
with comments added, the progn expression looks like
this:
(progn (goto-char ; First expression inprogn. (if (> arg 0) ; Ifargis positive, (1- (point)) ; move back one character; (1+ (point)))) ; else move forward one character. (point)) ; Second expression inprogn: ; return position of point.
The progn expression does this: when the search is
forward (arg is positive), Emacs leaves point just
after the searched-for character. By moving point back one
position, the character is uncovered. In this case, the
expression in the progn reads as follows:
(goto-char (1- (point))). This moves point one
character back. (The 1- function subtracts one from
its argument, just as 1+ adds ones to its argument.)
On the other hand, if the argument to
zap-to-character is negative, the search will be
backwards. The if detects this and the expression
reads: (goto-char (1+ (point))). (The
1+ function adds one to its argument.)
The second and last argument to progn is the
expression (point). This expression returns the
value of the position to which point is moved by the first
argument to progn. This value is then returned by
the if expression of which it is a part and is
passed to kill-region as kill-region's
second argument.
In brief, the function works like this: the first argument to
kill-region is the position of the cursor when the
zap-to-char command is given--the value of point at
that time. The search function then moves point if the search is
successful. The progn expression moves point just
enough so the zapped to character is not removed, and returns the
value of point after all this is done. The
kill-region function then removes the region.
Finally, the else-part of the if expression takes
care of the situation in which the zapped-towards character is
not found. If the argument to the zap-to-char
function is positive (or if none is given) and the zapped-to
character is not found, all the text is removed from the current
position of point to the end of the accessible region of the
buffer (the end of the buffer if there is no narrowing). If the
arg is negative and the zapped-to character is not
found, the operation goes to the beginning of the accessible
region. The code for this is a simple if clause:
(if (> arg 0) (point-max) (point-min))
This says that if arg is a positive number, return
the value of point-max, otherwise, return the value
of point-min.
For review, here is the code involving kill-region,
with comments:
(kill-region
(point) ; beginning-of-region
(if (search-forward
(char-to-string char) ; target
nil ; limit-of-search: none
t ; Return nil if fail.
arg) ; repeat-count.
(progn ; then-part
(goto-char
(if (> arg 0)
(1- (point))
(1+ (point))))
(point))
(if (> arg 0) ; else-part
(point-max)
(point-min))))
As you can see, the version 19 implementation does slightly less than the version 18 implementation, but is much simpler.
kill-region
The zap-to-char function uses the
kill-region function. This function is very simple;
leaving out part of its documentation string, it looks like this:
(defun kill-region (beg end) "Kill between point and mark. The text is deleted but saved in the kill ring." (interactive "*r") (copy-region-as-kill beg end) (delete-region beg end))
The main point to note is that it uses the
delete-region and copy-region-as-kill
functions which are described in following sections.
delete-region: A Digression into
C
The zap-to-char command uses the
kill-region function, which in turn uses two other
functions, copy-region-as-kill and
delete-region. The copy-region-as-kill
function will be described in a following section; it puts a copy
of the region in the kill ring so it can be yanked back. (See
section 8.5
copy-region-as-kill.)
The delete-region function removes the contents of a
region and you cannot get it back.
Unlike the other code discussed here, delete-region
is not written in Emacs Lisp; it is written in C and is one of
the primitives of the GNU Emacs system. Since it is very simple,
I will digress briefly from Lisp and describe it here.
Like many of the other Emacs primitives,
delete-region is written as an instance of a C
macro, a macro being a template for code. The first section of
the macro looks like this:
DEFUN ("delete-region", Fdelete_region, Sdelete_region, 2, 2, "r",
"Delete the text between point and mark.\n\
When called from a program, expects two arguments,\n\
character numbers specifying the stretch to be deleted.")
Without getting into the details of the macro writing process,
let me point out that this macro starts with the word
DEFUN. The word DEFUN was chosen since
the code serves the same purpose as defun does in
Lisp. The word DEFUN is followed by seven parts
inside of parentheses:
delete-region.
Fdelete_region. By convention, it starts with
`F'. Since C does not use hyphens in names, an
underscore is used instead.
interactive declaration in a function written in
Lisp: a letter followed, perhaps, by a prompt. In this case, the
letter is "r" which indicates that the two arguments
to the function will be the position of the beginning and end of
a region in the buffer. In this code, there isn't any prompt.
The formal parameters come next, with a statement of what kind of
object they are, and then what might be called the `body' of the
macro. For delete-region the `body' consists of the
following three lines:
validate_region (&b, &e); del_range (XINT (b), XINT (e)); return Qnil;
The first function, validate_region checks whether
the values passed as the beginning and end of the region are the
proper type and are within range. The second function,
del_range, actually deletes the text. If the
function completes its work without error, the third line returns
Qnil to indicate this.
del_range is a complex function we will not look
into. It updates the buffer and does other things. However, it is
worth looking at the two arguments passed to
del_range. These are XINT (b) and
XINT (e). As far as the C language is concerned,
b and e are two thirty-two bit integers
that mark the beginning and end of the region to be deleted. But
like other numbers in Emacs Lisp, only twenty-four bits of the
thirty-two bits are used for the number; the remaining eight bits
are used for keeping track of the type of information and other
purposes. (On certain machines, only six bits are so used.) In
this case, the eight bits are used to indicate that these numbers
are for buffer positions. When bits of a number are used this
way, they are called a tag. The use of the eight
bit tag on each thirty-two bit integer made it possible to write
Emacs to run much faster than it would otherwise. On the other
hand, with numbers limited to twenty-four bits, Emacs buffers are
limited to approximately eight megabytes. (You can sharply
increase the maximum buffer size by adding defines for
VALBITS and GCTYPEBITS in the
`emacs/src/config.h' file before compiling. See the note
in the `emacs/etc/FAQ' file that is part of the Emacs
distribution.)
`XINT' is C macro that extracts the 24 bit number
from the thirty-two bit Lisp object; the eight bits used for
other purposes are discarded. So del_range (XINT (b), XINT
(e)) deletes the region between the beginning position,
b, and the ending position, e.
From the point of view of the person writing Lisp, Emacs is all very simple; but hidden underneath is a great deal of complexity to make it all work.
defvar
Unlike the delete-region function, the
copy-region-as-kill function is written in Emacs
Lisp. It copies a region in a buffer and saves it in a variable
called the kill-ring. This section describes how
this variable is created and initialized.
(Again we note that the term kill-ring is a
misnomer. The text that is clipped out of the buffer can be
brought back; it is not a ring of corpses, but a ring of
resurrectable text.)
In Emacs Lisp, a variable such as the kill-ring is
created and given an initial value by using the
defvar special form. The name comes from "define
variable".
The defvar special form is similar to
setq in that it sets the value of a variable. It is
unlike setq in two ways: first, it only sets the
value of the variable if the variable does not already have a
value. If the variable already has a value, defvar
does not override the existing value. Second, defvar
has a documentation string.
You can see the current value of a variable, any variable, by
using the describe-variable function, which is
usually invoked by typing C-h v. If you type C-h
v and then kill-ring (followed by
RET) when prompted, you will see what is in your
current kill ring--this may be quite a lot! Conversely, if you
have been doing nothing this Emacs session except read this
document, you may have nothing in it. At the end of the
`*Help*' buffer, you will see the documentation for
kill-ring:
Documentation: List of killed text sequences.
The kill ring is defined by the defvar in the
following way:
(defvar kill-ring nil "List of killed text sequences.")
In this variable definition, the variable is given an initial
value of nil, which makes sense, since if you have
saved nothing, you want nothing back if you give a
yank command. The documentation string is written
just like the documentation string of a defun. As
with the documentation string of the defun, the
first line of the documentation should be a complete sentence,
since some commands, like apropos, print only the
first line of documentation. Succeeding lines should not be
indented; otherwise they look odd when you use C-h v
(describe-variable).
Most variables are internal to Emacs, but
some are intended as options that you can readily set with the
edit-options command. (These settings last only
for the duration of an editing session; to set a value
permanently, write a `.emacs' file. See section
16 Your
`.emacs' File.)
A readily settable variable is distinguished from others in Emacs by an asterisk, `*', in the first column of its documentation string.
For example:
(defvar line-number-mode nil "*Non-nil means display line number in mode line.")
This means that you can use the edit-options command
to change the value of line-number-mode.
Of course, you can also change the value of
line-number-mode by evaluating it within a
setq expression, like this:
(setq line-number-mode t)
See section 1.9.2 Using
setq.
copy-region-as-kill
The copy-region-as-kill function copies a region of
text from a buffer and saves it in a variable called the
kill-ring.
If you call copy-region-as-kill immediately after a
kill-region command, Emacs appends the newly copied
text to the previously copied text. This means that if you yank
back the text, you get it all, from both this and the previous
operation. On the other hand, if some other command precedes the
copy-region-as-kill, the function copies the text
into a separate entry in the kill ring.
Here is the complete text of the version 18
copy-region-as-kill, formatted for clarity with
several comments added:
(defun copy-region-as-kill (beg end)
"Save the region as if killed, but don't kill it."
(interactive "r")
(if (eq last-command 'kill-region)
;; then-part: Combine newly copied text
;; with previously copied text.
(kill-append (buffer-substring beg end) (< end beg))
;; else-part: Add newly copied text as a new element
;; to the kill ring and shorten the kill ring if necessary.
(setq kill-ring
(cons (buffer-substring beg end) kill-ring))
(if (> (length kill-ring) kill-ring-max)
(setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))
(setq this-command 'kill-region)
(setq kill-ring-yank-pointer kill-ring))
As usual, this function can be divided into its component parts:
(defun copy-region-as-kill (argument-list) "documentation..." (interactive "r") body...)
The arguments are beg and end and the
function is interactive with "r", so the two
arguments must refer to the beginning and end of the region. If
you have been reading though this document from the beginning,
understanding these parts of a function is almost becoming
routine.
The documentation is somewhat confusing unless you remember that the word `kill' has a meaning different from its usual meaning.
The body of the function starts with an if clause.
What this clause does is distinguish between two different
situations: whether or not this command is executed immediately
after a previous kill-region command. In the first
case, the new region is appended to the previously copied text.
Otherwise, it is inserted into the beginning of the kill ring as
a separate piece of text from the previous piece.
The last two lines of the function are two setq
expressions. One of them sets the variable
this-command to kill-region and the
other sets the variable kill-ring-yank-pointer to
point to the kill ring.
The body of copy-region-as-kill merits discussion in
detail.
copy-region-as-kill
The copy-region-as-kill function is written so that
two or more kills in a row combine their text into a single
entry. If you yank back the text from the kill ring, you get it
all in one piece. Moreover, kills that kill forward from the
current position of the cursor are added to the end of the
previously copied text and commands that copy text backwards add
it to the beginning of the previously copied text. This way, the
words in the text stay in the proper order.
The function makes use of two variables that keep track of the
current and previous Emacs command. The two variables are
this-command and last-command.
Normally, whenever a function is executed, Emacs sets the value
of this-command to the function being executed
(which in this case would be copy-region-as-kill).
At the same time, Emacs sets the value of
last-command to the previous value of
this-command. However, the
copy-region-as-kill command is different; it sets
the value of this-command to
kill-region, which is the name of the function that
calls copy-region-as-kill.
In the first part of the body of the
copy-region-as-kill function, an if
expression determines whether the value of
last-command is kill-region. If so, the
then-part of the if expression is evaluated; it uses
the kill-append function to concatinate the text
copied at this call to the function with the text already in the
first element (the CAR) of the kill ring. On the other hand, if
the value of last-command is not
kill-region, then the
copy-region-as-kill function attaches a new element
to the kill ring.
The if expression reads as follows; it uses
eq, which is a function we have not yet seen:
(if (eq last-command 'kill-region)
;; then-part
(kill-append (buffer-substring beg end) (< end beg))
The eq function tests whether
its first argument is the same Lisp object as its second
argument. The eq function is similar to the
equal function in that it is used to test for
equality, but differs in that it determines whether two
representations are actually the same object inside the
computer, but with different names. equal
determines whether the structure and contents of two
expressions are the same.
kill-append function
The kill-append function looks like this:
(defun kill-append (string before-p)
(setcar kill-ring
(if before-p
(concat string (car kill-ring))
(concat (car kill-ring) string))))
We can look at this function in parts. The setcar
function uses concat to concatinate the new text to
the CAR of the kill ring. Whether it prepends or appends the text
depends on the results of an if expression:
(if before-p ; if-part
(concat string (car kill-ring)) ; then-part
(concat (car kill-ring) string)) ; else-part
If the region being killed is before the region that was killed
in the last command, then it should be prepended before the
material that was saved in the previous kill; and conversely, if
the killed text follows what was just killed, it should be
appended after the previous text. The if expression
depends on the predicate before-p to decide whether
the newly saved text should be put before or after the previously
saved text.
The symbol before-p is the name of one of the
arguments to kill-append. When the
kill-append function is evaluated, it is bound to
the value returned by evaluating the actual argument. In this
case, this is the expression (< end beg). This
expression does not directly determine whether the killed text in
this command is located before or after the kill text of the last
command; what is does is determine whether the value of the
variable end is less than the value of the variable
beg. If it is, it means that the user is most likely
heading towards the beginning of the buffer. Also, the result of
evaluating the predicate expression, (< end beg),
will be true and the text will be prepended before the previous
text. On the other hand, if the value of the variable
end is greater than the value of the variable
beg, the text will be appended after the previous
text.
When the newly saved text will be prepended, then the string with the new text will be concatenated before the old test:
(concat string (car kill-ring))
But if the text will be appended, it will be concatenated after the old text:
(concat (car kill-ring) string))
To understand how this works, we first need to review the
concat function. The concat function
links together or unites two strings of text. The result is a
string. For example:
(concat "abc" "def")
=> "abcdef"
(concat "new "
(car '("first element" "second element")))
=> "new first element"
(concat (car
'("first element" "second element")) " modified")
=> "first element modified"
We can now make sense of kill-append: it modifies
the contents of the kill ring. The kill ring is a list, each
element of which is saved text. The setcar function
actually changes the first element of this list. It does this by
using concat to replace the old first element of the
kill ring (the CAR of the kill ring) with a new first element
made by concatenating together the old saved text and the newly
saved text. The newly saved text is put in front of the old text
or after the old text, depending on whether it is in front of or
after the old text in the buffer from which it is cut. The whole
concatination becomes the new first element of the kill ring.
Incidentally, this is what the beginning of my current kill ring looks like:
("concatenating together" "saved text" "element" ...
copy-region-as-kill
Now, back to the explanation of copy-region-as-kill:
If the last command is not kill-region, then instead
of calling kill-append, it calls the else-part of
the following code:
(if true-or-false-test-tests-false
what-is-done-if-test-returns-true
;; else-part
(setq kill-ring
(cons (buffer-substring beg end) kill-ring))
(if (> (length kill-ring) kill-ring-max)
(setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))
The setq line of the else-part sets the new value of
the kill ring to what results from adding the string being killed
to the old kill ring.
We can see how this works with a little example:
(setq example-list '("here is a clause" "another clause"))
After evaluating this expression with C-x C-e, you can
evaluate example-list and see what it returns:
example-list
=> ("here is a clause" "another clause")
Now, we can add a new element on to this list by evaluating the following expression:
(setq example-list (cons "a third clause" example-list))
When we evaluate example-list, we find its value is:
example-list
=> ("a third clause" "here is a clause" "another clause")
Thus, the third clause was added to the list by
cons.
This is exactly similar to what the setq and
cons do in the function, except that
buffer-substring is used to pull out a copy of a
region of text and hand it to the cons. Here is the
line again:
(setq kill-ring (cons (buffer-substring beg end) kill-ring))
The next segment of the else-part of
copy-region-as-kill is another if
clause. This if clause keeps the kill ring from
growing too long. It reads as follows:
(if (> (length kill-ring) kill-ring-max)
(setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))
This code checks whether the length of the kill ring is greater
than the maximum permitted length. This is the value of
kill-ring-max (which is 30, by default). If the
length of the kill ring is too long, then this code sets the last
element of the kill ring to nil. It does this by
using two functions, nthcdr and setcdr.
We looked at setcdr earlier (see section 7.5 setcdr). It
sets the CDR of a list, just as setcar sets the CAR
of a list. In this case, however, setcdr will not be
setting the cdr of the whole kill ring; the
nthcdr function is used to cause it to set the
cdr of the next to last element of the kill
ring--this means that since the cdr of the next to
last element is the last element of the kill ring, it will set
the last element of the kill ring.
The nthcdr function works by repeatedly taking the
CDR of a list--it takes the CDR of the CDR of the CDR ... It does
this N times and returns the results.
Thus, if we had a four element list that was supposed to be three
elements long, we could set the CDR of the next to last element
to nil, and thereby shorten the list.
You can see this by evaluating the following three expressions in
turn. First set the value of trees to (maple
oak pine birch), then set the CDR of its second CDR to
nil and then find the value of trees:
(setq trees '(maple oak pine birch))
=> (maple oak pine birch)
(setcdr (nthcdr 2 trees) nil)
=> nil
trees
=> (maple oak pine)
(The value returned by the setcdr expression is
nil since that is what the CDR is set to.)
In brief, in copy-region-as-kill, the
nthcdr function takes the CDR a number of times that
is one less than the maximum permitted size of the kill ring and
sets the CDR of that element (which will be the rest of the
elements in the kill ring) to nil. This prevents the
kill ring from growing too long.
The next to last line of the copy-region-as-kill
function is
(setq this-command 'kill-region)
This line is not part of either the inner or the outer
if expression, so it is evaluated every time
copy-region-as-kill is called. Here we find the
place where this-command is set to
kill-region. As we saw earlier, when the next
command is given, the variable last-command will be
given this value.
Finally, the last line of the copy-region-as-kill
function is:
(setq kill-ring-yank-pointer kill-ring)
The kill-ring-yank-pointer is a global variable that
is set to be the kill-ring.
Even though the kill-ring-yank-pointer is called a
`pointer', it is a variable just like the kill ring.
However, the name has been chosen to help humans understand how
the variable is used. The variable is used in functions such as
yank and yank-pop (see section 10 Yanking Text Back).
This leads us to the code for bringing back text that has been cut out of the buffer--the yank commands. However, before discussing the yank commands, it is better to learn how lists are implemented in a computer. This will make clear such mysteries as the use of the term `pointer'.
Here is a brief summary of some recently introduced functions.
car
cdr
car returns the first element of a list;
cdr returns the second and subsequent elements of
a list. For example:
(car '(1 2 3 4 5 6 7))
=> 1
(cdr '(1 2 3 4 5 6 7))
=> (2 3 4 5 6 7)
cons
cons constructs a list by prepending its first
argument to its second argument. For example:
(cons 1 '(2 3 4))
=> (1 2 3 4)
nthcdr
cdr `n' times on a
list. The `rest of the rest' as it were. For example:
(nthcdr 3 '(1 2 3 4 5 6 7))
=> (4 5 6 7)
setcar
setcdr
setcar changes the first element of a list;
setcdr changes the second and subsequent elements
of a list. For example:
(setq triple '(1 2 3))
(setcar triple '37)
triple
=> (37 2 3)
(setcdr triple '("foo" "bar"))
triple
=> (37 "foo" "bar")
progn
(progn 1 2 3 4)
=> 4
save-restriction
search-forward
nil or an error message.
kill-region
delete-region
copy-region-as-kill
kill-region cuts the text between point and mark
from the buffer and stores that text in the kill ring, so you
can get it back by yanking. delete-region removes
the text between point and mark from the buffer and throws it
away. You cannot get it back. copy-region-as-kill
copies the text between point and mark into the kill ring, from
which you can get it by yanking. The function does not cut or
remove the text from the buffer.
Write an interactive function that searches for a string. If the
search find the string, leave point after it and display a
message that says "Found!". (Do not use
search-forward for the name of this function; if you
do, you will overwrite the existing version of
search-forward that comes with Emacs. Use a name
such as test-search instead.)
Write a function that prints the third element of the kill ring in the echo area, if any; if the kill ring does not contain a third element, print an appropriate message.
As of version 19.29, copy-region-as-kill no longer
sets this-command. What are the consequences of this
change? What do you suppose motivated it?
In Lisp, atoms are recorded in a straightforward fashion; if the
implementation is not straightforward in practice, it is,
nonetheless, straightforward in theory. The atom
`rose', for example, is recorded as the four
contiguous letters `r', `o',
`s', `e'. A list on the other hand, is
kept differently. The mechanism is equally simple, but it takes a
moment to get used to the idea. A list is kept using a series of
pairs of pointers. In the series, the first pointer in each pair
points to an atom or to another list, and the second pointer in
each pair points to the next pair, or to the symbol
nil, which marks the end of the list.
A pointer itself is quite simply the electronic address of what is pointed to. Hence, a list is kept as a series of electronic addresses.
For example, the list (rose violet buttercup) has
three elements, `rose', `violet', and
`buttercup'. In the computer, the electronic address
of `rose' is recorded in a segment of computer
memory along with the address that gives the electronic address
of where the atom `violet' is located; and that
address (the one that tells where `violet' is
located) is kept along with an address that tells where the
address for the atom `buttercup' is located.
This sounds more complicated than it is and is easier seen in a diagram:
___ ___ ___ ___ ___ ___
|___|___|--> |___|___|--> |___|___|--> nil
| | |
| | |
--> rose --> violet --> buttercup
In the diagram, each box represents a word of computer memory
that holds a Lisp object, usually in the form of a memory
address. The boxes, i.e. the addresses, are in pairs. Each arrow
points to what the address is the address of, either an atom or
another pair of addresses. The first box is the electronic
address of `rose' and the arrow points to
`rose'; the second box is the address of the next
pair of boxes, the first part of which is the address of
`violet' and the second part of which is the address
of the next pair. The very last box does points to the symbol
nil, which marks the end of the list.
When a variable is set to a list with a function such as
setq, it stores the address of the first box in the
variable. Thus, evaluation of the expression
(setq bouquet '(rose violet buttercup))
creates a situation like this:
bouquet
|
| ___ ___ ___ ___ ___ ___
--> |___|___|--> |___|___|--> |___|___|--> nil
| | |
| | |
--> rose --> violet --> buttercup
In this case, the symbol bouquet holds the address
of the first pair of boxes. Indeed, the symbol
bouquet consists of a group of address-boxes, one of
which is the address of the printed word `bouquet',
a second of which is the address of a function definition
attached to the symbol, if any, a third of which is the address
of the first pair of address-boxes for the list (rose
violet buttercup), and so on.
This same list can be illustrated in a different sort of box notation like this:
bouquet
|
| -------------- --------------- ----------------
| | car | cdr | | car | cdr | | car | cdr |
-->| rose | o------->| violet | o------->| butter- | nil |
| | | | | | | cup | |
-------------- --------------- ----------------
In an earlier section, I suggested that you might imagine a symbol as being a chest of drawers. The function definition is put in one drawer, the value in another, and so on. What is put in the drawer holding the value can be changed without affecting the contents of the drawer holding the function definition, and vice-versa. Actually, what is put in each drawer is the address of the value or function definition. It is as if you found an old chest in the attic, and in one of its drawers you found a map giving you directions to where the buried treasure lies.
(In addition to its name, symbol definition, and variable value, a symbol has a `drawer' for a property list which can be used to record other information. Property lists are not discussed here; see section `Property Lists' in The GNU Emacs Lisp Reference Manual.)
Here is a fanciful representation:
Chest of Drawers Contents of Drawers
---------------------
| |
| symbol name | bouquet
| |
---------------------
| |
| symbol definition | [none]
| |
---------------------
| |
| variable value | (rose violet buttercup)
| |
---------------------
| |
| property list | [not described here]
| |
---------------------
|/ \|
If symbol is set to the CDR of a list, the list itself is not changed; the symbol simply has an address further down the list. (In the jargon, CAR and CDR are `non-destructive'.) Thus, evaluation of the following expression
(setq flowers (cdr bouquet))
produces this:
bouquet flowers
| |
| ___ ___ | ___ ___ ___ ___
--> | | | --> | | | | | |
|___|___|----> |___|___|--> |___|___|--> nil
| | |
| | |
--> rose --> violet --> buttercup
The value of flowers is (violet
buttercup), which is to say, the symbol
flowers holds the address of the pair of
address-boxes, the first of which holds the address of
violet, and the second of which holds the address of
buttercup.
A pair of address-boxes is called a cons cell or dotted pair. See section `List Type' in The GNU Emacs Lisp Reference Manual, and section `Dotted Pair Notation' in The GNU Emacs Lisp Reference Manual, for more information about cons cells and dotted pairs.
The function cons adds a new pair of addresses to
the front of a series of addresses like that shown above. For
example, evaluating the expression
(setq bouquet (cons 'lilly bouquet))
produces:
bouquet flowers
| |
| ___ ___ ___ ___ | ___ ___ ___ ___
--> | | | | | | --> | | | | | |
|___|___|----> |___|___|----> |___|___|---->|___|___|--> nil
| | | |
| | | |
--> lilly --> rose --> violet --> buttercup
However, this does not change the value of the symbol
flowers, as you can see by evaluating the following,
(eq (cdr (cdr bouquet)) flowers)
which returns t for true.
Until it is reset, flowers still has the value
(violet buttercup); that is, it has the address of
the cons cell whose first address is of violet.
Also, this does not alter any of the pre-existing cons cells;
they are all still there.
Thus, in Lisp, to get the CDR of a list, you just get the address
of the next cons cell in the series; to get the CAR of a list,
you get the address of the first element of the list; to
cons a new element on a list, you add a new cons
cell to the front of the list. That is all there is to it! The
underlying structure of Lisp is brilliantly simple!
And what does the last address in a series of cons cells refer
to? It is the address of the empty list, of nil.
In summary, when a Lisp variable is set to a value, it is provided with the address of the list to which the variable refers.
Set flowers to violet and
buttercup. Cons two more flowers on to this list and
set this new list to more-flowers. Set the CAR of
flowers to a fish. What does the
more-flowers list now contain?
Whenever you cut text out of a buffer with a `kill' command in GNU Emacs, you can bring it back with a `yank' command. The text that is cut out of the buffer is put in the kill ring and the yank commands insert the appropriate contents of the kill ring back into a buffer (not necessarily the original buffer).
A simple C-y (yank) command inserts the
first item from the kill ring into the current buffer. If the
C-y command is followed immediately by M-y,
the first element is replaced by the second element. Successive
M-y commands replace the second element with the
third, fourth, or fifth element, and so on. When the last element
in the kill ring is reached, it is replaced by the first element
and the cycle is repeated. (Thus the kill ring is called a `ring'
rather than just a `list'. However, the actual data structure
that holds the text is a list. See section B Handling the Kill Ring, for
the details of how the list is handled as a ring.)
The kill ring is a list of textual strings. This is what it looks like:
("some text" "a different piece of text" "yet more text")
If this were the contents of my kill ring and I pressed C-y, the string of characters saying `some text' would be inserted in this buffer where my cursor is located.
The yank command is also used for duplicating text
by copying it. The copied text is not cut from the buffer, but a
copy of it is put on the kill ring and is inserted by yanking it
back.
Three functions are used for bringing text back from the kill
ring: yank, which is usually bound to
C-y; yank-pop, which is usually bound to
M-y; and rotate-yank-pointer, which is
used by the two other functions.
These functions refer to the kill ring through a variable called
the kill-ring-yank-pointer. Indeed, the insertion
code for both the yank and yank-pop
functions is:
(insert (car kill-ring-yank-pointer))
To begin to understand how yank and
yank-pop work, it is first necessary to look at the
kill-ring-yank-pointer variable and the
rotate-yank-pointer function.
kill-ring-yank-pointer
Variable
kill-ring-yank-pointer is a variable, just as
kill-ring is a variable. It points to something by
being bound to the value of what it points to, like any other
Lisp variable.
Thus, if the value of the kill ring is:
("some text" "a different piece of text" "yet more text")
and the kill-ring-yank-pointer points to the second
clause, the value of kill-ring-yank-pointer is:
("a different piece of text" "yet more text")
As explained in the previous chapter (see section 9 How Lists are Implemented),
the computer does not keep two different copies of the text being
pointed to by both the kill-ring and the
kill-ring-yank-pointer. The words "a different piece
of text" and "yet more text" are not duplicated. Instead, the two
Lisp variables point to the same pieces of text. Here is a
diagram:
kill-ring kill-ring-yank-pointer
| |
| ___ ___ | ___ ___ ___ ___
---> | | | --> | | | | | |
|___|___|----> |___|___|--> |___|___|--> nil
| | |
| | |
| | --> "yet more text"
| |
| --> "a different piece of text
|
--> "some text"
Both the variable kill-ring and the variable
kill-ring-yank-pointer are pointers. But the kill
ring itself is usually described as if it were actually what it
is composed of. The kill-ring is spoken of as if it
were the list rather than that it points to the list. Conversely,
the kill-ring-yank-pointer is spoken of as pointing
to a list.
These two ways of talking about the same thing sound confusing at
first but make sense on reflection. The kill ring is generally
thought of as the complete structure of data that holds the
information of what has recently been cut out of the Emacs
buffers. The kill-ring-yank-pointer on the other
hand, serves to indicate--that is, to `point to'---that part of
the kill ring of which the first element (the CAR) will be
inserted.
The rotate-yank-pointer function changes the element
in the kill ring to which the kill-ring-yank-pointer
points; when the pointer is set to point to the next element
beyond the end of the kill ring, it automatically sets it to
point to the first element of the kill ring. This is how the list
is transformed into a ring. The rotate-yank-pointer
function itself is not difficult, but contains many details. It
and the much simpler yank and yank-pop
functions are described in an appendix. See section B Handling the Kill Ring.
yank and
nthcdr
Using C-h v (describe-variable), look at
the value of your kill ring. Add several items to your kill ring;
look at its value again. Using M-y
(yank-pop), move all the way around the kill ring.
How many items were in your kill ring? Find the value of
kill-ring-max. Was your kill ring full, or could you
have kept more blocks of text within it?
Using nthcdr and car, construct a
series of expressions to return the first, second, third, and
fourth elements of a list.
Emacs Lisp has two primary ways to cause an expression, or a
series of expressions, to be evaluated repeatedly: one uses a
while loop, and the other uses
recursion.
Repetition can be very valuable. For example, to move forward four sentences, you need only write a program that will move forward one sentence and then repeat the process four times. Since a computer does not get bored or tired, such repetitive action does not have the deleterious effects that excessive or the wrong kinds of repetition can have on humans.
while
The while special form tests whether the value
returned by evaluating its first argument is true or false. This
is similar to what the Lisp interpreter does with an
if; what the interpreter does next, however, is
different.
In a while expression, if the value returned by
evaluating the first argument is false, the Lisp interpreter
skips the rest of the expression (the body of
the expression) and does not evaluate it. However, if the value
is true, the Lisp interpreter evaluates the body of the
expression and then again tests whether the first argument to
while is true or false. If the value returned by
evaluating the first argument is again true, the Lisp interpreter
again evaluates the body of the expression.
The template for a while expression looks like this:
(while true-or-false-test body...)
So long as the true-or-false-test of the while
expression returns a true value when it is evaluated, the body is
repeatedly evaluated. This process is called a loop since the
Lisp interpreter repeats the same thing again and again, like an
airplane doing a loop. When the result of evaluating the
true-or-false-test is false, the Lisp interpreter does not
evaluate the rest of the while expression and `exits
the loop'.
Clearly, if the value returned by evaluating the first argument
to while is always true, the arguments following
will be evaluated again and again ... and again ... forever.
Conversely, if the value returned is never true, the expressions
in the body will never be evaluated. The craft of writing a
while loop consists of choosing a mechanism such
that the true-or-false-test returns true just the number of times
that you want the subsequent expressions to be evaluated; and
then have the test return false.
The value returned by evaluating a while is the
value of the true-or-false-test. An interesting consequence of
this is that a while loop that evaluates without
error will return nil or false regardless of whether
it has looped 1 or 100 times or none at all. A while
expression that evaluates successfully never returns a true
value! What this means is that while is always
evaluated for its side effects, which is to say, the consequences
of evaluating the expressions within the body of the
while loop. This makes sense. It is not the mere act
of looping that is desired, but the consequences of what happens
when the expressions in the loop are repeatedly evaluated.
while Loop and a List
A common way to control a while loop is to test
whether a list has any elements. If it does, the loop is
repeated; but if it does not, the repetition is ended. Since this
is an important technique, we will create a short example to
illustrate it.
A simple way to test whether a list has elements is to evaluate
the list: if it has no elements, it is an empty list and will
return the empty list, (), which is a synonym for
nil or false. On the other hand, a list with
elements will return those elements when it is evaluated. Since
Lisp considers as true any value that is not nil, a
list that returns elements will test true in a while
loop.
For example, you can set the variable empty-list to
nil by evaluating the following setq
expression:
(setq empty-list ())
After evaluating the setq expression, you can
evaluate the variable empty-list in the usual way,
by placing the cursor after the symbol and typing C-x
C-e; nil will appear in your echo area:
empty-list
On the other hand, if you set a variable to be a list with elements, the list will appear when you evaluate the variable, as you can see by evaluating the following two expressions:
(setq animals '(giraffe gazelle lion tiger)) animals
Thus, to create a while loop that tests whether
there are any items in the list animals, the first
part of the loop will be written like this:
(while animals
...
When the while tests its first argument, the
variable animals is evaluated. It returns a list. So
long as the list has elements, the while considers
the results of the test to be true; but when the list is empty,
considers the results of the test to be false.
To prevent the while loop from running forever, some
mechanism needs to be provided to empty the list eventually. An
oft-used technique is to have one of the subsequent forms in the
while expression set the value of the list to be the
CDR of the list. Each time the cdr function is
evaluated, the list will be made shorter, until eventually only
the empty list will be left. At this point, the test of the
while loop will return false, and the arguments to
the while will no longer be evaluated.
For example, the list of animals bound to the variable
animals can be set to be the CDR of the original
list with the following expression:
(setq animals (cdr animals))
If you have evaluated the previous expressions and then evaluate
this expression, you will see (gazelle lion tiger)
appear in the echo area. If you evaluate the expression again,
(lion tiger) will appear in the echo area. If you
evaluate it again and yet again, (tiger) appears and
then the empty list, shown by nil.
A template for a while loop that uses the
cdr function repeatedly to cause the
true-or-false-test eventually to test false looks like this:
(while test-whether-list-is-empty body... set-list-to-cdr-of-list)
This test and use of cdr can be put together in a
function that goes through a list and prints each element of the
list on a line of its own.
print-elements-of-list
The print-elements-of-list function illustrates a
while loop with a list.
The function requires several lines for its
output. Since the echo area is only one line, we cannot
illustrate how it works in the same way we have been
illustrating functions in the past, by evaluating them inside
Info. Instead, you need to copy the necessary expressions to
your `*scratch*' buffer and evaluate them there. You
can copy the expressions by marking the beginning of the
region with C-SPC (set-mark-command),
moving the cursor to the end of the region and then copying
the region using M-w
(copy-region-as-kill). In the
`*scratch*' buffer, you can yank the expressions back
by typing C-y (yank).
After you have copied the expressions to the `*scratch*'
buffer, evaluate each expression in turn. Be sure to evaluate the
last expression, (print-elements-of-list animals),
by typing C-u C-x C-e, that is, by giving an argument
to eval-last-sexp. This will cause the result of the
evaluation to be printed in the `*scratch*' buffer
instead of being printed in the echo area. (Otherwise you will
see something like this in your echo area:
^Jgiraffe^J^Jgazelle^J^Jlion^J^Jtiger^Jnil, in which
each `^J' stands for the newline that in the
`*scratch*' buffer puts each word on its own line. You
can evaluate these expressions right now in the Info buffer, if
you like, to see this effect.)
(setq animals '(giraffe gazelle lion tiger))
(defun print-elements-of-list (list)
"Print each element of LIST on a line of its own."
(while list
(print (car list))
(setq list (cdr list))))
(print-elements-of-list animals)
When you evaluate the three expressions in sequence in the `*scratch*' buffer, this will be printed in the buffer:
giraffe gazelle lion tiger nil
Each element of the list is printed on a line of its own (that is
what the function print does) and then the value
returned by the function is printed. Since the last expression in
the function is the while loop, and since
while loops always return nil, a
nil is printed after the last element of the list.
A loop is not useful unless it stops when it ought. Besides controlling a loop with a list, a common way of stopping a loop is to write the first argument as a test that returns false when the correct number of repetitions are complete. This means that the loop must have counter--an expression that counts how many times the loop repeats itself.
The test can be an expression such as (< count
desired-number) which returns t for true if
the value of count is less than the
desired-number of repetitions and nil
for false if the value of count is equal to or is
greater than the desired-number. The expression that
increments the count can be a simple setq such as
(setq count (1+ count)), where 1+ is a
built-in function in Emacs Lisp that adds 1 to its argument. (The
expression (1+ count) has the same result as
(+ count 1), but is easier for a human to read.)
The template for a while loop controlled by an
incrementing counter looks like this:
set-count-to-initial-value (while (< count desired-number) ; true-or-false-test body... (setq (1+ count))) ; incrementer
Note that you need to set the initial value of
count; usually it is set to 1.
Suppose you are playing on the beach and decide to make a triangle of pebbles, putting one pebble in the first row, two in the second row, three in the third row and so on, like this:
*
* *
* * *
* * * *
(About 2500 years ago, Pythagoras and others developed the beginnings of number theory by considering questions such as this.)
Suppose you want to know how many pebbles you will need to make a triangle with 7 rows?
Clearly, what you need to do is add up the numbers from 1 to 7.
There are two ways to do this; start with the smallest number,
one, and add up the list in sequence, 1, 2, 3, 4 and so on; or
start with the largest number and add the list going down: 7, 6,
5, 4 and so on. Because both mechanisms illustrate common ways of
writing while loops, we will create two examples,
one counting up and the other counting down. In this first
example, we will start with 1 and add 2, 3, 4 and so on.
If you are just adding up a short list of numbers, the easiest way to do it is to add up all the numbers at once. However, if you do not know ahead of time how many numbers your list will have, or if you want to be prepared for a very long list, then you need to design your addition so that what you do is repeat a simple process many times instead of doing a more complex process once.
For example, instead of adding up all the pebbles all at once, what you can do is add the number of pebbles in the first row, 1, to the number in the second row, 2, and then add the total of those two rows to the third row, 3. Then you can add the number in the fourth row, 4, to the total of the first three rows; and so on.
The critical characteristic of the process is that each repetitive action is simple. In this case, at each step we add only two numbers, the number of pebbles in the row and the total already found. This process of adding two numbers is repeated again and again until the last row has been added to the total of all the preceding rows. In a more complex loop the repetitive action might not be so simple, but it will be simpler than doing everything all at once.
The preceding analysis gives us the bones of our function
definition: first, we will need a variable that we can call
total that will be the total number of pebbles. This
will be the value returned by the function.
Second, we know that the function will require an argument: this
argument will be the total number of rows in the triangle. It can
be called number-of-rows.
Finally, we need a variable to use as a counter. We could call
this variable counter, but a better name is
row-number. That is because what the counter does is
count rows, and a program should be written to be understandable
as possible.
When the Lisp interpreter first starts evaluating the expressions
in the function, the value of total should be set to
zero, since we have not added anything to it. Then the function
should add the number of pebbles in the first row to the total,
and then add the number of pebbles in the second to the total,
and then add the number of pebbles in the third row to the total,
and so on, until there are no more rows left to add.
Both total and row-number are used only
inside the function, so they can be declared as local variables
with let and given initial values. Clearly, the
initial value for total should be 0. The initial
value of row-number should be 1, since we start with
the first row. This means that the let statement
will look like this:
(let ((total 0)
(row-number 1))
body...)
After the internal variables are declared and bound to their
initial values, we can begin the while loop. The
expression that serves as the test should return a value of
t for true so long as the row-number is
less than or equal to the number-of-rows. (If the
expression tests true only so long as the row number is less than
the number of rows in the triangle, the last row will never be
added to the total; hence the row number has to be either less
than or equal to the number of rows.)
Lisp provides the <= function
that returns true if the value of its first argument is less
than or equal to the value of its second argument and false
otherwise. So the expression that the while will
evaluate as its test should look like this:
(<= row-number number-of-rows)
The total number of pebbles can be found by repeatedly adding the number of pebbles in a row to the total already found. Since the number of pebbles in the row is equal to the row number, the total can be found by adding the row number to the total. (Clearly, in a more complex situation, the number of pebbles in the row might be related to the row number in a more complicated way; if this were the case, the row number would be replaced by the appropriate expression.)
(setq total (+ total row-number)
What this does is set the new value of total to be
equal to the sum of adding the number of pebbles in the row to
the previous total.
After setting the value of total, the conditions
need to be established for the next repetition of the loop, if
there is one. This is done by incrementing the value of the
row-number variable, which serves as a counter.
After the row-number variable has been incremented,
the true-or-false-test at the beginning of the while
loop tests whether its value is still less than or equal to the
value of the number-of-rows and if it is, adds the
new value of the row-number variable to the
total of the previous repetition of the loop.
The built-in Emacs Lisp function 1+ adds 1 to a
number, so the row-number variable can be
incremented with this expression:
(setq row-number (1+ row-number))
We have created the parts for the function definition; now we need to put them together.
First, the contents of the while expression:
(while (<= row-number number-of-rows) ; true-or-false-test (setq total (+ total row-number)) (setq row-number (1+ row-number))) ; incrementer
Along with the let expression varlist, this very
nearly completes the body of the function definition. However, it
requires one final element, the need for which is somewhat
subtle.
The final touch is to place the variable total on a
line by itself after the while expression.
Otherwise, the value returned by the whole function is the value
of the last expression that is evaluated in the body of the
let, and this is the value returned by the
while, which is always nil.
This may not be evident at first sight. It almost looks as if the
incrementing expression is the last expression of the whole
function. But that expression is part of the body of the
while; it is the last element of the list that
starts with the symbol while. Moreover, the whole of
the while loop is a list within the body of the
let.
In outline, the function will look like this:
(defun name-of-function (argument-list)
"documentation..."
(let (varlist)
(while (true-or-false-test)
body-of-while... )
... ) ; Need final expression here.
The result of evaluating the let is what is going to
be returned by the defun since the let
is not embedded within any containing list, except for the
defun as a whole. However, if the while
is the last element of the let expression, the
function will always return nil. This is not what we
want! Instead, what we want is the value of the variable
total. This is returned by simply placing the symbol
as the last element of the list starting with let.
It gets evaluated after the preceding elements of the list are
evaluated, which means it gets evaluated after it has been
assigned the correct value for the total.
It may be easier to see this by printing the list starting with
let all on one line. This format makes it evident
that the varlist and while expressions
are the second and third elements of the list starting with
let, and the total is the last element:
(let (varlist) (while (true-or-false-test) body-of-while... ) total)
Putting everything together, the triangle function
definition looks like this:
(defun triangle (number-of-rows) ; Version with
; incrementing counter.
"Add up the number of pebbles in a triangle.
The first row has one pebble, the second row two pebbles,
the third row three pebbles, and so on.
The argument is NUMBER-OF-ROWS."
(let ((total 0)
(row-number 1))
(while (<= row-number number-of-rows)
(setq total (+ total row-number))
(setq row-number (1+ row-number)))
total))
After you have installed triangle by evaluating the
function, you can try it out. Here are two examples:
(triangle 4) (triangle 7)
The sum of the first four numbers is 10 and the sum of the first seven numbers is 28.
Another common way to write a while loop is to write
the test so that it determines whether a counter is greater than
zero. So long as the counter is greater than zero, the loop is
repeated. But when the counter is equal to or less than zero, the
loop is stopped. For this to work, the counter has to start out
greater than zero and then be made smaller and smaller by one of
the forms that is evaluated repeatedly.
The test will be an expression such as (> counter
0) which returns t for true if the value of
counter is greater than zero, and nil
for false if the value of counter is equal to or
less than zero. The expression that makes the number smaller and
smaller can be a simple setq such as (setq
counter (1- counter)), where 1- is a built-in
function in Emacs Lisp that subtracts 1 from its argument.
The template for a decrementing while loop looks
like this:
(while (> counter 0) ; true-or-false-test body... (setq counter (1- counter))) ; decrementer
To illustrate a loop with a decrementing counter, we will rewrite
the triangle function so the counter decreases to
zero.
This is the reverse of the earlier version of the function. In this case, to find out how many pebbles are needed to make a triangle with 3 rows, add the number of pebbles in the third row, 3, to the number in the preceding row, 2, and then add the total of those two rows to the row that precedes them, which is 1.
Likewise, to find the number of pebbles in a triangle with 7 rows, add the number of pebbles in the seventh row, 7, to the number in the preceding row, which is 6, and then add the total of those two rows to the row that precedes them, which is 5, and so on. As in the previous example, each addition only involves adding two numbers, the total of the rows already added up and the number of pebbles in the row that is being added to the total. This process of adding two numbers is repeated again and again until there are no more pebbles to add.
We know how many pebbles to start with: the number of pebbles in the last row is equal to the number of rows. If the triangle has seven rows, the number of pebbles in the last row is 7. Likewise, we know how many pebbles are in the preceding row: it is one less than the number in the row.
We start with three variables: the total number of rows in the
triangle; the number of pebbles in a row; and the total number of
pebbles, which is what we want to calculate. These variables can
be named number-of-rows,
number-of-pebbles-in-row, and total,
respectively.
Both total and number-of-pebbles-in-row
are used only inside the function and are declared with
let. The initial value of total should,
of course, be zero. However, the initial value of
number-of-pebbles-in-row should be equal to the
number of rows in the triangle, since the addition will start
with the longest row.
This means that the beginning of the let expression
will look like this:
(let ((total 0)
(number-of-pebbles-in-row number-of-rows))
body...)
The total number of pebbles can be found by repeatedly adding the number of pebbles in a row to the total already found, that is, by repeatedly evaluating the following expression:
(setq total (+ total number-of-pebbles-in-row))
After the number-of-pebbles-in-row is added to the
total, the number-of-pebbles-in-row
should be decremented by one, since the next time the loop
repeats, the preceding row will be added to the total.
The number of pebbles in a preceding row is one less than the
number of pebbles in a row, so the built-in Emacs Lisp function
1- can be used to compute the number of pebbles in
the preceding row. This can be done with the following
expression:
(setq number-of-pebbles-in-row
(1- number-of-pebbles-in-row))
Finally, we know that the while loop should stop
making repeated additions when there are no pebbles in a row. So
the test for the while loop is simply:
(while (> number-of-pebbles-in-row 0)
Putting these expressions together, we have a function definition that looks like this:
;;; First subtractive version.
(defun triangle (number-of-rows)
"Add up the number of pebbles in a triangle."
(let ((total 0)
(number-of-pebbles-in-row number-of-rows))
(while (> number-of-pebbles-in-row 0)
(setq total (+ total number-of-pebbles-in-row))
(setq number-of-pebbles-in-row
(1- number-of-pebbles-in-row)))
total))
As written, this function works.
However, it turns out that one of the local variables,
number-of-pebbles-in-row, is unneeded!
When the triangle function is
evaluated, the symbol number-of-rows will be
bound to a number, giving it an initial value. That number can
be changed in the body of the function as if it were a local
variable, without any fear that such a change will effect the
value of the variable outside of the function. This is a very
useful characteristic of Lisp; it means that the variable
number-of-rows can be used anywhere in the
function where number-of-pebbles-in-row is used.
Here is a second version of the function written a bit more cleanly:
(defun triangle (number) ; Second version.
"Return sum of numbers 1 through NUMBER inclusive."
(let ((total 0))
(while (> number 0)
(setq total (+ total number))
(setq number (1- number)))
total))
In brief, a properly written while loop will consist
of three parts:
A recursive function contains code that tells itself to evaluate itself. When the function evaluates itself, it again finds the code that tells itself to evaluate itself, so the function evaluates itself again ... and again ... A recursive function will keep telling itself to evaluate itself again forever unless it is also provided with a stop condition.
A recursive function contains a conditional expression which has three parts:
Recursive functions are often much simpler than any other kind of function. Indeed, when people first start to use them, they often look so mysteriously simple as to be incomprehensible. Like riding a bicycle, reading a recursive function definition takes a certain knack which is hard at first but then seems simple.
The template for a recursive function looks like this:
(defun name-of-recursive-function (argument-list)
"documentation..."
body...
(if do-again-test
(name-of-recursive-function
next-step-expression)))
Each time the recursive function is evaluated, an argument is bound to the value of the next-step-expression; and that value is used in the do-again-test. The next-step-expression is designed so that the do-again-test returns false when the function should no longer be repeated.
The do-again-test is sometimes called the stop condition, since it stops the repetitions when it tests false.
The example of a while loop that printed the
elements of a list of numbers can be written recursively. Here is
the code, including an expression to set the value of the
variable animals to a list.
This example must be copied to the `*scratch*' buffer
and each expression must be evaluated there. Use C-u C-x
C-e to evaluate the (print-elements-recursively
animals) expression so that the results are printed in the
buffer; otherwise the Lisp interpreter will try to squeeze the
results into the one line of the echo area.
Also, place your cursor immediately after the last closing
parenthesis of the print-elements-recursively
function, before the comment. Otherwise, the Lisp interpreter
will try to evaluate the comment.
(setq animals '(giraffe gazelle lion tiger))
(defun print-elements-recursively (list)
"Print each element of LIST on a line of its own.
Uses recursion."
(print (car list)) ; body
(if list ; do-again-test
(print-elements-recursively ; recursive call
(cdr list)))) ; next-step-expression
(print-elements-recursively animals)
The print-elements-recursively function first prints
the first element of the list, the CAR of the list. Then, if the
list is not empty, the function invokes itself, but gives itself
as its argument, not the whole list, but the second and
subsequent elements of the list, the CDR of the list.
When this evaluation occurs, the function prints the first
element of the list it receives as its argument (which is the
second element of the original list). Then, the if
expression is evaluated and when true, the function calls itself
with the CDR of the list it is invoked with, which (the second
time around) is the CDR of the CDR of the original list.
Each time the function invokes itself, it invokes itself on a
shorter version of the original list. Eventually, the function
invokes itself on an empty list. The print function
prints the empty list as nil. Next, the conditional
expression tests the value of list. Since the value
of list is nil, the if
expression tests false so the then-part is not evaluated. The
function as a whole then returns nil. Consequently,
you see nil twice when you evaluate the function.
When you evaluate (print-elements-recursively
animals) in the `*scratch*' buffer, you see this
result:
giraffe gazelle lion tiger nil nil
(The first nil is the value of the empty list that
is printed; the second nil is the value returned by
the whole function.)
The triangle function described in a previous
section can also be written recursively. It looks like this:
(defun triangle-recursively (number)
"Return the sum of the numbers 1 through NUMBER inclusive.
Uses recursion."
(if (= number 1) ; do-again-test
1 ; then-part
(+ number ; else-part
(triangle-recursively ; recursive call
(1- number))))) ; next-step-expression
(triangle-recursively 7)
You can install this function by evaluating it and then try it by
evaluating (triangle-recursively 7). (Remember to
put your cursor immediately after the last parenthesis of the
function definition, before the comment.)
To understand how this function works, let's consider what happens in the various cases when the function is passed 1, 2, 3, or 4 as the value of its argument.
First, what happens if the value of the argument is 1?
The function has an if expression after the
documentation string. It tests whether the value of
number is equal to 1; if so, Emacs evaluates the
then-part of the if expression, which returns the
number 1 as the value of the function. (A triangle with one row
has one pebble in it.)
Suppose, however, that the value of the argument is 2. In this
case, Emacs evaluates the else-part of the if
expression.
The else-part consists of an addition, the recursive call to
triangle-recursively and a decrementing action; and
it looks like this:
(+ number (triangle-recursively (1- number)))
When Emacs evaluates this expression, the innermost expression is evaluated first; then the other parts in sequence. Here are the steps in detail:
(1- number) so Emacs
decrements the value of number from 2 to 1.
triangle-recursively
function.
triangle-recursively function
In this case, Emacs evaluates triangle-recursively
with an argument of 1. This means that this evaluation of
triangle-recursively returns 1.
number.
number is the second element of the
list that starts with +; its value is 2.
+ expression.
+ expression receives two arguments, the first
from the evaluation of number (Step 3) and the
second from the evaluation of triangle-recursively
(Step 2). The result of the addition is the sum of 2 plus 1,
and the number 3 is returned, which is correct. A triangle with
two rows has three pebbles in it.
Suppose that triangle-recursively is called with an
argument of 3.
if expression is evaluated first. This is the
do-again test and returns false, so the else-part of the
if expression is evaluated. (Note that in this
example, the do-again-test causes the function to call itself
when it tests false, not when it tests true.)
triangle-recursively
function.
triangle-recursively
function. We know what happens when Emacs evaluates
triangle-recursively with an argument of 2. After
going through the sequence of actions described earlier, it
returns a value of 3. So that is what will happen here.
The value returned by the function as a whole will be 6.
Now that we know what will happen when
triangle-recursively is called with an argument of
3, it is evident what will happen if it is called with an
argument of 4:
In the recursive call, the evaluation of
(triangle-recursively (1- 4))will return the value of evaluating
(triangle-recursively 3)which is 6 and this value will be added to 4 by the addition in the third line.
The value returned by the function as a whole will be 10.
Each time triangle-recursively is evaluated, it
evaluates a version of itself with a smaller argument, until the
argument is small enough so that it does not evaluate itself.
cond
The version of triangle-recursively described
earlier is written with the if special form. It can
also be written using another special form called
cond. The name of the special form cond
is an abbreviation of the word `conditional'.
Although the cond special form is not used as often
in the Emacs Lisp sources as if, it is used often
enough to justify explaining it.
The template for a cond expression looks like this:
(cond body...)
where the body is a series of lists.
Written out more fully, the template looks like this:
(cond ((first-true-or-false-test first-consequent) (second-true-or-false-test second-consequent) (third-true-or-false-test third-consequent) ...)
When the Lisp interpreter evaluates the cond
expression, it evaluates the first element (the CAR or
true-or-false-test) of the first expression in a series of
expressions within the body of the cond.
If the true-or-false-test returns nil the rest of
that expression, the consequent, is skipped and the
true-or-false-test of the next expression is evaluated. When an
expression is found whose true-or-false-test returns a value that
is not nil, the consequent of that expression is
evaluated. The consequent can be one or more expressions. If the
consequent consists of more than one expression, the expressions
are evaluated in sequence and the value of the last one is
returned. If the expression does not have a consequent, the value
of the true-or-false-test is returned.
If none of the true-or-false-tests test true, the
cond expression returns nil.
Written using cond, the triangle
function looks like this:
(defun triangle-using-cond (number)
(cond ((<= number 0) 0)
((= number 1) 1)
((> number 1)
(+ number (triangle-using-cond (1- number))))))
In this example, the cond returns 0 if the number is
less than or equal to 0, it returns 1 if the number is 1 and it
evaluates (+ number (triangle-using-cond (1-
number))) if the number is greater than 1.
Write a function similar to triangle in which each
row has a value which is the square of the row number. Use a
while loop.
Write a function similar to triangle that multiplies
instead of adds the values.
Rewrite these two functions recursively. Rewrite these functions
using cond.
Write a function for Texinfo mode that creates an index entry at the beginning of a paragraph for every `@dfn' within the paragraph. (In a Texinfo file, `@dfn' marks a definition. For more information, see "Indicating Definitions, Commands, etc." in Texinfo, The GNU Documentation Format.)
Regular expression searches are used extensively in GNU Emacs.
The two functions, forward-sentence and
forward-paragraph illustrate these searches well.
Regular expression searches are described in section `Regular
Expression Search' in The GNU Emacs Manual, as well
as in section `Regular Expressions' in The GNU Emacs Lisp
Reference Manual. In writing this chapter, I am presuming
that you have at least a mild acquaintance with them. The major
point to remember is that regular expressions permit you to
search for patterns as well as for literal strings of characters.
For example, the code in forward-sentence searches
for the pattern of possible characters that could mark the end of
a sentence, and moves point to that spot.
Before looking at the code for the forward-sentence
function, it is worth considering what the pattern that marks the
end of a sentence must be. The pattern is discussed in the next
section; following that is a description of the regular
expression search function, re-forward-search. The
forward-sentence function is described in the
section following. Finally, the forward-paragraph
function is described in the last section of this chapter.
forward-paragraph is a complex function that
introduces several new features.
sentence-end
The symbol sentence-end is bound to the pattern that
marks the end of a sentence. What should this regular expression
be?
Clearly, a sentence may be ended by a period, a question mark, or an exclamation mark. Indeed, only clauses that end with one of those three characters should be considered the end of a sentence. This means that the pattern should include the character set:
[.?!]
However, we do not want forward-sentence merely to
jump to a period, a question mark, or an exclamation mark,
because such a character might be used in the middle of a
sentence. A period, for example, is used after abbreviations. So
other information is needed.
According to convention, you type two spaces after every sentence, but only one space after a period, a question mark, or an exclamation mark in the body of a sentence. So a period, a question mark, or an exclamation mark followed by two spaces is a good indicator of an end of sentence. However, in a file, the two spaces may instead be a tab or the end of a line. This means that the regular expression should include these three items as alternatives. This group of alternatives will look like this:
\\($\\| \\| \\)
^ ^^
TAB SPC
Here, `$' indicates the end of the line, and I have pointed out where the tab and two spaces are inserted in the expression. Both are inserted by putting the actual characters into the expression.
Two backslashes, `\\', are required before the parentheses and vertical bars: the first backslash to quote the following backslash in Emacs; and the second to indicate that the following character, the parenthesis or the vertical bar, is special.
Also, a sentence may be followed by one or more carriage returns, like this:
[ ]*
Like tabs and spaces, a carriage return is inserted into a regular expression by inserting it literally. The asterisk indicates that the RET is repeated zero or more times.
But a sentence end does not consist only of a period, a question mark or an exclamation mark followed by appropriate space: a closing quotation mark or a closing brace of some kind may precede the space. Indeed more than one such mark or brace by precede the space. These require a expression that looks like this:
[]\"')}]*
In this expression, the first `]' is the first character in the expression; the second character is `"', which is preceded by a `\' to tell Emacs the `"' is not special. The last three characters are `'', `)', and `}'.
All this suggests what the regular expression pattern for
matching the end of a sentence should be; and, indeed, if we
evaluate sentence-end we find that it returns the
following value:
sentence-end
=> "[.?!][]\"')}]*\\($\\| \\| \\)[
]*"
re-search-forward
Function
The re-search-forward function is very like the
search-forward function. (See section 8.1.3 The
search-forward Function.)
re-search-forward searches for a regular expression.
If the search is successful, it leaves point immediately after
the last character in the target. If the search is backwards, it
leaves point just before the first character in the target. You
may tell re-search-forward to return t
for true. (Moving point is therefore a `side effect'.)
Like search-forward, the
re-search-forward function takes four arguments:
nil as the third argument
causes the function to signal an error (and print a message) when
the search fails; any other value causes it to return
nil if the search fails and t if the
search succeeds.
re-search-forward to search
backwards.
The template for re-search-forward looks like this:
(re-search-forward "regular-expression"
limit-of-search
what-to-do-if-search-fails
repeat-count)
The second, third, and fourth arguments are optional. However, if you want to pass a value to either or both of the last two arguments, you must also pass a value to all the preceding arguments. Otherwise, the Lisp interpreter will mistake which argument you are passing the value to.
In the forward-sentence function, the regular
expression will be the value of the variable
sentence-end, namely:
"[.?!][]\"')}]*\\($\\| \\| \\)[ ]*"
The limit of the search will be the end of the paragraph (since a
sentence cannot go beyond a paragraph). If the search fails, the
function will return nil; and the repeat count will
be provided by the argument to the forward-sentence
function.
forward-sentence
The command to move the cursor forward a sentence is a straightforward illustration of how to use regular expression searches in Emacs Lisp. Indeed, the function looks longer and more complicated than it is; this is because the function is designed to go backwards as well as forwards; and, optionally, over more than one sentence. The function is usually bound to the key command M-e.
Here is the code for forward-sentence:
(defun forward-sentence (&optional arg)
"Move forward to next sentence-end. With argument, repeat.
With negative argument, move backward repeatedly to sentence-beginning.
Sentence ends are identified by the value of sentence-end
treated as a regular expression. Also, every paragraph boundary
terminates sentences as well."
(interactive "p")
(or arg (setq arg 1))
(while (< arg 0)
(let ((par-beg
(save-excursion (start-of-paragraph-text) (point))))
(if (re-search-backward
(concat sentence-end "[^ \t\n]") par-beg t)
(goto-char (1- (match-end 0)))
(goto-char par-beg)))
(setq arg (1+ arg)))
(while (> arg 0)
(let ((par-end
(save-excursion (end-of-paragraph-text) (point))))
(if (re-search-forward sentence-end par-end t)
(skip-chars-backward " \t\n")
(goto-char par-end)))
(setq arg (1- arg))))
The function looks long at first sight and it is best to look at its skeleton first, and then its muscle. The way to see the skeleton is to look at the expressions that start in the left-most columns:
(defun forward-sentence (&optional arg)
"documentation..."
(interactive "p")
(or arg (setq arg 1))
(while (< arg 0)
body-of-while-loop
(while (> arg 0)
body-of-while-loop
This looks much simpler! The function definition consists of
documentation, an interactive expression, an
or expression, and while loops.
Let's look at each of these parts in turn.
We note that the documentation is thorough and understandable.
The function has an interactive "p" declaration.
This means that the processed prefix argument, if any, is passed
to the function as its argument. (This will be a number.) If the
function is not passed an argument (it is optional) then the
argument arg will be bound to 1. When
forward-sentence is called non-interactively without
an argument, arg is bound to nil.
The or expression handles the prefix argument. What
it does is either leave the value of arg as it is,
but only if arg is bound to a value; or it sets the
value of arg to 1, in the case when arg
is bound to nil.
while loops
Two while loops follow the or
expression. The first while has a true-or-false-test
that tests true if the prefix argument for
forward-sentence is a negative number. This is for
going backwards. The body of this loop is similar to the body of
the second while clause, but it is not exactly the
same. We will skip this while loop and concentrate
on the second while loop.
The second while loop is for moving point forward.
Its skeleton looks like this:
(while (> arg 0) ; true-or-false-test
(let varlist
(if (true-or-false-test)
then-part
else-part
(setq arg (1- arg)))) ; while loop decrementer
The while loop is of the decrementing kind. (See
section 11.1.4 Loop with a
Decrementing Counter.) It has a true-or-false-test that tests
true so long as the counter (in this case, the variable
arg) is greater than zero; and it has a decrementer
that subtracts 1 from the value of the counter every time the
loop repeats.
If no prefix argument is given to forward-sentence,
which is the most common way the command is used, this
while loop will run once, since the value of
arg will be 1.
The body of the while loop consists of a
let expression, which creates and binds a local
variable, and has, as its body, an if expression.
The body of the while loop looks like this:
(let ((par-end
(save-excursion (end-of-paragraph-text) (point))))
(if (re-search-forward sentence-end par-end t)
(skip-chars-backward " \t\n")
(goto-char par-end)))
The let expression creates and binds the local
variable par-end. As we shall see, this local
variable is designed to provide a bound or limit to the regular
expression search. If the search fails to find a proper sentence
ending in the paragraph, it will stop on reaching the end of the
paragraph.
But first, let us examine how par-end is bound to
the value of the end of the paragraph. What happens is that the
let sets the value of par-end to the
value returned when the Lisp interpreter evaluates the expression
(save-excursion (end-of-paragraph-text) (point))
In this expression, (end-of-paragraph-text) moves
point to the end of the paragraph, (point) returns
the value of point, and then save-excursion restores
point to its original position. Thus, the let binds
par-end to the value returned by the
save-excursion expression, which is the position of
the end of the paragraph. (The
(end-of-paragraph-text) function uses
forward-paragraph, which we will discuss shortly.)
Emacs next evaluates the body of the let, which is
an if expression that looks like this:
(if (re-search-forward sentence-end par-end t) ; if-part
(skip-chars-backward " \t\n") ; then-part
(goto-char par-end))) ; else-part
The if tests whether its first argument is true and
if so, evaluates its then-part; otherwise, the Emacs Lisp
interpreter evaluates the else-part. The true-or-false-test of
the if expression is the regular expression search.
It may seem odd to have what looks like the `real work' of the
forward-sentence function buried here, but this is a
common way this kind of operation is carried out in Lisp.
The re-search-forward function searches for the end
of the sentence, that is, for the pattern defined by the
sentence-end regular expression. If the pattern is
found--if the end of the sentence is found--then the
re-search-forward function does two things:
re-search-forward function carries out a
side effect, which is to move point to the end of the occurrence
found.
re-search-forward function returns a value
of true. This is the value received by the if, and
means that the search was successful.
The side effect, the movement of point, is completed before the
if function is handed the value returned by the
successful conclusion of the search.
When the if function receives the value of true from
a successful call to re-search-forward, the
if evaluates the then-part, which is the expression
(skip-chars-backward " \t\n"). This expression moves
backwards over any blank spaces, tabs or carriage returns until a
printed character is found and then leaves point after the
character. Since point has already been moved to the end of the
pattern that marks the end of the sentence, this action leaves
point right after the closing printed character of the sentence,
which is usually a period.
On the other hand, if the re-search-forward function
fails to find a pattern marking the end of the sentence, the
function returns false. The false then causes the if
to evaluate its third argument, which is (goto-char
par-end): it moves point to the end of the paragraph.
Regular expression searches are exceptionally useful and the
pattern illustrated by re-search-forward, in which
the search is the test of an if expression, is
handy. You will see or write code incorporating this pattern
often.
forward-paragraph: a Goldmine
of Functions
The forward-paragraph function moves point forward
to the end of the paragraph. It is usually bound to
M-} and makes use of a number of functions that are
important in themselves, including let*,
match-beginning, and looking-at.
The function definition for forward-paragraph is
considerably longer than the function definition for
forward-sentence because it works with a paragraph,
each line of which may begin with a fill prefix.
A fill prefix consists of a string of characters that are repeated at the beginning of each line. For example, in Lisp code, it is a convention to start each line of a paragraph-long comment with `;;; '. In Text mode, four blank spaces make up another common fill prefix, creating an indented paragraph. (See section `Fill Prefix' in The GNU Emacs Manual, for more information about fill prefixes.)
The existence of a fill prefix means that in addition to being
able to find the end of a paragraph whose lines begin on the
left-most column, the forward-paragraph function
must be able to find the end of a paragraph when all or many of
the lines in the buffer begin with the fill prefix.
Moreover, it is sometimes practical to ignore a fill prefix that exists, especially when blank lines separate paragraphs. This is an added complication.
Rather than print all of the forward-paragraph
function, we will only print parts of it. Read without
preparation, the function can be daunting!
In outline, the function looks like this:
(defun forward-paragraph (&optional arg)
"documentation..."
(interactive "p")
(or arg (setq arg 1))
(let*
varlist
(while (< arg 0) ; backward-moving-code
...
(setq arg (1+ arg)))
(while (> arg 0) ; forward-moving-code
...
(setq arg (1- arg)))))
The first parts of the function are routine: the function's argument list consists of one optional argument. Documentation follows.
The lower case `p' in the interactive
declaration means that the processed prefix argument, if any, is
passed to the function. This will be a number, and is the repeat
count of how many paragraphs point will move. The or
expression in the next line handles the common case when no
argument is passed the function, which occurs if the function is
called from other code rather than interactively. This case was
described earlier. (See section 12.3
forward-sentence.) Now we reach the end of the
familiar part of this function.
let* expression
The next line of the forward-paragraph function
begins a let* expression. This is a different kind
of expression than we have seen so far. The symbol is
let* not let.
The let* special form is like let
except that Emacs sets each variable in sequence, one after
another, and variables in the latter part of the varlist can make
use of the values to which Emacs set variables in the earlier
part of the varlist.
In the let* expression in this function, Emacs binds
two variables: fill-prefix-regexp and
paragraph-separate. The value to which
paragraph-separate is bound depends on the value of
fill-prefix-regexp.
Let's look at each in turn. The symbol
fill-prefix-regexp is set to the value returned by
evaluating the following list:
(and fill-prefix
(not (equal fill-prefix ""))
(not paragraph-ignore-fill-prefix)
(regexp-quote fill-prefix))
This is an expression whose first element is the function
and.
The and function evaluates each of its arguments
until one of the arguments returns of value of nil,
in which case the and expression returns
nil; however, if none of the arguments returns a
value of nil, the value resulting from evaluating
the last argument is returned. (Since such a value is not
nil, it is considered true in Lisp.) In other words,
an and expression returns a true value only if all
its arguments are true.
In this case, the variable fill-prefix-regexp is
bound to a non-nil value only if the following four
expressions produce a true (i.e., a non-nil) value
when they are evaluated; otherwise,
fill-prefix-regexp is bound to nil.
fill-prefix
nil.
(not (equal fill-prefix "")
(not paragraph-ignore-fill-prefix)
nil if the variable
paragraph-ignore-fill-prefix has been turned on by
being set to a true value such as t.
(regexp-quote fill-prefix)
and function. If
all the arguments to the and are true, the value
resulting from evaluating this expression will be returned by
the and expression and bound to the variable
fill-prefix-regexp,
The result of evaluating this
and expression successfully is that
fill-prefix-regexp will be bound to the value of
fill-prefix as modified by the
regexp-quote function. What
regexp-quote does is read a string and return a
regular expression that will exactly match the string and
match nothing else. This means that
fill-prefix-regexp will be set to a value that
will exactly match the fill prefix if the fill prefix exists.
Otherwise, the variable will be set to nil.
The second local variable in the let* expression is
paragraph-separate. It is bound to the value
returned by evaluating the expression:
(if fill-prefix-regexp
(concat paragraph-separate
"\\|^" fill-prefix-regexp "[ \t]*$")
paragraph-separate)))
This expression shows why let* rather than
let was used. The true-or-false-test for the
if depends on whether the variable
fill-prefix-regexp evaluates to nil or
some other value.
If fill-prefix-regexp does not have a value, Emacs
evaluates the else-part of the if expression and
binds paragraph-separate to its local value.
(paragraph-separate is a regular expression that
matches what separates paragraphs.)
But if fill-prefix-regexp does have a value, Emacs
evaluates the then-part of the if expression and
binds paragraph-separate to a regular expression
that includes the fill-prefix-regexp as part of the
pattern.
Specifically, paragraph-separate is set to the
original value of the paragraph separate regular expression
concatinated with an alternative expression that consists of the
fill-prefix-regexp followed by a blank line. The
`^' indicates that the
fill-prefix-regexp must begin a line, and the
optional whitespace to the end of the line is defined by "[
\t]*$".) The `\\|' defines this portion of
the regexp as an alternative to paragraph-separate.
Now we get into the body of the let*. The first part
of the body of the let* deals with the case when the
function is given a negative argument and is therefore moving
backwards. We will skip this section.
while loop
The second part of the body of the let* deals with
forward motion. It is a while loop that repeats
itself so long as the value of arg is greater than
zero. In the most common use of the function, the value of the
argument is 1, so the body of the while loop is
evaluated exactly once, and the cursor moves forward one
paragraph.
This part handles three situations: when point is between paragraphs, when point is within a paragraph and there is a fill prefix, and when point is within a paragraph and there is no fill prefix.
The while loop looks like this:
(while (> arg 0)
(beginning-of-line)
;; between paragraphs
(while (prog1 (and (not (eobp))
(looking-at paragraph-separate))
(forward-line 1)))
;; within paragraphs, with a fill prefix
(if fill-prefix-regexp
;; There is a fill prefix; it overrides paragraph-start.
(while (and (not (eobp))
(not (looking-at paragraph-separate))
(looking-at fill-prefix-regexp))
(forward-line 1))
;; within paragraphs, no fill prefix
(if (re-search-forward paragraph-start nil t)
(goto-char (match-beginning 0))
(goto-char (point-max))))
(setq arg (1- arg)))
We can see immediately that this is a decrementing counter
while loop, using the expression (setq (1-
arg)) as the decrementer. The body of the loop consists of
three expressions:
;; between paragraphs
(beginning-of-line)
(while
body-of-while)
;; within paragraphs, with fill prefix
(if true-or-false-test
then-part
;; within paragraphs, no fill prefix
else-part
When the Emacs Lisp interpreter evaluates the body of the
while loop, the first thing it does is evaluate the
(beginning-of-line) expression and move point to the
beginning of the line. Then there is an inner while
loop. This while loop is designed to move the cursor
out of the blank space between paragraphs, if it should happen to
be there. Finally there is an if expression that
actually moves point to the end of the paragraph.
First, let us look at the inner while loop. This
loop handles the case when point is between paragraphs; it uses
three functions that are new to us: prog1,
eobp and looking-at.
prog1 is similar to the progn
function, except that prog1 evaluates its
arguments in sequence and then returns the value of its first
argument as the value of the whole expression.
(progn returns the value of its last argument as
the value of the expression.) The second and subsequent
arguments to prog1 are evaluated only for their
side effects.
eobp is an abbreviation of `End Of Buffer
P' and is a function that returns true if point is at
the end of the buffer.
looking-at is a function that returns true if the
text following point matches the regular expression passed
looking-at as its argument.
The while loop we are studying looks like this:
(while (prog1 (and (not (eobp))
(looking-at paragraph-separate))
(forward-line 1)))
This is a while loop with no body! The
true-or-false-test of the loop is the expression:
(prog1 (and (not (eobp))
(looking-at paragraph-separate))
(forward-line 1)))
The first argument to the prog1 is the
and expression. It has within in it a test of
whether point is at the end of the buffer and also a test of
whether the pattern following point matches the regular
expression for separating paragraphs.
If the cursor is not at the end of the buffer and if the
characters following the cursor mark the separation between two
paragraphs, then the and expression is true. After
evaluating the and expression, the Lisp interpreter
evaluates the second argument to prog1, which is
forward-line. This moves point forward one line. The
value returned by the prog1 however, is the value of
its first argument, so the while loop continues so
long as point is not at the end of the buffer and is between
paragraphs. When, finally, point is moved to a paragraph, the
and expression tests false. Note however, that the
forward-line command is carried out anyhow. This
means that when point is moved from between paragraphs to a
paragraph, it is left at the beginning of the second line of the
paragraph.
The next expression in the outer while loop is an
if expression. The Lisp interpreter evaluates the
then-part of the if when the
fill-prefix-regexp variable has a value other than
nil, and it evaluates the else-part when the value
of if fill-prefix-regexp is nil, that
is, when there is no fill prefix.
It is simplest to look at the code for the case when there is no
fill prefix first. This code consists of yet another inner
if expression, and reads as follows:
(if (re-search-forward paragraph-start nil t)
(goto-char (match-beginning 0))
(goto-char (point-max)))
This expression actually does the work that most people think of
as the primary purpose of the forward-paragraph
command: it causes a regular expression search to occur that
searches forward to the start of the next paragraph and if it is
found, moves point there; but if the start of another paragraph
if not found, it moves point to the end of the accessible region
of the buffer.
The only unfamiliar part of this is the use of
match-beginning. This is another function that is
new to us. The match-beginning function returns a
number specifying the location of the start of the text that was
matched by the last regular expression search.
The match-beginning function is used here because of
a characteristic of a forward search: a successful forward
search, regardless of whether it is a plain search or a regular
expression search, will move point to the end of the text that is
found. In this case, a successful search will move point to the
end of the pattern for paragraph-start, which will
be the beginning of the next paragraph rather than the end of the
current one.
However, we want to put point at the end of the current paragraph, not at the beginning of the next one. The two positions may be different, because there may be several blank lines between paragraphs.
When given an argument of 0,
match-beginning returns the position that is the
start of the text that the most recent regular expression
search matched. In this case, the most recent regular
expression search is the one looking for
paragraph-start, so match-beginning
returns the beginning position of the pattern, rather than the
end of the pattern. The beginning position is the end of the
paragraph.
(Incidentally, when passed a positive number as an argument, the
match-beginning function will place point at that
parenthesized expression in the last regular expression. It is a
useful function.)
The inner if expression just discussed is the
else-part of an enclosing if expression which tests
whether there is a fill prefix. If there is a fill prefix, the
then-part of this if is evaluated. It looks like
this:
(while (and (not (eobp))
(not (looking-at paragraph-separate))
(looking-at fill-prefix-regexp))
(forward-line 1))
What this expression does is move point forward line by line so long as three conditions are true:
The last condition may be puzzling, until you remember that point
was moved to the beginning of the line early in the
forward-paragraph function. This means that if the
text has a fill prefix, the looking-at function will
see it.
In summary, when moving forward, the
forward-paragraph function does the following:
For review, here is the code we have just been discussing, formatted for clarity:
(interactive "p")
(or arg (setq arg 1))
(let* (
(fill-prefix-regexp
(and fill-prefix (not (equal fill-prefix ""))
(not paragraph-ignore-fill-prefix)
(regexp-quote fill-prefix)))
(paragraph-separate
(if fill-prefix-regexp
(concat paragraph-separate
"\\|^"
fill-prefix-regexp
"[ \t]*$")
paragraph-separate)))
backward-moving-code (omitted) ...
(while (> arg 0) ; forward-moving-code
(beginning-of-line)
(while (prog1 (and (not (eobp))
(looking-at paragraph-separate))
(forward-line 1)))
(if fill-prefix-regexp
(while (and (not (eobp)) ; then-part
(not (looking-at paragraph-separate))
(looking-at fill-prefix-regexp))
(forward-line 1))
; else-part: the inner-if
(if (re-search-forward paragraph-start nil t)
(goto-char (match-beginning 0))
(goto-char (point-max))))
(setq arg (1- arg))))) ; decrementer
The full definition for the forward-paragraph
function not only includes this code for going forwards, but also
code for going backwards.
If you are reading this inside of GNU Emacs and you want to see
the whole function, you can type M-.
(find-tag) and the name of the function when
prompted for it. If the find-tag function first asks
you for the name of a `TAGS' table, give it the name of
the `TAGS' file in your `emacs/src' directory,
which will have a pathname such as
`/usr/local/emacs/src/TAGS'. (The exact path to the
`emacs/src' directory depends on how your copy of Emacs
was installed. If you don't know the path, you can sometimes find
out by typing C-h i to enter Info and then typing
C-x C-f to see the path to the `emacs/info'
directory. The path to the `TAGS' file is often the
corresponding `emacs/src' path; sometimes, however, Info
files are stored elsewhere.)
You can also create your own `TAGS' file for directories that lack one.
You can create your own `TAGS' file to help you jump to
sources. For example, if you have a large number of files in your
`~/emacs' directory, as I do--I have 137 `.el'
files in it, of which I load 17-- you will find it easier to jump
to specific functions if you create a `TAGS' file for
that directory than if you search for the function name with
grep or some other tool.
You can create a `TAGS' file by calling the
etags program that comes as a part of the Emacs
distribution. Usually, etags is compiled and
installed when Emacs is built. (etags is not an
Emacs Lisp function or a part of Emacs; it is a C program.)
To create a `TAGS' file, first switch to the directory
in which you want to create the file. In Emacs you can do this
with the M-x cd command, or by visiting a file in the
directory, or by listing the directory with C-x d
(dired). Then type
M-! etags *.el
to create a `TAGS' file. The etags program
takes all the usual shell `wildcards'. For example, if you have
two directories for which you want a single `TAGS file',
type the command like this, where `../elisp/' is the
second directory:
M-! etags *.el ../elisp/*.el
Type
M-! etags --help
to see a list of the options accepted by etags.
The etags program handles Emacs Lisp, Common Lisp,
Scheme, C, Fortran, Pascal, LaTeX, and most assemblers. The
program has no switches for specifying the language; it
recognizes the language in an input file according to its file
name and contents.
Also, `etags' is very helpful when you are writing code
yourself and want to refer back to functions you have already
written. Just run etags again at intervals as you
write new functions, so they become part of the `TAGS'
file.
Here is a brief summary of some recently introduced functions.
while
nil. (The expression is evaluated only for its
side effects.) For example:
(let ((foo 2))
(while (> foo 0)
(insert (format "foo is %d.\n" foo))
(setq foo (1- foo))))
=> foo is 2.
foo is 1.
nil
(The insert function inserts its arguments at point;
the format function returns a string formatted from its
arguments the way message formats its arguments;
\n produces a new line.)
re-search-forward
search-forward:
nil or an error message.
let*
(let* ((foo 7)
(bar (* 3 foo)))
(message "`bar' is %d." bar))
=> `bar' is 21.
match-beginning
looking-at
t for true if the text after point matches
the argument, which should be a regular expression.
eobp
t for true if point is at the end of the
accessible part of a buffer. The end of the accessible part is
the end of the buffer if the buffer is not narrowed; it is the
end of the narrowed part if the buffer is narrowed.
prog1
(prog1 1 2 3 4)
=> 1
re-search-forward
Write a function to search for a regular expression that matches two or more blank lines in sequence.
Write a function to search for duplicated words, such as `the
the'. See section `Syntax of Regular Expressions' in The
GNU Emacs Manual, for information on how to write a regexp
(a regular expression) to match a string that is composed of two
identical halves. You can devise several regexps; some are better
than others. The function I use is described in an appendix,
along with several regexps. See section A The the-the
Function.
Repetition and regular expression searches are powerful tools
that you often use when you write code in Emacs Lisp. This
chapter illustrates the use of regular expression searches
through the construction of word count commands using
while loops and recursion.
The standard Emacs distribution contains a function for counting the number of lines within a region. However, there is no corresponding function for counting words.
Certain types of writing ask you to count words. Thus, if you
write an essay, you may be limited to 800 words; if you write a
novel, you may discipline yourself to write 1000 words a day. It
seems odd to me that Emacs lacks a word count command. Perhaps
people use Emacs mostly for code or types of documentation that
do not require word counts; or perhaps they restrict themselves
to the operating system word count command, wc.
Alternatively, people may follow the publishers' convention and
compute a word count by dividing the number of characters in a
document by five. In any event, here are commands to count words.
count-words-region
Function
A word count command could count words in a line, paragraph,
region, or buffer. What should the command cover? You could
design the command to count the number of words in a complete
buffer. However, the Emacs tradition encourages flexibility--you
may want to count words in just a section, rather than all of a
buffer. So it makes more sense to design the command to count the
number of words in a region. Once you have a
count-words-region command, you can, if you wish,
count words in a whole buffer by marking it with C-x h
(mark-whole-buffer).
Clearly, counting words is a repetitive act: starting from the
beginning of the region, you count the first word, then the
second word, then the third word, and so on, until you reach the
end of the region. This means that word counting is ideally
suited to recursion or to a while loop.
First, we will implement the word count command with a
while loop, then with recursion. The command will,
of course, be interactive.
The template for an interactive function definition is, as always:
(defun name-of-function (argument-list) "documentation..." (interactive-expression...) body...)
What we need to do is fill in the slots.
The name of the function should be self-explanatory and similar
to the existing count-lines-region name. This makes
the name easier to remember. count-words-region is a
good choice.
The function counts words within a region. This means that the
argument list must contain symbols that are bound to the two
positions, the beginning and end of the region. These two
positions can be called `beginning' and
`end' respectively. The first line of the
documentation should be a single sentence, since that is all that
is printed as documentation by a command such as
apropos. The interactive expression will be of the
form `(interactive "r")', since that will cause
Emacs to pass the beginning and end of the region to the
function's argument list. All this is routine.
The body of the function needs to be written so as to do three
tasks: first to set up conditions under which the
while loop can count words, second to run the
while loop, and, third, to send a message to the
user.
When a user calls count-words-region, point may be
at the beginning or the end of the region. However, the counting
process must start at the beginning of the region. These means we
will want to put point there if it is not already there.
Executing (goto-char beginning) ensures this. Of
course, we will want to return point to its expected position
when the function finishes its work. For this reason, the body
must be enclosed in a save-excursion expression.
The central part of the body of the function consists of a
while loop in which one expression jumps point
forward word by word, and another expression counts those jumps.
The true-or-false-test of the while loop should test
true so long as point should jump forward, and false when point
is at the end of the region.
We could use (forward-word 1) as the expression for
moving point forward word by word, but it is easier to see what
Emacs identifies as a `word' if we use a regular expression
search.
A regular expression search that finds the pattern for which it is searching leaves point after the last character matched. This means that a succession of successful word searches will move point forward word by word.
As a practical matter, we want the regular expression search to jump over whitespace and punctuation between words as well as over the words themselves. A regexp that refuses to jump over interword whitespace would never jump more than one word! This means that the regexp should include the whitespace and punctuation that follows a word, if any, as well as the word itself. (A word may end a buffer and not have any following whitespace or punctuation, so that part of the regexp must be optional.)
Thus, what we want for the regexp is a pattern defining one or more word constituent characters followed, optionally, by one or more characters that are not word constituents. The regular expression for this is:
\w+\W*
The buffer's syntax table determines which characters are and are not word constituents. (See section 14.2 What Constitutes a Word or Symbol?, for more about syntax. Also, see section `The Syntax Table' in The GNU Emacs Manual, and, section `Syntax Tables' in The GNU Emacs Lisp Reference Manual.)
The search expression looks like this:
(re-search-forward "\\w+\\W*")
(Note that paired backslashes precede the `w' and `W'. A single backslash has special meaning to the Emacs Lisp interpreter. It indicates that the following character is interpreted differently than usual. For example, the two characters, `\n', stand for `newline', rather than for a backslash followed by `n'. Two backslashes in a row stand for an ordinary, `unspecial' backslash.)
We need a counter to count how many words there are; this
variable must first be set to 0 and then incremented each time
Emacs goes around the while loop. The incrementing
expression is simply:
(setq count (1+ count))
Finally, we want to tell the user how many words there are in the
region. The message function is intended for
presenting this kind of information to the user. The message has
to be phrased so that it reads properly regardless of how many
words there are in the region: we don't want to say that "there
are 1 words in the region". The conflict between singular and
plural is ungrammmatical. We can solve this problem by using a
conditional expression that evaluates different messages
depending on the number of words in the region. There are three
possibilities: no words in the region, one word in the region,
and more than one word. This means that the cond
special form is appropriate.
All this leads to the following function definition:
;;; First version; has bugs!
(defun count-words-region (beginning end)
"Print number of words in the region.
Words are defined as at least one word-constituent
character followed by at least one character that
is not a word-constituent. The buffer's syntax
table determines which characters these are."
(interactive "r")
(message "Counting words in region ... ")
;;; 1. Set up appropriate conditions.
(save-excursion
(goto-char beginning)
(let ((count 0))
;;; 2. Run the while loop.
(while (< (point) end)
(re-search-forward "\\w+\\W*")
(setq count (1+ count)))
;;; 3. Send a message to the user.
(cond ((zerop count)
(message
"The region does NOT have any words."))
((= 1 count)
(message
"The region has 1 word."))
(t
(message
"The region has %d words." count))))))
As written, the function works, but not in all circumstances.
count-words-region
The count-words-region command described in the
preceding section has two bugs, or rather, one bug with two
manifestations. First, if you mark a region containing only
whitespace in the middle of some text, the
count-words-region command tells you that the region
contains one word! Second, if you mark a region containing only
whitespace at the end of the buffer or the accessible portion of
a narrowed buffer, the command displays an error message that
looks like this:
Search failed: "\\w+\\W*"
If you are reading this in Info in GNU Emacs, you can test for these bugs yourself.
First, evaluate the function in the usual manner to install it.
If you wish, you can also install this keybinding by evaluating it, too:
(global-set-key "\C-c=" 'count-words-region)
To conduct the first test, set mark and point to the beginning and end of the following line and then type C-c = (or M-x count-words-region if you have not bound C-c =):
one two three
Emacs will tell you, correctly, that the region has three words.
Repeat the test, but place mark at the beginning of the line and place point just before the word `one'. Again type the command C-c = (or M-x count-words-region). Emacs should tell you that the region has no words, since it is composed only of the whitespace at the beginning of the line. But instead Emacs tells you that the region has one word!
For the third test, copy the sample line to the end of the `*scratch*' buffer and then type several spaces at the end of the line. Place mark right after the word `three' and point at the end of line. (The end of the line will be the end of the buffer.) Type C-c = (or M-x count-words-region) as you did before. Again, Emacs should tell you that the region has no words, since it is composed only of the whitespace at the end of the line. Instead, Emacs displays an error message saying `Search failed'.
The two bugs stem from the same problem.
Consider the first manifestation of the bug, in which the command
tells you that the whitespace at the beginning of the line
contains one word. What happens is this: The M-x
count-words-region command moves point to the beginning of
the region. The while tests whether the value of
point is smaller than the value of end, which it is.
Consequently, the regular expression search looks for and finds
the first word. It leaves point after the word.
count is set to one. The while loop
repeats; but this time the value of point is larger than the
value of end, the loop is exited; and the function
displays a message saying the number of words in the region is
one. In brief, the regular expression search looks for and finds
the a word even though it is outside the marked region.
In the second manifestation of the bug, the region is whitespace
at the end of the buffer. Emacs says `Search
failed'. What happens is that the true-or-false-test in
the while loop tests true, so the search expression
is executed. But since there are no more words in the buffer, the
search fails.
In both manifestations of the bug, the search extends or attempts to extend outside of the region.
The solution is to limit the search to the region--this is a fairly simple action, but as you may have come to expect, it is not quite as simple as you might think.
As we have seen, the re-search-forward function
takes a search pattern as its first argument. But in addition to
this first, mandatory argument, it accepts three optional
arguments. The optional second argument bounds the search. The
optional third argument, if t, causes the function
to return nil rather than signal an error if the
search fails. The optional fourth argument is a repeat count. (In
Emacs, you can get a function's documentation by typing C-h
f, the name of the function, and then RET.)
In the count-words-region definition, the value of
the end of the region is held by the variable end
which is passed as an argument to the function. Thus, we can add
end as an argument to the regular expression search
expression:
(re-search-forward "\\w+\\W*" end)
However, if you make only this change to the
count-words-region definition and then test the new
version of the definition on a stretch of whitespace, you will
receive an error message saying `Search failed'.
What happens is this: the search is limited to the region, and fails as you expect because there are no word-constituent characters in the region. Since it fails, we receive an error message. But we do not want to receive an error message in this case; we want to receive the message that "The region does NOT have any words."
The solution to this problem is to provide
re-search-forward with a third argument of
t, which causes the function to return
nil rather than signal an error if the search fails.
However, if you make this change and try it, you will see the
message "Counting words in region ... " and ... you will keep on
seeing that message ..., until you type C-g
(keyboard-quit).
Here is what happens: the search is limited to the region, as
before, and it fails because there are no word-constituent
characters in the region, as expected. Consequently, the
re-search-forward expression returns
nil. It does nothing else. In particular, it does
not move point, which it does as a side effect if it finds the
search target. After the re-search-forward
expression returns nil, the next expression in the
while loop is evaluated. This expression increments
the count. Then the loop repeats. The true-or-false-test tests
true because the value of point is still less than the value of
end, since the re-search-forward expression did not
move point. ... and the cycle repeats ...
The count-words-region definition requires yet
another modification, to cause the true-or-false-test of the
while loop to test false if the search fails. Put
another way, there are two conditions that must be satisfied in
the true-or-false-test before the word count variable is
incremented: point must still be within the region and the search
expression must have found a word to count.
Since both the first condition and the second condition must be
true together, the two expressions, the region test and the
search expression, can be joined with an and
function and embedded in the while loop as the
true-or-false-test, like this:
(and (< (point) end) (re-search-forward "\\w+\\W*" end t))
(For information about and, see section 12.4
forward-paragraph: a Goldmine of Functions.)
The re-search-forward expression returns
t if the search succeeds and as a side effect moves
point. Consequently, as words are found, point is moved through
the region. When the search expression fails to find another
word, or when point reaches the end of the region, the
true-or-false-test tests false, the while loop
exists, and the count-words-region function displays
one or other of its messages.
After incorporating these final changes, the
count-words-region works without bugs (or at least,
without bugs that I have found!). Here is what it looks like:
;;; Final version: while
(defun count-words-region (beginning end)
"Print number of words in the region."
(interactive "r")
(message "Counting words in region ... ")
;;; 1. Set up appropriate conditions.
(save-excursion
(let ((count 0))
(goto-char beginning)
;;; 2. Run the while loop.
(while (and (< (point) end)
(re-search-forward "\\w+\\W*" end t))
(setq count (1+ count)))
;;; 3. Send a message to the user.
(cond ((zerop count)
(message
"The region does NOT have any words."))
((= 1 count)
(message
"The region has 1 word."))
(t
(message
"The region has %d words." count))))))
You can write the function for counting words recursively as well
as with a while loop. Let's see how this is done.
First, we need to recognize that the
count-words-region function has three jobs: it sets
up the appropriate conditions for counting to occur; it counts
the words in the region; and it sends a message to the user
telling how many words there are.
If we write a single recursive function to do everything, we will receive a message for every recursive call. If the region contains 13 words, we will receive thirteen messages, one right after the other. We don't want this! Instead, we must write two functions to do the job, one of which (the recursive function) will be used inside of the other. One function will set up the conditions and display the message; the other will return the word count.
Let us start with the function that causes the message to be
displayed. We can continue to call this
count-words-region.
This is the function that the user will call. It will be
interactive. Indeed, it will be similar to our previous versions
of this function, except that it will call
recursive-count-words to determine how many words
are in the region.
We can readily construct a template for this function, based on our previous versions:
;; Recursive version; uses regular expression search
(defun count-words-region (beginning end)
"documentation..."
(interactive-expression...)
;;; 1. Set up appropriate conditions.
(explanatory message)
(set-up functions...
;;; 2. Count the words.
recursive call
;;; 3. Send a message to the user.
message providing word count))
The definition looks straightforward, except that somehow, the
count returned by the recursive call must be passed to the
message displaying the word count. A little thought suggests that
this can be done by making use of a let expression:
we can bind a variable in the varlist of a let
expression to the number of words in the region, as returned by
the recursive call; and then the cond expression,
using binding, can display the value to the user.
Often, one thinks of the binding within a let
expression as somehow secondary to the `primary' work of a
function. But in this case, what you might consider the `primary'
job of the function, counting words, is done within the
let expression.
Using let, the function definition looks like this:
(defun count-words-region (beginning end)
"Print number of words in the region."
(interactive "r")
;;; 1. Set up appropriate conditions.
(message "Counting words in region ... ")
(save-excursion
(goto-char beginning)
;;; 2. Count the words.
(let ((count (recursive-count-words end)))
;;; 3. Send a message to the user.
(cond ((zerop count)
(message
"The region does NOT have any words."))
((= 1 count)
(message
"The region has 1 word."))
(t
(message
"The region has %d words." count))))))
Next, we need to write the recursive counting function.
A recursive function has at least three parts: the `do-again-test', the `next-step-expression', and the recursive call.
The do-again-test determines whether the function will or will
not be called again. Since we are counting words in a region and
can use a function that moves point forward for every word, the
do-again-test can check whether point is still within the region.
The do-again-test should find the value of point and determine
whether point is before, at, or after the value of the end of the
region. We can use the point function to locate
point. Clearly, we must pass the value of the end of the region
to the recursive counting function as an argument.
In addition, the do-again-test should also test whether the search finds a word. If it does not, the function should not call itself again.
The next-step-expression changes a value so that when the recursive function is supposed to stop calling itself, it stops. More precisely, the next-step-expression changes a value so that at the right time, the do-again-test stops the recursive function from calling itself again. In this case, the next-step-expression can be the expression that moves point forward word by word.
The third part of a recursive function is the recursive call.
Somewhere, also, we also need a part that does the `work' of the function, a part that does the counting. A vital part!
But already, we have an outline of the recursive counting function:
(defun recursive-count-words (region-end) "documentation..." do-again-test next-step-expression recursive call)
Now we need to fill in the slots. Let's start with the simplest cases first: if point is at or beyond the end of the region, there cannot be any words in the region, so the function should return zero. Likewise, if the search fails, there are no words to count, so the function should return zero.
On the other hand, if point is within the region and the search succeeds, the function should call itself again.
Thus, the do-again-test should look like this:
(and (< (point) region-end)
(re-search-forward "\\w+\\W*" region-end t))
Note that the search expression is part of the do-again-test--the
function returns t if its search succeeds and
nil if it fails. (See section 13.1.1 The Whitespace Bug in
count-words-region, for an explanation of how
re-search-forward works.)
The do-again-test is the true-or-false test of an if
clause. Clearly, if the do-again-test succeeds, the then-part of
the if clause should call the function again; but if
it fails, the else-part should return zero since either point is
outside the region or the search failed because there were not
words to find.
But before considering the recursive call, we need to consider the next-step-expression. What is it? Interestingly, it is the search part of the do-again-test.
In addition to returning t or nil for
the do-again-test, re-search-forward moves point
forward as a side effect of a successful search. This is the
action that changes the value of point so that the recursive
function stops calling itself when point completes its movement
through the region. Consequently, the
re-search-forward expression is the
next-step-expression.
In outline, then, the body of the
recursive-count-words function looks like this:
(if do-again-test-and-next-step-combined
;; then
recursive-call-returning-count
;; else
return-zero)
How to incorporate the mechanism that counts?
If you are not used to writing recursive functions, a question like this can be troublesome. But it can and should be approached systematically.
We know that the counting mechanism should be associated in some
way with the recursive call. Indeed, since the
next-step-expression moves point forward by one word, and since a
recursive call is made for each word, the counting mechanism must
be an expression that adds one to the value returned by a call to
recursive-count-words.
Consider several cases:
From the sketch we can see that the else-part of the
if returns zero for the case of no words. This means
that the then-part of the if must return a value
resulting from adding one to the value returned from a count of
the remaining words.
The expression will look like this, where 1+ is a
function that adds one to its argument.
(1+ (recursive-count-words region-end))
The whole recursive-count-words function will then
look like this:
(defun recursive-count-words (region-end)
"documentation..."
;;; 1. do-again-test
(if (and (< (point) region-end)
(re-search-forward "\\w+\\W*" region-end t))
;;; 2. then-part: the recursive call
(1+ (recursive-count-words region-end))
;;; 3. else-part
0))
Let's examine how this works:
If there are no words in the region, the else part of the
if expression is evaluated and consequently the
function returns zero.
If there is one word in the region, the value of point is less
than the value of region-end and the search
succeeds. In this case, the true-or-false-test of the
if expression tests true, and the then-part of the
if expression is evaluated. The counting expression
is evaluated. This expression returns a value (which will be the
value returned by the whole function) that is the sum of one
added to the value returned by a recursive call.
Meanwhile, the next-step-expression has caused point to jump over
the first (and in this case only) word in the region. This means
that when (recursive-count-words region-end) is
evaluated a second time, as a result of the recursive call, the
value of point will be equal to or greater than the value of
region end. So this time, recursive-count-words will
return zero. The zero will be added to one, and the original
evaluation of recursive-count-words will return one
plus zero, which is one, which is the correct amount.
Clearly, if there are two words in the region, the first call to
recursive-count-words returns one added to the value
returned by calling recursive-count-words on a
region containing the remaining word--that is, it adds one to
one, producing two, which is the correct amount.
Similarly, if there are three words in the region, the first call
to recursive-count-words returns one added to the
value returned by calling recursive-count-words on a
region containing the remaining two words--and so on and so on.
With full documentation the two functions look like this:
The recursive function:
(defun recursive-count-words (region-end)
"Number of words between point and REGION-END."
;;; 1. do-again-test
(if (and (< (point) region-end)
(re-search-forward "\\w+\\W*" region-end t))
;;; 2. then-part: the recursive call
(1+ (recursive-count-words region-end))
;;; 3. else-part
0))
The wrapper:
;;; Recursive version
(defun count-words-region (beginning end)
"Print number of words in the region.
Words are defined as at least one word-constituent
character followed by at least one character that is
not a word-constituent. The buffer's syntax table
determines which characters these are."
(interactive "r")
(message "Counting words in region ... ")
(save-excursion
(goto-char beginning)
(let ((count (recursive-count-words end)))
(cond ((zerop count)
(message
"The region does NOT have any words."))
((= 1 count)
(message "The region has 1 word."))
(t
(message
"The region has %d words." count))))))
Using a while loop, write a function to count the
number of punctuation marks in a region--period, comma,
semi-colon, colon, exclamation mark, question mark. Do the same
using recursion.
defun
Our next project is to count the number of words in a function
definition. Clearly, this can be done using some variant of
count-word-region. See section 13 Counting: Repetition and
Regexps. If we are just going to count the words in one
definition, it is easy enough to mark the definition with the
C-M-h (mark-defun) command, and then call
count-word-region.
However, I am more ambitious: I want to count the words and symbols in every definition in the Emacs sources and then print a graph that shows how many functions there are of each length: how many contain 40 to 49 words or symbols, how many contain 50 to 59 words or symbols, and so on. I have often been curious how long a typical function is, and this will tell.
Described in one phrase, the histogram project is daunting; but divided into numerous small steps, each of which we can take one at a time, the project becomes less fearsome. Let us consider what the steps must be:
count-words-in-defun function.
This is quite a project! But if we take each step slowly, it will not be difficult.
When we first start thinking about how to count the words in a
function definition, the first question is (or ought to be) what
are we going to count? When we speak of `words' with respect to a
Lisp function definition, we are actually speaking, in large
part, of `symbols'. For example, the following
multiply-by-seven function contains the five symbols
defun, multiply-by-seven,
number, *, and 7. In
addition, in the documentation string, it contains the four words
`Multiply', `NUMBER',
`by', and `seven'. The symbol
`number' is repeated, so the definition contains a
total of ten words and symbols.
(defun multiply-by-seven (number) "Multiply NUMBER by seven." (* 7 number))
However, if we mark the multiply-by-seven definition
with C-M-h (mark-defun), and then call
count-words-region on it, we will find that
count-words-region claims the definition has eleven
words, not ten! Something is wrong!
The problem is twofold: count-words-region does not
count the `*' as a word, and it counts the single
symbol, multiply-by-seven, as containing three
words. The hyphens are treated as if they were interword spaces
rather than intraword connectors:
`multiply-by-seven' is counted as if it were written
`multiply by seven'.
The cause of this confusion is the regular expression search
within the count-words-region definition that moves
point forward word by word. In the canonical version of
count-words-region, the regexp is:
"\\w+\\W*"
This regular expression is a pattern defining one or more word constituent characters possibly followed by one or more characters that are not word constituents. What is meant by `word constituent characters' brings us to the issue of syntax, which is worth a section of its own.
Emacs treats different characters as belonging to different syntax categories. For example, the regular expression, `\\w+', is a pattern specifying one or more word constituent characters. Word constituent characters are members of one syntax category. Other syntax categories include the class of punctuation characters, such as the period and the comma, and the class of whitespace characters, such as the blank space and the tab character. (For more information, see section `The Syntax Table' in The GNU Emacs Manual, and, section `Syntax Tables' in The GNU Emacs Lisp Reference Manual.)
Syntax tables specify which characters belong to which
categories. Usually, a hyphen is not specified as a `word
constituent character'. Instead, it is specified as being in the
`class of characters that are part of symbol names but not
words.' This means that the count-words-region
function treats it in the same way it treats an interword white
space, which is why count-words-region counts
`multiply-by-seven' as three words.
There are two ways to cause Emacs to count `multiply-by-seven' as one symbol: modify the syntax table or modify the regular expression.
We could redefine a hyphen as a word constituent character by modifying the syntax table that Emacs keeps for each mode. This action would serve our purpose, except that a hyphen is merely the most common character within symbols that is not typically a word constituent character; there are others, too.
Alternatively, we can redefine the regular expression used in the
count-words definition so as to include symbols.
This procedure has the merit of clarity, but the task is a little
tricky.
The first part is simple enough: the pattern must match "at least one character that is a word or symbol constituent". Thus:
\\(\\w\\|\\s_\\)+
The `\\(' is the first part of the grouping construct that includes the `\\w' and the `\\s_' as alternatives, separated by the `\\|'. The `\\w' matches any word-constituent character and the `\\s_' matches any character that is part of a symbol name but not a word-constituent character. The `+' following the group indicates that the word or symbol constituent characters must be matched at least once.
However, the second part of the regexp is more difficult to design. What we want is to follow the first part with "optionally one or more characters that are not constituents of a word or symbol". At first, I thought I could define this with the following:
\\(\\W\\|\\S_\\)*"
The upper case `W' and `S' match characters that are not word or symbol constituents. Unfortunately, this expression matches any character that is either not a word constituent or not a symbol constituent. This matches any character!
I then noticed that every word or symbol in my test region was followed by white space (blank space, tab, or newline). So I tried placing a pattern to match one or more blank spaces after the pattern for one or more word or symbol constituents. This failed, too. Words and symbols are often separated by whitespace, but in actual code parentheses may follow symbols and punctuation may follow words. So finally, I designed a pattern in which the word or symbol constituents are followed optionally by characters that are not white space and then followed optionally by white space.
Here is the full regular expression:
"\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*"
count-words-in-defun
Function
We have seen that there are several ways to write a
count-word-region function. To write a
count-words-in-defun, we need merely adapt one of
these versions.
The version that uses a while loop is easy to
understand, so I am going to adapt that. Because
count-words-in-defun will be part of a more complex
program, it need not be interactive and it need not display a
message but just return the count. These considerations simplify
the definition a little.
On the other hand, count-words-in-defun will be used
within a buffer that contains function definitions. Consequently,
it is reasonable to ask that the function determine whether it is
called when point is within a function definition, and if it is,
to return the count for that definition. This adds complexity to
the definition, but saves us from needing to pass arguments to
the function.
These considerations lead us to prepare the following template:
(defun count-words-in-defun ()
"documentation..."
(set up...
(while loop...)
return count)
As usual, our job is to fill in the slots.
First, the set up.
We are presuming that this function will be called within a
buffer containing function definitions. Point will either be
within a function definition or not. For
count-words-in-defun to work, point must move to the
beginning of the definition, a counter must start at zero, and
the counting loop must stop when point reaches the end of the
definition.
The beginning-of-defun function searches backwards
for an opening delimiter such as a `(' at the
beginning of a line, and moves point to that position, or else to
the limit of the search. In practice, this means that
beginning-of-defun moves point to the beginning of
an enclosing or preceding function definition, or else to the
beginning of the buffer. We can use
beginning-of-defun to place point where we wish to
start.
The while loop requires a counter to keep track of
the words or symbols being counted. A let expression
can be used to create a local variable for this purpose, and bind
it to an initial value of zero.
The end-of-defun function works like
beginning-of-defun except that it moves point to the
end of the definition. end-of-defun can be used as
part of an expression that determines the position of the end of
the definition.
The set up for count-words-in-defun takes shape
rapidly: first we move point to the beginning of the definition,
then we create a local variable to hold the count, and, finally,
we record the position of the end of the definition so the
while loop will know when to stop looping.
The code looks like this:
(beginning-of-defun)
(let ((count 0)
(end (save-excursion (end-of-defun) (point))))
The code is simple. The only slight complication is likely to
concern end: it is bound to the position of the end
of the definition by a save-excursion expression
that returns the value of point after end-of-defun
temporarily moves it to the end of the definition.
The second part of the count-words-in-defun, after
the set up, is the while loop.
The loop must contain an expression that jumps point forward word
by word and symbol by symbol, and another expression that counts
the jumps. The true-or-false-test for the while loop
should test true so long as point should jump forward, and false
when point is at the end of the definition. We have already
redefined the regular expression for this (see section 14.2 What Constitutes a Word or
Symbol?), so the loop is straightforward:
(while (and (< (point) end)
(re-search-forward
"\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*" end t)
(setq count (1+ count)))
The third part of the function definition returns the count of
words and symbols. This part is the last expression within the
body of the let expression, and can be, very simply,
the local variable count, which when evaluated
returns the count.
Put together, the count-words-in-defun definition
looks like this:
(defun count-words-in-defun ()
"Return the number of words and symbols in a defun."
(beginning-of-defun)
(let ((count 0)
(end (save-excursion (end-of-defun) (point))))
(while
(and (< (point) end)
(re-search-forward
"\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*"
end t))
(setq count (1+ count)))
count))
How to test this? The function is not interactive, but it is easy
to put a wrapper around the function to make it interactive; we
can use almost the same code as for the recursive version of
count-words-region:
;;; Interactive version.
(defun count-words-defun ()
"Number of words and symbols in a function definition."
(interactive)
(message
"Counting words and symbols in function definition ... ")
(let ((count (count-words-in-defun)))
(cond
((zerop count)
(message
"The definition does NOT have any words or symbols."))
((= 1 count)
(message
"The definition has 1 word or symbol."))
(t
(message
"The definition has %d words or symbols." count)))))
Let's re-use C-c = as a convenient keybinding:
(global-set-key "\C-c=" 'count-words-defun)
Now we can try out count-words-defun: install both
count-words-in-defun and
count-words-defun, and set the keybinding, and then
place the cursor within the following definition:
(defun multiply-by-seven (number)
"Multiply NUMBER by seven."
(* 7 number))
=> 10
Success! The definition has 10 words and symbols.
The next problem is to count the numbers of words and symbols in several definitions within a single file.
defuns Within a
File
A file such as `simple.el' may have 80 or more function definitions within it. Our long term goal is to collect statistics on many files, but as a first step, our immediate goal is to collect statistics on one file.
The information will be a series of numbers, each number being the length of a function definition. We can store the numbers in a list.
We know that we will want to incorporate the information regarding one file with information about many other files; this means that the function for counting definition lengths within one file need only return the lengths' list. It need not and should not display any messages.
The word count commands contain one expression to jump point forward word by word and another expression to count the jumps. The definitions' lengths' function can be designed to work the same way, with one expression to jump point forward definition by definition and another expression to construct the lengths' list.
This statement of the problem makes it elementary to write the
function definition. Clearly, we will start the count at the
beginning of the file, so the first command will be
(goto-char (point-min)). Next, we start the
while loop; and the true-or-false test of the loop
can be a regular expression search for the next function
definition--so long as the search succeeds, point is moved
forward and then the body of the loop is evaluated. The body
needs an expression that constructs the lengths' list.
cons, the list construction command, can be used to
create the list. That is almost all there is to it.
Here is what this fragment of code looks like:
(goto-char (point-min))
(while (re-search-forward "^(defun" nil t)
(setq lengths-list
(cons (count-words-in-defun) lengths-list)))
What we have left out is the mechanism for finding the file that contains the function definitions.
In previous examples, we either used this, the Info file, or we switched back and forth to some other buffer, such as the `*scratch*' buffer.
Finding a file is a new process that we have not yet discussed.
To find a file in Emacs, you use the C-x C-f
(find-file) command. This command is almost, but not
quite right for the lengths problem.
Let's look at the source for find-file (you can use
the find-tag command to find the source of a
function):
(defun find-file (filename) "Edit file FILENAME. Switch to a buffer visiting file FILENAME, creating one if none already exists." (interactive "FFind file: ") (switch-to-buffer (find-file-noselect filename)))
The definition possesses short but complete documentation and an
interactive specification that prompts you for a file name when
you use the command interactively. The body of the definition
contains two functions, find-file-noselect and
switch-to-buffer.
According to its documentation as shown by C-h f (the
describe-function command), the
find-file-noselect function reads the named file
into a buffer and returns the buffer. However, the buffer is not
selected. Emacs does not switch its attention (or yours if you
are using find-file-noselect) to the named buffer.
That is what switch-to-buffer does: it switches the
buffer to which Emacs attention is directed; and it switches the
buffer displayed in the window to the new buffer. We have
discussed buffer switching elsewhere. (See section 2.3 Switching Buffers.)
In this histogram project, we do not need to display each file on
the screen as the program determines the length of each
definition within it. Instead of employing
switch-to-buffer, we can work with
set-buffer, which redirects the attention of the
computer program to a different buffer but does not redisplay it
on the screen. So instead of calling on find-file to
do the job, we must write our own expression.
The task is easy: use find-file-noselect and
set-buffer.
lengths-list-file in Detail
The core of the lengths-list-file function is a
while loop containing a function to move point
forward `defun by defun' and a function to count the number of
words and symbols in each defun. This core must be surrounded by
functions that do various other tasks, including finding the
file, and ensuring that point starts out at the beginning of the
file. The function definition looks like this:
(defun lengths-list-file (filename)
"Return list of definitions' lengths within FILE.
The returned list is a list of numbers.
Each number is the number of words or
symbols in one function definition."
(message "Working on `%s' ... " filename)
(save-excursion
(let ((buffer (find-file-noselect filename))
(lengths-list))
(set-buffer buffer)
(setq buffer-read-only t)
(widen)
(goto-char (point-min))
(while (re-search-forward "^(defun" nil t)
(setq lengths-list
(cons (count-words-in-defun) lengths-list)))
(kill-buffer buffer)
lengths-list)))
The function is passed one argument, the name of the file on which it will work. It has four lines of documentation, but no interactive specification. Since people worry that a computer is broken if they don't see anything going on, the first line of the body is a message.
The next line contains a save-excursion that returns
Emacs attention to the current buffer when the function
completes. This is useful in case you embed this function in
another function that presumes point is restored to the original
buffer.
In the varlist of the let expression, Emacs finds
the file and binds the local variable buffer to the
buffer containing the file. At the same time, Emacs creates
lengths-list as a local variable.
Next, Emacs switches its attention to the buffer.
In the following line, Emacs makes the buffer read-only. Ideally, this line is not necessary. None of the functions for counting words and symbols in a function definition should change the buffer. Besides, the buffer is not going to be saved, even if it were changed. This line is entirely the consequence of great, perhaps excessive, caution. The reason for the caution is that this function and those it calls work on the sources for Emacs and it is very inconvenient if they are inadvertently modified. It goes without saying that I did not realize a need for this line until an experiment went awry and started to modify my Emacs source files ...
Next comes a call to widen the buffer if it is narrowed. This function is usually not needed--Emacs creates a fresh buffer if none already exists; but if a buffer visiting the file already exists Emacs returns that one. In this case, the buffer may be narrowed and must be widened. If we wanted to be fully `user-friendly', we would arrange to save the restriction and the location of point, but we won't.
The (goto-char (point-min)) expression moves point
to the beginning of the buffer.
Then comes a while loop in which the `work' of the
function is carried out. In the loop, Emacs determines the length
of each definition and constructs a lengths' list containing the
information.
Emacs kills the buffer after working through it. This is to save
space inside of Emacs. My version of Emacs 19 contains over 300
source files of interest. Another function will apply
lengths-list-file to each of them. If Emacs visits
all of them and deletes none, my computer may run out of virtual
memory.
Finally, the last expression within the let
expression is the lengths-list variable; its value
is returned as the value of the whole function.
You can try this function by installing it in the usual fashion.
Then place your cursor after the following expression and type
C-x C-e (eval-last-sexp).
(lengths-list-file "../lisp/debug.el")
(You may need to change the pathname of the file; the one here
works if this Info file and the Emacs sources are in customary
places, such as /usr/local/emacs/info and
/usr/local/emacs/lisp. To change the expression,
copy it to the `*scratch*' buffer and edit it. Then
evaluate it.)
On my version of Emacs, the lengths' list for `debug.el' takes seven seconds to produce and looks like this:
(75 41 80 62 20 45 44 68 45 12 34 235)
Note that the length of the last definition in the file is first in the list.
defuns in
Different Files
In the previous section, we created a function that returns a list of the lengths of each definition in a file. Now, we want to define a function to return a master list of the lengths of the definitions in a list of files.
Working on each of a list of files is a repetitious act, so we
can use either a while loop or recursion.
The design using a while loop is routine. The
argument passed the function is a list of files. As we saw
earlier (see section 11.1.1 A while Loop
and a List), you can write a while loop so that
the body of the loop is evaluated if such a list contains
elements, but to exit the loop if the list is empty. For this
design to work, the body of the loop must contain an expression
that shortens the list each time the body is evaluated, so that
eventually the list is empty. The usual technique is to set the
value of the list to the value of the CDR of the list each time
the body is evaluated.
The template looks like this:
(while test-whether-list-is-empty body... set-list-to-cdr-of-list)
Also, we remember that a while loop returns
nil (the result of evaluating the
true-or-false-test), not the result of any evaluation within its
body. (The evaluations within the body of the loop are done for
their side effects.) However, the expression that sets the
lengths' list is part of the body--and that is the value that we
want returned by the function as a whole. To do this, we enclose
the while loop within a let expression,
and arrange that the last element of the let
expression contains the value of the lengths' list. (See section
Example with incrementing
counter.)
These considerations lead us directly to the function itself:
;;; Use while loop.
(defun lengths-list-many-files (list-of-files)
"Return list of lengths of defuns in LIST-OF-FILES."
(let (lengths-list)
;;; true-or-false-test
(while list-of-files
(setq lengths-list
(append
lengths-list
;;; Generate a lengths' list.
(lengths-list-file
(expand-file-name (car list-of-files)))))
;;; Make files' list shorter.
(setq list-of-files (cdr list-of-files)))
;;; Return final value of lengths' list.
lengths-list))
expand-file-name is a built-in function that
converts a file name to its absolute, long, path name form. Thus,
debug.el
becomes
/usr/local/emacs/lisp/debug.el
The only other new element of this function definition is the as
yet unstudied function append, which merits a short
section for itself.
append Function
The append function attaches one list to another.
Thus,
(append '(1 2 3 4) '(5 6 7 8))
produces the list
(1 2 3 4 5 6 7 8)
This is exactly how we want to attach two lengths' lists produced
by lengths-list-file to each other. The results
contrast with cons,
(cons '(1 2 3 4) '(5 6 7 8))
which constructs a new list in which the first argument to
cons becomes the first element of the new list:
((1 2 3 4) 5 6 7 8)
Besides a while loop, you can work on each of a list
of files with recursion. A recursive version of
lengths-list-many-files is short and simple.
The recursive function has the usual parts: the `do-again-test',
the `next-step-expression', and the recursive call. The
`do-again-test' determines whether the function should call
itself again, which it will do if the list-of-files
contains any remaining elements; the `next-step-expression'
resets the list-of-files to the CDR of itself, so
eventually the list will be empty; and the recursive call calls
itself on the shorter list. The complete function is shorter than
this description!
(defun recursive-lengths-list-many-files (list-of-files)
"Return list of lengths of each defun in LIST-OF-FILES."
(if list-of-files ; do-again-test
(append
(lengths-list-file
(expand-file-name (car list-of-files)))
(recursive-lengths-list-many-files
(cdr list-of-files)))))
In a sentence, the function returns the lengths' list for the
first of the list-of-files appended to the result of
calling itself on the rest of the list-of-files.
Here is a test of recursive-lengths-list-many-files,
along with the results of running lengths-list-file
on each of the files individually.
Install recursive-lengths-list-many-files and
lengths-list-file, if necessary, and then evaluate
the following expressions. You may need to change the files'
pathnames; those here work when this Info file and the Emacs
sources are located in their customary places. To change the
expressions, copy them to the `*scratch*' buffer, edit
them, and then evaluate them.
The results are shown after the `=>'. (These results are for files from Emacs Version 18.57; files from other versions of Emacs may produce different results.)
(lengths-list-file
"../lisp/macros.el")
=> (176 154 86)
(lengths-list-file
"../lisp/mailalias.el")
=> (116 122 265)
(lengths-list-file
"../lisp/makesum.el")
=> (85 179)
(recursive-lengths-list-many-files
'("../lisp/macros.el"
"../lisp/mailalias.el"
"../lisp/makesum.el"))
=> (176 154 86 116 122 265 85 179)
The recursive-lengths-list-many-files function
produces the output we want.
The next step is to prepare the data in the list for display in a graph.
The recursive-lengths-list-many-files function
returns a list of numbers. Each number records the length of a
function definition. What we need to do now is transform this
data into a list of numbers suitable for generating a graph. The
new list will tell how many functions definitions contain less
than 10 words and symbols, how many contain between 10 and 19
words and symbols, how many contain between 20 and 29 words and
symbols, and so on.
In brief, we need to go through the lengths' list produced by the
recursive-lengths-list-many-files function and count
the number of defuns within each range of lengths, and produce a
list of those numbers.
Based on what we have done before, we can readily foresee that it should not be too hard to write a function that `CDRs' down the lengths' list, looks at each element, determines which length range it is in, and increments a counter for that range.
However, before beginning to write such a function, we should consider the advantages of sorting the lengths' list first, so the numbers are ordered from smallest to largest. First, sorting will make it easier to count the numbers in each range, since two adjacent numbers will either be in the same length range or in adjacent ranges. Second, by inspecting a sorted list, we can discover the highest and lowest number, and thereby determine the largest and smallest length range that we will need.
Emacs contains a function to sort lists, called (as you might
guess) sort. The sort function takes
two arguments, the list to be sorted, and a predicate that
determines whether the first of two list elements is "less" than
the second.
As we saw earlier (see section 1.8.4 Using the Wrong Type Object
as an Argument), a predicate is a function that determines
whether some property is true or false. The sort
function will reorder a list according to whatever property the
predicate uses; this means that sort can be used to
sort non-numeric lists by non-numeric criteria--it can, for
example, alphabetize a list.
The < function is used when sorting a numeric
list. For example,
(sort '(4 8 21 17 33 7 21 7) '<)
produces this:
(4 7 7 8 17 21 21 33)
(Note that in this example, both the arguments are quoted so that
the symbols are not evaluated before being passed to
sort as arguments.)
Sorting the list returned by the
recursive-lengths-list-many-files function is
straightforward:
(sort
(recursive-lengths-list-many-files
'("../lisp/macros.el"
"../lisp/mailalias.el"
"../lisp/makesum.el"))
'<)
which produces:
(85 86 116 122 154 176 179 265)
(Note that in this example, the first argument to
sort is not quoted, since the expression must be
evaluated so as to produce the list that is passed to
sort.)
The recursive-lengths-list-many-files function
requires a list of files as its argument. For our test examples,
we constructed such a list by hand; but the Emacs Lisp source
directory is too large for us to do for that. Instead, we need to
use the directory-files function to construct a list
for us.
The directory-files function takes three arguments:
the first argument is the name of a directory, a string; a
non-nil second argument causes the function to
return the files' absolute pathnames; and the third argument is a
selector. If it contains a regular expression (rather than
nil), only pathnames that match that regular
expression are returned.
Thus, on my system,
(length (directory-files "../lisp" t "\\.el$"))
tells me that my version 19.25 Lisp sources directory contains 307 `.el' files.
An expression to sort the list returned by
recursive-lengths-list-many-files looks like this:
(sort (recursive-lengths-list-many-files (directory-files "../lisp" t "\\.el$")) '<)
Our immediate goal is to generate a list that tells us how many
function definitions contain fewer than 10 words and symbols, how
many contain between 10 and 19 words and symbols, how many
contain between 20 and 29 words and symbols, and so on. With a
sorted list of numbers, this is easy: count how many elements of
the list are smaller than 10, then, after moving past the numbers
just counted, count how many are smaller than 20, then, after
moving past the numbers just counted, count how many are smaller
than 30, and so on. Each of the numbers, 10, 20, 30, 40, and the
like, is one larger than the top of that range. We can call the
list of such numbers the top-of-ranges list.
If we wanted to, we could generate this list automatically, but it is simpler to write a list manually. Here it is:
(defvar top-of-ranges '(10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300) "List specifying ranges for `defuns-per-range'.")
To change the ranges, we edit this list.
Next, we need to write the function that creates the list of the
number of definitions within each range. Clearly, this function
must take the sorted-lengths and the
top-of-ranges lists as arguments.
The defuns-per-range function must do two things
again and again: it must count the number of definitions within a
range specified by the current top-of-range value; and it must
shift to the next higher value in the top-of-ranges
list after counting the number of definitions in the current
range. Since each of these actions is repetitive, we can use
while loops for the job. One loop counts the number
of definitions in the range defined by the current top-of-range
value, and the other loop selects each of the top-of-range values
in turn.
Several entries of the sorted-lengths list are
counted for each range; this means that the loop for the
sorted-lengths list will be inside the loop for the
top-of-ranges list, like a small gear inside a big
gear.
The inner loop counts the number of definitions within the range.
It is a simple counting loop of the type we have seen before.
(See section 11.1.3 A Loop
with an Incrementing Counter.) The true-or-false test of the
loop tests whether the value from the sorted-lengths
list is smaller than the current value of the top of the range.
If it is, the function increments the counter and tests the next
value from the sorted-lengths list.
The inner loop looks like this:
(while length-element-smaller-than-top-of-range (setq number-within-range (1+ number-within-range)) (setq sorted-lengths (cdr sorted-lengths)))
The outer loop must start with the lowest value of the
top-of-ranges list, and then be set to each of the
succeeding higher values in turn. This can be done with a loop
like this:
(while top-of-ranges body-of-loop... (setq top-of-ranges (cdr top-of-ranges)))
Put together, the two loops look like this:
(while top-of-ranges
;; Count the number of elements within the current range.
(while length-element-smaller-than-top-of-range
(setq number-within-range (1+ number-within-range))
(setq sorted-lengths (cdr sorted-lengths)))
;; Move to next range.
(setq top-of-ranges (cdr top-of-ranges)))
In addition, in each circuit of the outer loop, Emacs should
record the number of definitions within that range (the value of
number-within-range) in a list. We can use
cons for this purpose. (See section 7.2 cons.)
The cons function works fine, except that the list
it constructs will contain the number of definitions for the
highest range at its beginning and the number of definitions for
the lowest range at its end. This is because cons
attaches new elements of the list to the beginning of the list,
and since the two loops are working their way through the
lengths' list from the lower end first, the
defuns-per-range-list will end up largest number
first. But we will want to print our graph with smallest values
first and the larger later. The solution is to reverse the order
of the defuns-per-range-list. We can do this using
the nreverse function, which reverses the order of a
list.
For example,
(nreverse '(1 2 3 4))
produces:
(4 3 2 1)
Note that the nreverse function is
"destructive"---that is, it changes the list to which it is
applied; this contrasts with the car and
cdr functions, which are non-destructive. In this
case, we do not want the original
defuns-per-range-list, so it does not matter that it
is destroyed. (The reverse function provides a
reversed copy of a list, leaving the original list as is.)
Put all together, the defuns-per-range looks like
this:
(defun defuns-per-range (sorted-lengths top-of-ranges)
"SORTED-LENGTHS defuns in each TOP-OF-RANGES range."
(let ((top-of-range (car top-of-ranges))
(number-within-range 0)
defuns-per-range-list)
;; Outer loop.
(while top-of-ranges
;; Inner loop.
(while (and
;; Need number for numeric test.
(car sorted-lengths)
(< (car sorted-lengths) top-of-range))
;; Count number of definitions within current range.
(setq number-within-range (1+ number-within-range))
(setq sorted-lengths (cdr sorted-lengths)))
;; Exit inner loop but remain within outer loop.
(setq defuns-per-range-list
(cons number-within-range defuns-per-range-list))
(setq number-within-range 0) ; Reset count to zero.
;; Move to next range.
(setq top-of-ranges (cdr top-of-ranges))
;; Specify next top of range value.
(setq top-of-range (car top-of-ranges)))
;; Exit outer loop and count the number of defuns larger than
;; the largest top-of-range value.
(setq defuns-per-range-list
(cons
(length sorted-lengths)
defuns-per-range-list))
;; Return a list of the number of definitions within each range,
;; smallest to largest.
(nreverse defuns-per-range-list)))
The function is straightforward except for one subtle feature. The true-or-false test of the inner loop looks like this:
(and (car sorted-lengths)
(< (car sorted-lengths) top-of-range))
instead of like this:
(< (car sorted-lengths) top-of-range)
The purpose of the test is to determine whether the first item in
the sorted-lengths list is less than the value of
the top of the range.
The simple version of the test works fine unless the
sorted-lengths list has a nil value. In
that case, the (car sorted-lengths) expression
function returns nil. The < function
cannot compare a number to nil, which is an empty
list, so Emacs signals an error and stops the function from
attempting to continue to execute.
The sorted-lengths list always becomes
nil when the counter reaches the end of the list.
This means that any attempt to use the
defuns-per-range function with the simple version of
the test will fail.
We solve the problem by using the (car
sorted-lengths) expression in conjunction with the
and expression. The (car
sorted-lengths) expression returns a non-nil
value so long as the list has at least one number within it, but
returns nil if the list is empty. The
and expression first evaluates the (car
sorted-lengths) expression, and if it is nil,
returns false without evaluating the <
expression. But if the (car sorted-lengths)
expression returns a non-nil value, the
and expression evaluates the <
expression, and returns that value as the value of the
and expression.
This way, we avoid an error. See section 12.4
forward-paragraph: a Goldmine of Functions, for
more information about and.
Here is a short test of the defuns-per-range
function. First, evaluate the expression that binds (a shortened)
top-of-ranges list to the list of values, then
evaluate the expression for binding the
sorted-lengths list, and then evaluate the
defuns-per-range function.
;; (Shorter list than we will use later.)
(setq top-of-ranges
'(110 120 130 140 150
160 170 180 190 200))
(setq sorted-lengths
'(85 86 110 116 122 129 154 176 179 200 265 300 300))
(defuns-per-range sorted-lengths top-of-ranges)
The list returned looks like this:
(2 2 2 0 0 1 0 2 0 0 4)
Indeed, there are two elements of the sorted-lengths
list smaller than 110, two elements between 110 and 119, two
elements between 120 and 129, and so on. There are four elements
with a value of 200 or larger.
Our goal is to construct a graph showing the numbers of function definitions of various lengths in the Emacs lisp sources.
As a practical matter, if you were creating a graph, you would
probably use a program such as gnuplot to do the
job. (gnuplot is nicely integrated into GNU Emacs.)
In this case, however, we create one from scratch, and in the
process we will reaquaint ourselves with some of what we learned
before and learn more.
In this chapter, we will first write a simple graph printing function. This first definition will be a prototype, a rapidly written function that enables us to reconnoiter this unknown graph-making territory. We will discover dragons, or find that they are myth. After scouting the terrain, we will feel more confident and enhance the function to label the axes automatically.
Since Emacs is designed to be flexible and work with all kinds of terminals, including character-only terminals, the graph will need to be made from one of the `typewriter' symbols. An asterisk will do; as we enhance the graph-printing function, we can make the choice of symbol a user option.
We can call this function graph-body-print; it will
take a numbers-list as its only argument. At this
stage, we will not label the graph, but only print its body.
The graph-body-print function inserts a vertical
column of asterisks for each element in the
numbers-list. The height of each line is determined
by the value of that element of the numbers-list.
Inserting columns is a repetitive act; that means that this
function can be written either with a while loop or
recursively.
Our first challenge is to discover how to print a column of asterisks. Usually, in Emacs, we print characters onto a screen horizontally, line by line, by typing. We have two routes we can follow: write our own column-insertion function or discover whether one exists in Emacs.
To see whether there is one in Emacs, we can use the M-x apropos command. This command is like the C-h a (command-apropos) command, except that the latter finds only those functions that are commands. The M-x apropos command lists all symbols that match a regular expression, including functions that are not interactive.
What we want to look for is some command that prints or inserts
columns. Very likely, the name of the function will contain
either the word `print' or the word `insert' or the word
`column'. Therefore, we can simply type M-x apropos RET
print\|insert\|column RET and look at the result. On my
system, this command takes quite some time, and then produces a
list of 79 functions and variables. Scanning down the list, the
only function that looks as if it might do the job is
insert-rectangle. Indeed, this is the function we
want; its documentation says:
insert-rectangle: Insert text of RECTANGLE with upper left corner at point. RECTANGLE's first line is inserted at point, its second line is inserted at a point vertically under point, etc. RECTANGLE should be a list of strings.
We can run a quick test, to make sure it does what we expect of it.
Here is the result of placing the cursor after the
insert-rectangle expression and typing C-u C-x
C-e (eval-last-sexp). The function inserts the
strings `"first"', `"second"', and
`"third"' at and below point. Also the function
returns nil.
(insert-rectangle '("first" "second" "third"))first
second
third
nil
Of course, we won't be inserting the text of the
insert-rectangle expression itself into the buffer
in which we are making the graph, but will call the function from
our program. We shall, however, have to make sure that point is
in the buffer at the place where the
insert-rectangle function will insert its column of
strings.
If you are reading this in Info, you can see how this works by
switching to another buffer, such as the `*scratch*'
buffer, placing point somewhere in the buffer, typing
M-ESC, typing the insert-rectangle
expression into the minibuffer at the prompt, and then typing
RET. This causes Emacs to evaluate the expression in
the minibuffer, but to use as the value of point the position of
point in the `*scratch*' buffer. (M-ESC is
the keybinding for eval-expression.)
We find when we do this that point ends up at the end of the last inserted line--that is to say, this function moves point as a side-effect. If we were to repeat the command, with point at this position, the next insertion would be below and to the right of the previous insertion. We don't want this! If we are going to make a bar graph, the columns need to be beside each other.
So we discover that each cycle of the column-inserting
while loop must reposition point to the place we
want it, and that place will be at the top, not the bottom, of
the column. Moreover, we remember that when we print a graph, we
do not expect all the columns to be the same height. This means
that the top of each column may be at a different height from the
previous one. We cannot simply reposition point to the same line
each time, but moved over to the right--or perhaps we can...
We are planning to make the columns of the bar graph out of
asterisks. The number of asterisks in the column is the number
specified by the current element of the
numbers-list. We need to construct a list of
asterisks of the right length for each call to
insert-rectangle. If this list consists solely of
the requisit number of asterisks, then we will have position
point the right number of lines above the base for the graph to
print correctly. This could be difficult.
Alternatively, if we can figure out some way to pass
insert-rectangle a list of the same length each
time, then we can place point on the same line each time, but
move it over one column to the right for each new column. If we
do this, however, some of the entries in the list passed to
insert-rectangle must be blanks rather than
asterisks. For example, if the maximum height of the graph is 5,
but the height of the column is 3, then
insert-rectangle requires an argument that looks
like this:
(" " " " "*" "*" "*")
This last proposal is not so difficult, so long as we can
determine the column height. There are two ways for us to specify
the column height: we can arbitrarily state what it will be,
which would work fine for graphs of that height; or we can search
through the list of numbers and use the maximum height of the
list as the maximum height of the graph. If the latter operation
were difficult, then the former procedure would be easiest, but
there is a function built into Emacs that determines the maximum
of its arguments. We can use that function. The function is
called max and it returns the largest of all its
arguments, which must be numbers. Thus, for example,
(max 3 4 6 5 7 3)
returns 7. (A corresponding function called min
returns the smallest of all its arguments.)
However, we cannot simply call max on the
numbers-list; the max function expects
numbers as its argument, not a list of numbers. Thus, the
following expression,
(max '(3 4 6 5 7 3))
produces the following error message;
Wrong type of argument: integer-or-marker-p, (3 4 6 5 7 3)
We need a function that passes a list of
arguments to a function. This function is apply.
This function `applies' its first argument (a function) to its
remaining arguments, the last of which may be a list.
For example,
(apply 'max 3 4 7 3 '(4 8 5))
returns 8.
(Incidentally, I don't know how you would learn of this function
without a book such as this. It is possible to discover other
functions, like search-forward or
insert-rectangle, by guessing at a part of their
names and then using apropos. Even though its base
in metaphor is clear---`apply' its first argument to the rest--I
doubt a novice would come up with that particular word when using
apropos or other aid. Of course, I could be wrong;
after all, the function was first named by someone who had to
invent it.)
The second and subsequent arguments to apply are
optional, so we can use apply to call a function and
pass the elements of a list to it, like this, which also returns
8:
(apply 'max '(4 8 5))
This latter way is how we will use apply. The
recursive-lengths-list-many-files function returns a
numbers' list to which we can apply max (we could
also apply max to the sorted numbers' list; it does
not matter whether the list is sorted or not.)
Hence, the operation for finding the maximum height of the graph is this:
(setq max-graph-height (apply 'max numbers-list))
Now we can return to the question of how to create a list of
strings for a column of the graph. Told the maximum height of the
graph and the number of asterisks that should appear in the
column, the function should return a list of strings for the
insert-rectangle command to insert.
Each column is made up of asterisks or blanks. Since the function
is passed the value of the height of the column and the number of
asterisks in the column, the number of blanks can be found by
subtracting the number of asterisks from the height of the
column. Given the number of blanks and the number of asterisks,
two while loops can be used to construct the list:
;;; First version.
(defun column-of-graph (max-graph-height actual-height)
"Return list of strings that is one column of a graph."
(let ((insert-list nil)
(number-of-top-blanks
(- max-graph-height actual-height)))
;; Fill in asterisks.
(while (> actual-height 0)
(setq insert-list (cons "*" insert-list))
(setq actual-height (1- actual-height)))
;; Fill in blanks.
(while (> number-of-top-blanks 0)
(setq insert-list (cons " " insert-list))
(setq number-of-top-blanks
(1- number-of-top-blanks)))
;; Return whole list.
insert-list))
If you install this function and then evaluate the following expression you will see that it returns the list as desired:
(column-of-graph 5 3)
returns
(" " " " "*" "*" "*")
As written, column-of-graph contains a major flaw:
the symbols used for the blank and for the marked entries in the
column are `hard-coded' as a space and asterisk. This is fine for
a prototype, but you, or another user, may wish to use other
symbols. For example, in testing the graph function, you many
want to use a period in place of the space, to make sure the
point is being repositioned properly each time the
insert-rectangle function is called; or you might
want to substitute a `+' sign or other symbol for
the asterisk. You might even want to make a graph-column that is
more than one display column wide. The program should be more
flexible. The way to do that is to replace the blank and the
asterisk with two variables that we can call
graph-blank and graph-symbol and define
those variables separately.
Also, the documentation is not well written. These considerations lead us to the second version of the function:
(defvar graph-symbol "*" "String used as symbol in graph, usually an asterisk.") (defvar graph-blank " " "String used as blank in graph, usually a blank space. graph-blank must be the same number of columns wide as graph-symbol.")
(For an explanation of defvar, see section 8.4 Initializing a Variable with
defvar.)
;;; Second version.
(defun column-of-graph (max-graph-height actual-height)
"Return list of MAX-GRAPH-HEIGHT strings;
ACTUAL-HEIGHT are graph-symbols.
The graph-symbols are contiguous entries at the end
of the list.
The list will be inserted as one column of a graph.
The strings are either graph-blank or graph-symbol."
(let ((insert-list nil)
(number-of-top-blanks
(- max-graph-height actual-height)))
;; Fill in graph-symbols.
(while (> actual-height 0)
(setq insert-list (cons graph-symbol insert-list))
(setq actual-height (1- actual-height)))
;; Fill in graph-blanks.
(while (> number-of-top-blanks 0)
(setq insert-list (cons graph-blank insert-list))
(setq number-of-top-blanks
(1- number-of-top-blanks)))
;; Return whole list.
insert-list))
If we wished, we could rewrite column-of-graph a
third time to provide optionally for a line graph as well as for
a bar graph. This would not be hard to do. One way to think of a
line graph is that it is no more than a bar graph in which the
part of each bar that is below the top is blank. To construct a
column for a line graph, the function first constructs a list of
blanks that is one shorter than the value, then it uses
cons to attach a graph symbol to the list; then it
uses cons again to attach the `top blanks' to the
list.
It is easy to see how to write such a function, but since we
don't need it, we will not do it. But the job could be done, and
if it were done, it would be done with
column-of-graph. Even more important, it is worth
noting that few changes would have to be made anywhere else. The
enhancement, if we ever wish to make it, is simple.
Now, finally, we come to our first actual graph printing
function. This prints the body of a graph, not the labels for the
vertical and horizontal axes, so we can call this
graph-body-print.
graph-body-print
Function
After our preparation in the preceding section, the
graph-body-print function is straightforward. The
function will print column after column of asterisks and blanks,
using the elements of a numbers' list to specify the number of
asterisks in each column. This is a repetitive act, which means
we can use a decrementing while loop or recursive
function for the job. In this section, we will write the
definition using a while loop.
The column-of-graph function requires the height of
the graph as an argument, so we should determine and record that
as a local variable.
This leads us to the following template for the
while loop version of this function:
(defun graph-body-print (numbers-list)
"documentation..."
(let ((height ...
...))
(while numbers-list
insert-columns-and-reposition-point
(setq numbers-list (cdr numbers-list)))))
We need to fill in the slots of the template.
Clearly, we can use the (apply 'max numbers-list)
expression to determine the height of the graph.
The while loop will cycle through the
numbers-list one element at a time. As it is
shortened by the (setq numbers-list (cdr
numbers-list)) expression, the CAR of each instance of the
list is the value of the argument for
column-of-graph.
At each cycle of the while loop, the
insert-rectangle function inserts the list returned
by column-of-graph. Since the
insert-rectangle function moves point to the lower
right of the inserted rectangle, we need to save the location of
point at the time the rectangle is inserted, move back to that
position after the rectangle is inserted, and then move
horizontally to the next place from which
insert-rectangle is called.
If the inserted columns are one character wide, as they will be
if single blanks and asterisks are used, the repositioning
command is simply (forward-char 1); however, the
width of a column may be greater than one. This means that the
repositioning command should be written (forward-char
symbol-width). The symbol-width itself is the
length of a graph-blank and can be found using the
expression (length graph-blank). The best place to
bind the symbol-width variable to the value of the
width of graph column is in the varlist of the let
expression.
These considerations lead to the following function definition:
(defun graph-body-print (numbers-list)
"Print a bar graph of the NUMBERS-LIST.
The numbers-list consists of the Y-axis values."
(let ((height (apply 'max numbers-list))
(symbol-width (length graph-blank))
from-position)
(while numbers-list
(setq from-position (point))
(insert-rectangle
(column-of-graph height (car numbers-list)))
(goto-char from-position)
(forward-char symbol-width)
;; Draw graph column by column.
(sit-for 0)
(setq numbers-list (cdr numbers-list)))
;; Place point for X axis labels.
(forward-line height)
(insert "\n")
))
The one unexpected expression in this function is the
(sit-for 0) expression in the while
loop. This expression makes the graph printing operation more
interesting to watch than it would be otherwise. The expression
causes Emacs to `sit' or do nothing for a zero length of time and
then redraw the screen. Placed here, it causes Emacs to redraw
the screen column by column. Without it, Emacs would not redraw
the screen until the function exits.
We can test graph-body-print with a short list of
numbers.
graph-symbol, graph-blank,
column-of-graph and graph-body-print.
(graph-body-print '(1 2 3 4 6 4 3 5 7 6 5 2 3))
eval-expression).
graph-body-print expression into the
minibuffer with C-y (yank).
graph-body-print expression.
Emacs will print a graph like this:
*
* **
* ****
*** ****
********* *
************
*************
recursive-graph-body-print
Function
The graph-body-print function may also be written
recursively. In this case, it is divided into two parts: an
outside `wrapper' that uses a let expression to
determine the values of several variables that need only be found
once, such as the maximum height of the graph, and a inside
function that is called recursively to print the graph.
The `wrapper' is uncomplicated:
(defun recursive-graph-body-print (numbers-list)
"Print a bar graph of the NUMBERS-LIST.
The numbers-list consists of the Y-axis values."
(let ((height (apply 'max numbers-list))
(symbol-width (length graph-blank))
from-position)
(recursive-graph-body-print-internal
numbers-list
height
symbol-width)))
The recursive function is a little more difficult. It has four
parts: the `do-again-test', the printing code, the recursive
call, and the `next-step-expression'. The `do-again-test' is an
if expression that determines whether the
numbers-list contains any remaining elements; if it
does, the function prints one column of the graph using the
printing code and calls itself again. The function calls itself
again according to the value produced by the
`next-step-expression' which causes the call to act on a shorter
version of the numbers-list.
(defun recursive-graph-body-print-internal
(numbers-list height symbol-width)
"Print a bar graph.
Used within recursive-graph-body-print function."
(if numbers-list
(progn
(setq from-position (point))
(insert-rectangle
(column-of-graph height (car numbers-list)))
(goto-char from-position)
(forward-char symbol-width)
(sit-for 0) ; Draw graph column by column.
(recursive-graph-body-print-internal
(cdr numbers-list) height symbol-width))))
After installation, this expression can be tested; here is a sample:
(recursive-graph-body-print '(3 2 5 6 7 5 3 4 6 4 3 2 1))
Here is what recursive-graph-body-print produces:
*
** *
**** *
**** ***
* *********
************
*************
Either of these two functions, graph-body-print or
recursive-graph-body-print, create the body of a
graph.
A graph needs printed axes, so you can orient yourself. For a do-once project, it may be reasonable to draw the axes by hand using Emacs's Picture mode; but a graph drawing function may be used more than once.
For this reason, I have written enhancements to the basic
print-graph-body function that automatically print
labels for the horizontal and vertical axes. Since the label
printing functions do not contain much new material, I have
placed their description in an appendix. See section C A Graph with Labelled Axes.
"You don't have to like Emacs to like it" -- this seemingly paradoxical statement is the secret of GNU Emacs. The plain, `out of the box' Emacs is a generic tool. Most people who use it, customize it to suit themselves.
GNU Emacs is mostly written in Emacs Lisp; this means that by writing expressions in Emacs Lisp you can change or extend Emacs.
There are those who appreciate Emacs's default configuration. After all, Emacs starts you in C mode when you edit a C file, starts you in Fortran mode when you edit a Fortran file, and starts you in Fundamental mode when you edit an unadorned file. This all makes sense, if you do not know who is going to use Emacs. Who knows what a person hopes to do with an unadorned file? Fundamental mode is the right default for such a file, just as C mode is the right default for editing C code. But when you do know who is going to use Emacs--you, yourself--then it makes sense to customize Emacs.
For example, I seldom want Fundamental mode when I edit an otherwise undistinguished file; I want Text mode. This is why I customize Emacs: so it suits me.
You can customize and extend Emacs by writing or adapting a `~/.emacs' file. This is your personal initialization file; its contents, written in Emacs Lisp, tell Emacs what to do.
This chapter describes a simple `.emacs' file; for more information, see section `The Init File' in The GNU Emacs Manual, and section `The Init File' in The GNU Emacs Lisp Reference Manual.
In addition to your personal initialization file, Emacs automatically loads various site-wide initialization files, if they exist. These have the same form as your `.emacs' file, but are loaded by everyone.
Two site-wide initialization files, `site-load.el' and `site-init.el', are loaded into Emacs and then `dumped' if a `dumped' version of Emacs is created, as is most common. (Dumped copies of Emacs load more quickly. However, once a file is loaded and dumped, a change to it does not lead to a change in Emacs unless you load it yourself or re-dump Emacs. See section `Building Emacs' in The GNU Emacs Lisp Reference Manual, and the `INSTALL' file.)
Three other site-wide initialization files are loaded automatically each time you start Emacs, if they exist. These are `site-start.el', which is loaded before your `.emacs' file, and `default.el', and the terminal type file, which are both loaded after your `.emacs' file.
Settings and definitions in your `.emacs' file will
overwrite conflicting settings and definitions in a
`site-start.el' file, if it exists; but the settings and
definitions in a `default.el' or terminal type file will
overwrite those in your `.emacs' file. (You can prevent
interference from a terminal type file by setting
term-file-prefix to nil. See section
16.10 A Simple Extension:
line-to-top-of-window.)
The `INSTALL' file that comes in the distribution contains descriptions of the `site-init.el' and `site-load.el' files.
The `loadup.el', `startup.el', and `loaddefs.el' files control loading. These files are in the `lisp' directory of the Emacs distribution and are worth perusing.
The `loaddefs.el' file contains a good many suggestions as to what to put into your own `.emacs' file, or into a site-wide initialization file.
My copy of Emacs version 19.23 has 392 options that you can set
with the edit-options command. These `options' are
no more than variables such as we have seen earlier and defined
using defvar.
Emacs determines whether a variable is intended to be easily
settable by looking at the first character in its documentation
string; if the first character is an asterisk, `*',
the variable is a user-settable option. (See section 8.4 Initializing a Variable with
defvar.)
The edit-options command lists all the variables in
Emacs that the people who wrote the Emacs Lisp libraries think
ought to be readily settable. It provides an easy-to-use
interface for resetting these variables.
On the other hand, options set using edit-options
are set only for the duration of your editing session. The new
values are not saved between sessions. Each time Emacs starts, it
reads the original defvar value in its source code.
To carry a changed setting from one session to the next, you need
to use a setq expression within a `.emacs'
file or other file that you load every time you start a session.
For me, the major use of the edit-options command is
to suggest variables I might want to set in my `.emacs'
file. I urge you to look through the list.
See section `Editing Variable Values' in The GNU Emacs Manual, for more information.
When you start Emacs, it loads your `.emacs' file unless
you tell it not to by specifying `-q' on the command
line. (The emacs -q command gives you a plain,
out-of-the-box Emacs.)
A `.emacs' file contains Lisp expressions. Often, these are no more than expressions to set values; sometimes they are function definitions.
See section `The Init File `~/.emacs'' in The GNU Emacs Manual, for a short description of initialization files.
This chapter goes over some of the same ground, but is a walk among extracts from a complete, long-used `.emacs' file--my own.
The first part of the file consists of comments: reminders to myself. By now, of course, I remember these things, but when I started, I did not.
;;;; Bob's .emacs file ; Robert J. Chassell ; 26 September 1985
Look at that date! I started this file a long time ago. I have been adding to it every since.
; Each section in this file is introduced by a ; line beginning with four semi-colons; and each ; entry is introduced by a line beginning with ; three semi-colons.
This describes the usual conventions for comments in Emacs Lisp. Everything on a line that follows a semi-colon is a comment. Two, three, and four semi-colons are used as section and subsection markers. (See section `Comments' in The GNU Emacs Lisp Reference Manual, for more about comments.)
;;;; The Help Key ; Control-h is the help key; ; after typing control-h, type a letter to ; indicate the subject about which you want help. ; For an explanation of the help facility, ; type control-h three times in a row.
Just remember: type C-h three times for help.
; To find out about any mode, type control-h m ; while in that mode. For example, to find out ; about mail mode, enter mail mode and then type ; control-h m.
`Mode help', as I call this, is very helpful. Usually, it tells you all you need to know.
Of course, you don't need to include comments like these in your `.emacs' file. I included them in mine because I kept forgetting about Mode help or the conventions for comments--but I was able to remember to look here to remind myself.
Now we come to the part that `turns on' Text mode and Auto Fill mode.
;;; Text mode and Auto Fill mode ; The next two lines put Emacs into Text mode ; and Auto Fill mode, and are for writers who ; want to start writing prose rather than code. (setq default-major-mode 'text-mode) (add-hook 'text-mode-hook 'turn-on-auto-fill)
Here is the first part of this `.emacs' file that does something besides remind a forgetful human!
The first of the two lines in parentheses tells Emacs to turn on Text mode when you find a file, unless that file should go into some other mode, such as C mode.
When Emacs reads a file, it looks at the extension to the file name, if any. (The extension is the part that comes after a `.'.) If the file ends with a `.c' or `.h' extension then Emacs turns on C mode. Also, Emacs looks at first nonblank line of the file; if the line says `-*- C -*-', Emacs turns on C mode. Emacs possesses a list of extensions and specifications that it uses automatically. In addition, Emacs looks near the last page for a per-buffer, "local variables list", if any.
See sections "How Major Modes are Chosen" and "Local Variables in Files" in The GNU Emacs Manual, for information.
Now, back to the `.emacs' file.
Here is the line again; how does it work?
(setq default-major-mode 'text-mode)
This line is a short, but complete Emacs Lisp expression.
We are already familiar with setq. It sets the
following variable, default-major-mode, to the
subsequent value, which is text-mode. The single
quote mark before text-mode tells Emacs to deal
directly with the text-mode variable, not with
whatever it might stand for. See section 1.9 Setting the Value of a
Variable, for a reminder of how setq works. The
main point is that there is no difference between the procedure
you use to set a value in your `.emacs' file and the
procedure you use anywhere else in Emacs.
Here is the second line:
(add-hook 'text-mode-hook 'turn-on-auto-fill)
In this line, the add-hook command, adds
turn-on-auto-fill to the variable called
text-mode-hook. turn-on-auto-fill is
the name of a program, that, you guessed it!, turns on Auto Fill
mode.
Every time Emacs turns on Text mode, Emacs runs the commands `hooked' onto Text mode. So every time Emacs turns on Text mode, Emacs also turns on Auto Fill mode.
In brief, the first line causes Emacs to enter Text mode when you edit a file, unless the file name extension, first non-blank line, or local variables tell Emacs otherwise.
Text mode among other actions, sets the syntax table to work conveniently for writers. In Text mode, Emacs considers an apostrophe as part of a word like a letter; but Emacs does not consider a period or a space as part of a word. Thus, M-f moves you over `it's'. On the other hand, in C mode, M-f stops just after the `t' of `it's'.
The second line causes Emacs to turn on Auto Fill mode when it turns on Text mode. In Auto Fill mode, Emacs automatically breaks a line that is too wide and brings the excessively wide part of the line down to the next line. Emacs breaks lines between words, not within them.
When Auto Fill mode is turned off, lines continue to the right as
you type them. Depending on how you set the value of
truncate-lines, the words you type either disappear
off the right side of the screen, or else are shown, in a rather
ugly and unreadable manner, as a continuation line on the screen.
Here is a setq to `turn on' mail aliases, along with
more reminders.
;;; Mail mode ; To enter mail mode, type `C-x m' ; To enter RMAIL (for reading mail), ; type `M-x rmail' (setq mail-aliases t)
This setq command sets the value
of the variable mail-aliases to t.
Since t means true, the line says, in effect,
"Yes, use mail aliases."
Mail aliases are convenient short names for long email addresses or for lists of email addresses. The file where you keep your `aliases' is `~/.mailrc'. You write an alias like this:
alias geo george@foobar.wiz.edu
When you write a message to George, address it to `geo'; the mailer will automatically expand `geo' to the full address.
By default, Emacs inserts tabs in place of multiple spaces when
it formats a region. (For example, you might indent many lines of
text all at once with the indent-region command.)
Tabs look fine on a terminal or with ordinary printing, but they
produce badly indented output when you use TeX or Texinfo since
TeX ignores tabs.
The following turns off Indent Tabs mode:
;;; Prevent Extraneous Tabs (setq-default indent-tabs-mode nil)
Note that this line uses setq-default rather than
the setq command that we have see before. The
setq-default command sets values only in buffers
that do not have their own local values for the variable.
See sections "Tabs vs. Spaces" and "Local Variables in Files" in The GNU Emacs Manual.
Now for some personal keybindings:
;;; Compare windows (global-set-key "\C-cw" 'compare-windows)
compare-windows is a nifty
command that compares the text in your current window with
text in the next window. It makes the comparison by starting
at point in each window, moving over text in each window as
far as they match. I use this command all the time.
This also shows how to set a key globally, for all modes.
The command is global-set-key.
It is followed by the keybinding. In a `.emacs' file,
the keybinding is written as shown: \C-c stands
for `control-c', which means `press the control key and the
c key at the same time'. The w means
`press the w key'. The keybinding is surrounded by
double quotation marks. In documentation, you would write this
as C-c w. (If you were binding a META
key, such as M-c, rather than a CTL key,
you would write \M-c. See section `Rebinding Keys
in Your Init File' in The GNU Emacs Manual, for
details.)
The command invoked by the keys is compare-windows.
Note that compare-windows is preceded by a single
quote; otherwise, Emacs would first try to evaluate the symbol to
determine its value.
These three things, the double quotation marks, the backslash before the `C', and the single quote mark are necessary parts of keybinding that I tend to forget. Fortunately, I have come to remember that I should look at my existing `.emacs' file, and adapt what is there.
As for the keybinding itself: C-c w. This combines the prefix key, C-c, with a single character, in this case, w. This set of keys, C-c followed by a single character, is strictly reserved for individuals' own use. If you ever write an extension to Emacs, please avoid taking any of these keys for public use. Create a key like C-c C-w instead. Otherwise, we will run out of `own' keys.
Here is another keybinding, with a comment:
;;; Keybinding for `occur' ; I use occur a lot, so let's bind it to a key: (global-set-key "\C-co" 'occur)
The occur command shows all the
lines in the current buffer that contain a match for a regular
expression. Matching lines are shown in a buffer called
`*Occur*'. That buffer serves as a menu to jump to
occurrences.
Here is how to unbind a key, so it does not work:
;;; Unbind `C-x f' (global-unset-key "\C-xf")
There is a reason for this unbinding: I found I inadvertently typed C-x f when I meant to type C-x C-f. Rather than find a file, as I intended, I accidentally set the width for filled text, almost always to a width I did not want. Since I hardly ever reset my default width, I simply unbound the key.
The following rebinds an existing key:
;;; Rebind `C-x C-b' for `buffer-menu' (global-set-key "\C-x\C-b" 'buffer-menu)
By default, C-x C-b runs the list-buffers
command. This command lists your buffers in another
window. Since I almost always want to do something in that
window, I prefer the buffer-menu command, which not
only lists the buffers, but moves point into that window.
Many people in the GNU Emacs community have written extensions to Emacs. As time goes by, these extensions are often included in new releases. For example, the Calendar and Diary packages are now part of the standard Emacs version 19 distribution; they were not part of the standard Emacs version 18 distribution.
(Calc, which I consider a vital part of Emacs, would be part of the standard distribution except that it is so large it is packaged separately.)
You can use a load command to evaluate a complete
file and thereby install all the functions and variables in the
file into Emacs. For example:
(load "~/emacs/kfill")
This evaluates, i.e. loads, the `kfill.el' file (or if it exists, the faster, byte compiled `kfill.elc' file) from the `emacs' sub-directory of your home directory.
(`kfill.el' was adapted from Kyle E. Jones' `filladapt.el' package by Bob Weiner and "provides no muss, no fuss word wrapping and filling of paragraphs with hanging indents, included text from news and mail messages, and Lisp, C++, PostScript or shell comments." I use it all the time and hope it is incorporated into the standard distribution.)
If you load many extensions, as I do, then instead of specifying
the exact location of the extension file, as shown above, you can
specify that directory as part of Emacs's load-path.
Then, when Emacs loads a file, it will search that directory as
well as its default list of directories. (The default list is
specified in `paths.h' when Emacs is built.)
The following command adds your `~/emacs' directory to the existing load path:
;;; Emacs Load Path (setq load-path (cons "~/emacs" load-path))
Incidentally, load-library is an interactive
interface to the load function. The complete
function looks like this:
(defun load-library (library) "Load the library named LIBRARY. This is an interface to the function `load'." (interactive "sLoad library: ") (load library))
The name of the function, load-library, comes from
the use of `library' as a conventional synonym for `file'. The
source for the load-library command is in the
`files.el' library.
Another interactive command that does a slightly different job is
load-file. See section `Libraries of Lisp Code for
Emacs' in The GNU Emacs Manual, for information on
the distinction between load-library and this
command.
Instead of installing a function by loading the file that contains it, or by evaluating the function definition, you can make the function available but not actually install it until it is first called. This is called autoloading.
When you execute an autoloaded function, Emacs automatically evaluates the file that contains the definition, and then calls the function.
Emacs starts quicker with autoloaded functions, since their libraries are not loaded right away; but you need to wait a moment when you first use such a function, while its containing file is evaluated.
Rarely used functions are frequently autoloaded. The
`loaddefs.el' library contains hundreds of autoloaded
functions, from bookmark-set to
wordstar-mode. Of course, you may come to use a
`rare' function frequently. In this case, you should load that
function's file with a load expression in your
`.emacs' file.
In my `.emacs' file for Emacs version 19.23, I load 17 libraries that contain functions that would otherwise be autoloaded. (Actually, it would have been better to include these files in my `dumped' Emacs when I built it, but I forgot. See section `Building Emacs' in The GNU Emacs Lisp Reference Manual, and the `INSTALL' file for more about dumping.)
You may also want to include autoloaded expressions in your
`.emacs' file. autoload is a built-in
function that takes up to five arguments, the final three of
which are optional. The first argument is the name of the
function to be autoloaded; the second is the name of the file to
be loaded. The third argument is documentation for the function,
and the fourth tells whether the function can be called
interactively. The fifth argument tells what type of
object---autoload can handle a keymap or macro as
well as a function (the default is a function).
Here is a typical example:
(autoload 'html-helper-mode "html-helper-mode" "Edit HTML documents" t)
This expression autoloads the html-helper-mode
function from the `html-helper-mode.el' file (or, if it
exists, from the byte compiled file
`html-helper-mode.elc'.) The file must be located in a
directory specified by load-path. The documentation
says that this is a mode to help you edit documents written in
the HyperText Markup Language. You can call this mode
interactively by typing M-x html-helper-mode. (You
need to duplicate the function's regular documentation in the
autoload expression because the regular function is not yet
loaded, so its documentation is not available.)
See section `Autoload' in The GNU Emacs Lisp Reference Manual, for more information.
line-to-top-of-window
Here is a simple extension to Emacs that moves the line point is on to the top of the window. I use this all the time, to make text easier to read.
You can put the following code into a separate file and then load it from your `.emacs' file, or you can include it within your `.emacs' file.
Here is the definition:
;;; Line to top of window; ;;; replace three keystroke sequence C-u 0 C-l (defun line-to-top-of-window () "Move the line point is on to top of window." (interactive) (recenter 0))
Now for the keybinding.
Although most of an Emacs version 18 `.emacs' file works with version 19, there are some differences (also, of course, there are new features in Emacs 19).
In version 19 Emacs, you can write a function key like this: `[f6]'. In version 18, you must specify the key strokes sent by the keyboard when you press that function key. For example, a Zenith 29 keyboard sends ESC P when I press its sixth function key; an Ann Arbor Ambassador keyboard sends ESC O F. Write these keystrokes as `\eP' and `\eOF', respectively.
In my version 18 `.emacs' file, I bind
line-to-top-of-window to a key that depends on the
type of terminal:
(defun z29-key-bindings () "Function keybindings for Z29 terminal." ;; ... (global-set-key "\eP" 'line-to-top-of-window)) (defun aaa-key-bindings () "Function keybindings for Ann Arbor Ambassador" ;; ... (global-set-key "\eOF" 'line-to-top-of-window))
(You can find out what a function key sends by typing the
function key, and then typing C-h l
(view-lossage) which displays the last 100 input
keystrokes.)
After specifying the key bindings, I evaluate an expression that chooses among keybindings, depending on the type of terminal I am using. However, before doing that, I turn off the predefined, default terminal-specific keybindings, which overwrite bindings in the `.emacs' if they clash.
;;; Turn Off Predefined Terminal Keybindings ; The following turns off the predefined ; terminal-specific keybindings such as the ; vt100 keybindings in lisp/term/vt100.el. ; If there are no predefined terminal ; keybindings, or if you like them, ; comment this out. (setq term-file-prefix nil)
Here is the selection expression itself:
(let ((term (getenv "TERM")))
(cond
((equal term "z29") (z29-key-bindings))
((equal term "aaa") (aaa-key-bindings))
(t (message
"No binding for terminal type %s."
term))))
In Emacs version 19, function keys (as well as mouse button
events and non-ASCII characters) are written within square
brackets, without quotation marks. I bind
line-to-top-of-window to my F6 function
key like this:
(global-set-key [f6] 'line-to-top-of-window)
Much simpler!
For more information, see section `Rebinding Keys in Your Init File' in The GNU Emacs Manual.
If you run both Emacs 18 and Emacs 19, you can select which code to evaluate with the following conditional:
(if (string=
(int-to-string 18)
(substring (emacs-version) 10 12))
;; evaluate version 18 code
(progn
...
;; else evaluate version 19 code
...
Emacs uses keymaps to record which keys call which commands. Specific modes, such as C mode or Text mode, have their own keymaps; the mode-specific keymaps override the global map that is shared by all buffers.
The global-set-key function binds, or rebinds, the
global keymap. For example, the following binds the key C-c
C-l to the function line-to-top-of-window:
(global-set-key "\C-c\C-l" 'line-to-top-of-window))
Mode-specific keymaps are bound using the define-key
function, which takes a specific keymap as an argument, as well
as the key and the command. For example, my `.emacs'
file contains the following expression to bind the
texinfo-insert-@group command to C-c C-c
g:
(define-key texinfo-mode-map "\C-c\C-cg" 'texinfo-insert-@group)
The texinfo-insert-@group function itself is a
little extension to Texinfo mode that inserts
`@group' into a Texinfo file. I use this command all
the time and prefer to type the three strokes C-c C-c
g rather than the six strokes @ g r o u p.
(`@group' and its matching `@end group'
are commands that keep all enclosed text together on one page;
many multi-line examples in this book are surrounded by
`@group ... @end group'.)
Here is the texinfo-insert-@group function
definition:
(defun texinfo-insert-@group () "Insert the string @group in a Texinfo buffer." (interactive) (beginning-of-line) (insert "@group\n"))
(Of course, I could have used Abbrev mode to save typing, rather than write a function to insert a word; but I prefer key strokes consistent with other Texinfo mode key bindings.)
You will see numerous define-key expressions in
`loaddefs.el' as well as in the various mode libraries,
such as `c-mode.el' and `lisp-mode.el'.
See section `Customizing Key Bindings' in The GNU Emacs Manual, and section `Keymaps' in The GNU Emacs Lisp Reference Manual, for more information about keymaps.
You can specify colors when you use Emacs version 19 with the MIT X Windowing system. (All the previous examples should work with both Emacs version 18 and Emacs version 19; this works only with Emacs version 19.)
I hate the default colors and specify my own.
Most of my specifications are in various X initialization files. I wrote notes to myself in my `.emacs' file to remind myself what I did:
;; I use TWM for window manager; ;; my ~/.xsession file specifies: ; xsetroot -solid navyblue -fg white
Actually, the root of the X window is not part of Emacs at all, but I like the reminder anyhow.
;; My ~/.Xresources file specifies: ; XTerm*Background: sky blue ; XTerm*Foreground: white ; emacs*geometry: =80x40+100+0 ; emacs*background: blue ; emacs*foreground: grey97 ; emacs*cursorColor: white ; emacs*pointerColor: white
Here are the expressions in my `.emacs' file that set values:
;;; Set highlighting colors for isearch and drag
(set-face-foreground 'highlight "white" )
(set-face-background 'highlight "slate blue")
(set-face-background 'region "slate blue")
(set-face-background
'secondary-selection "turquoise")
;; Set calendar highlighting colors
(setq calendar-load-hook
'(lambda ()
(set-face-foreground 'diary-face "skyblue")
(set-face-background 'holiday-face "slate blue")
(set-face-foreground 'holiday-face "white")))
The various shades of blue soothe my eye and prevent me from seeing the screen flicker.
Here are a few miscellaneous settings for version 19 Emacs:
(resize-minibuffer-mode 1) (setq resize-minibuffer-mode t)
(setq search-highlight t)
(setq default-frame-alist
'((menu-bar-lines . 1)
(auto-lower . t)
(auto-raise . t)))
; Cursor shapes are defined in
; `/usr/include/X11/cursorfont.h';
; for example, the `target' cursor is number 128;
; the `top_left_arrow' cursor is number 132.
(let ((mpointer (x-get-resource "*mpointer"
"*emacs*mpointer")))
;; If you have not set your mouse pointer
;; then sent it, otherwise leave as is:
(if (eq mpointer nil)
(setq mpointer "132")) ; top_left_arrow
(setq x-pointer-shape (string-to-int mpointer))
(set-mouse-color "white"))
Finally, a feature I really like: a modified mode line.
Since I sometimes work over a network, I replaced the `Emacs: ' that is normally written on the left hand side of the mode line by the name of the system--otherwise, I forget which machine I am using. In addition, I list the default directory lest I lose track of where I am, and I specify the line point is on, with `Line' spelled out. My `.emacs' file looks like this:
(setq mode-line-system-identification
(substring (system-name) 0
(string-match "\\..+" (system-name))))
(setq default-mode-line-format
(list ""
'mode-line-modified
"<"
'mode-line-system-identification
"> "
"%14b"
" "
'default-directory
" "
"%[("
'mode-name
'minor-mode-alist
"%n"
'mode-line-process
")%]--"
"Line %l--"
'(-3 . "%P")
"-%-"))
;; Start with new default.
(setq mode-line-format default-mode-line-format)
I set the default mode line format so as to permit
various modes, such as Info, to override it. Many elements in the
list are self-explanatory: mode-line-modified is a
variable the tells whether the buffer has been modified,
mode-name tells the name of the mode, and so on.
The `"%14b"' displays the current buffer name (using
the buffer-name function with which we are
familiar); the `14' specifies the maximum number of characters
that will be displayed. When a name has fewer characters,
whitespace is added to fill out to this number. `%['
and `%]' cause a pair of square brackets to appear
for each recursive editing level. `%n' says `Narrow'
when narrowing is in effect. `%P' tells you the
percentage of the buffer that is above the bottom of the window,
or `Top', `Bottom', or `All'. (A lower case `p' tell
you the percentage above the top of the window.)
`%-' inserts enough dashes to fill out the line.
In and after Emacs version 19.29, you can use
frame-title-format to set the title of an Emacs
frame. This variable has the same structure as
mode-line-format.
Mode line formats are described in section `Mode Line Format' in The GNU Emacs Lisp Reference Manual.
Remember, "You don't have to like Emacs to like it" -- your own Emacs can have different colors, different commands, and different keys than a default Emacs.
On the other hand, if you want to bring up a plain `out of the box' Emacs, with no customization, type:
emacs -q
This will start an Emacs that does not load your `~/.emacs' initialization file. A plain, default Emacs. Nothing more.
GNU Emacs has two debuggers, debug and
edebug. The first is built into the internals of
Emacs and is always with you; the second is an extension to Emacs
that has become part of the standard distribution in version 19.
Both debuggers are described extensively in section `Debugging Lisp Programs' in The GNU Emacs Lisp Reference Manual. In this chapter, I will walk through a short example of each.
debug
Suppose you have written a function definition that is intended
to return the sum of the numbers 1 through a given number. (This
is the triangle function discussed earlier. See
section Example with
decrementing counter, for a discussion.)
However, your function definition has a bug. You have mistyped `1=' for `1-'. Here is the broken definition:
(defun triangle-bugged (number)
"Return sum of numbers 1 through NUMBER inclusive."
(let ((total 0))
(while (> number 0)
(setq total (+ total number))
(setq number (1= number))) ; Error here.
total))
If you are reading this in Info, you can evaluate this definition
in the normal fashion. You will see triangle-bugged
appear in the echo area.
Now evaluate the triangle-bugged function with an
argument of 4:
(triangle-bugged 4)
You will produce an error message that says: Symbol's function definition is void: 1=
In practice, for a bug as simple as this, this error message will tell you what you need to know to correct the definition. However, suppose you are not quite certain what is going on?
You can turn on debugging by setting the
value of debug-on-error to t:
(setq debug-on-error t)
This causes Emacs to enter the debugger next time it encounters an error.
You can turn off debug-on-error by setting it to
nil:
(setq debug-on-error nil)
Set debug-on-error to t and evaluate
the following:
(triangle-bugged 4)
This time, Emacs will create a buffer called `*Backtrace*' that looks like this:
---------- Buffer: *Backtrace* ----------
Signalling: (void-function 1=)
(1= number))
(setq number (1= number)))
(while (> number 0) (setq total (+ total number))
(setq number (1= number))))
(let ((total 0)) (while (> number 0) (setq total ...)
(setq number ...)) total))
triangle-bugged(4)
eval((triangle-bugged 4))
eval-last-sexp(nil)
* call-interactively(eval-last-sexp)
---------- Buffer: *Backtrace* ----------
(I have reformatted this example slightly; the debugger does not fold long lines.)
You read the `*Backtrace*' buffer
from the bottom up; it tells you what Emacs did that led to
the error. In this case, what Emacs did was make an
interactive call to C-x C-e
(eval-last-sexp), which led to the evaluation of
the triangle-bugged expression. Each line above
tells you what the Lisp interpreter evaluated next.
The third line from the top of the buffer is
(setq number (1= number))
Emacs tried to evaluate this expression; in order to do so, it tried to evaluate the inner expression shown on the second line from the top:
(1= number)
This is where the error occurred; as the top line says:
Signalling: (void-function 1=)
You can correct the mistake, re-evaluate the function definition, and then run your test again.
If you are reading this in Info, you can now turn off
debug-on-error by setting it to nil:
(setq debug-on-error nil)
debug-on-entry
A second way to start debug on a function is to
enter the debugger when you call the function. You can do this by
calling debug-on-entry.
Type:
M-x debug-on-entry RET triangle-bugged RET
Now, evaluate the following:
(triangle-bugged 5)
Emacs will create a `*Backtrace*' buffer and tell you
that it is beginning to evaluate the triangle-bugged
function:
---------- Buffer: *Backtrace* ---------- Entering: * triangle-bugged(5) eval((triangle-bugged 5)) eval-last-sexp(nil) * call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ----------
In the `*Backtrace*' buffer, type d. Emacs
will evaluate the first expression in
triangle-bugged; the buffer will look like this:
---------- Buffer: *Backtrace* ----------
Beginning evaluation of function call form:
* (let ((total 0)) (while (> number 0) (setq total ...)
(setq number ...)) total))
triangle-bugged(5)
* eval((triangle-bugged 5))
eval-last-sexp(nil)
* call-interactively(eval-last-sexp)
---------- Buffer: *Backtrace* ----------
Now, type d again, eight times, slowly. Each time you type d, Emacs will evaluate another expression in the function definition. Eventually, the buffer will look like this:
---------- Buffer: *Backtrace* ----------
Beginning evaluation of function call form:
* (setq number (1= number)))
* (while (> number 0) (setq total (+ total number))
(setq number (1= number))))
* (let ((total 0)) (while (> number 0)
(setq total ...) (setq number ...)) total))
triangle-bugged(5)
* eval((triangle-bugged 5))
eval-last-sexp(nil)
* call-interactively(eval-last-sexp)
---------- Buffer: *Backtrace* ----------
Finally, after you type d two more times, Emacs will reach the error, and the top two lines of the `*Backtrace*' buffer will look like this:
---------- Buffer: *Backtrace* ---------- Signalling: (void-function 1=) * (1= number)) ... ---------- Buffer: *Backtrace* ----------
By typing d, you were able to step through the function.
You can quit a `*Backtrace*' buffer by typing
q; this quits the trace, but does not cancel
debug-on-entry.
To cancel the effect of
debug-on-entry, call
cancel-debug-on-entry and the name of the
function, like this:
M-x cancel-debug-on-entry RET triangle-debugged RET
(If you are reading this in Info, cancel
debug-on-entry now.)
debug-on-quit and
(debug)
In addition to setting debug-on-error or calling
debug-on-entry, there are two other ways to start
debug.
You can start debug whenever you
type C-g (keyboard-quit) by setting
the variable debug-on-quit to t.
This is useful for debugging infinite loops.
Or, you can insert a line that says
(debug) into your code where you want the
debugger to start, like this:
(defun triangle-bugged (number)
"Return sum of numbers 1 through NUMBER inclusive."
(let ((total 0))
(while (> number 0)
(setq total (+ total number))
(debug) ; Start debugger.
(setq number (1= number))) ; Error here.
total))
The debug function is described in detail in section
`The Lisp Debugger' in The GNU Emacs Lisp Reference
Manual.
edebug Source Level
Debugger
Edebug normally displays the source of the code you are debugging, with an arrow at the left that shows which line you are currently executing.
You can walk through the execution of a function, line by line, or run quickly until reaching a breakpoint where execution stops.
Edebug is described in section `Edebug' in The GNU Emacs Lisp Reference Manual.
Here is a bugged function definition for
triangle-recursively. See section 11.2.2 Recursion in Place of a
Counter, for a review of it. This example is presented
without indentation to the left of the defun, as
explained below.
(defun triangle-recursively-bugged (number)
"Return sum of numbers 1 through NUMBER inclusive.
Uses recursion."
(if (= number 1)
1
(+ number
(triangle-recursively-bugged
(1= number))))) ; Error here.
Normally, you would install this definition by positioning your
cursor after the function's closing parenthesis and typing
C-x C-e (eval-last-sexp) or else by
positioning your cursor within the definition and typing
C-M-x (eval-defun). (By default, the
eval-defun command works only in Emacs Lisp mode or
in Lisp Interactive mode.)
However, to prepare this function definition for Edebug, you must first instrument the code using a different command. In Emacs version 19, you can do this by positioning your cursor within the definition and typing the following:
M-x edebug-defun RET
This will cause Emacs to load Edebug automatically if it is not
already loaded, and properly instrument the function. (After
loading Edebug, you can use its standard keybindings, such as
C-u C-M-x (eval-defun with a prefix
argument) for edebug-defun.)
In Emacs version 18, you need to load Edebug yourself; you can do
this by putting the appropriate load command in your
`.emacs' file.
If you are reading this in Info, you can instrument the
triangle-recursively-bugged function shown above.
edebug-defun fails to locate the bounds of a
definition whose defun line is indented; so the
example is presented without the usual spaces to the left of the
defun.
After instrumenting the function, place your cursor after the
following expression and type C-x C-e
(eval-last-sexp):
(triangle-recursively-bugged 3)
You will be jumped back to the source for
triangle-recursively-bugged and the cursor
positioned at the beginning of the if line of the
function. Also, you will see an arrow at the left hand side of
that line that looks like this: `=>'. The arrow
marks the line where the function is executing.
=>-!-(if (= number 1)
In the example, the location of point is displayed with a star, `-!-' (in Info, it is displayed as `-!-').
If you now press SPC, point will move to the next expression to be executed; the line will look like this:
=>(if -!-(= number 1)
As you continue to press SPC, point will move from
expression to expression. At the same time, whenever an
expression returns a value, that value will be displayed in the
echo area. For example, after you move point past
number, you will see the following:
Result: 3 = C-c
This means the value of number is 3, which is ASCII
CTL-C, the third ASCII code.
You can continue moving through the code until you reach the line with the error. Before evaluation, that line looks like this:
=> -!-(1= number))))) ; Error here.
When you press SPC once again, you will produce an error message that says:
Symbol's function definition is void: 1=
This is the bug.
Press `q' to quit Edebug.
To remove instrumentation from a function definition, simply re-evaluate it with a command that does not instrument it. For example, you could place your cursor after the definition's closing parenthesis and type C-x C-e.
Edebug does a great deal more than walk with you through a function. You can set it so it races through on its own, stopping only at an error or at specified stopping points; you can cause it to display the changing values of various expressions; you can find out how many times a function is called, and more.
Edebug is described in section `Edebug' in The GNU Emacs Lisp Reference Manual.
Install the count-words-region function and then
cause it to enter the built-in debugger when you call it. Run the
command on a region containing two words. You will need to press
d a remarkable number of times. On your system, is a
`hook' called after the command finishes? (For information on
hooks, see section `Command Loop Overview' in The GNU Emacs
Lisp Reference Manual.)
Copy count-words-region into the
`*scratch*' buffer, remove white space before the
defun line if necessary, instrument the function for
Edebug, and walk through its execution. The function does not
need to have a bug, although you can introduce one if you wish.
If the function lacks a bug, the walk-through completes without
problems.
While running Edebug, type ? to see a list of all the
Edebug commands. (The global-edebug-prefix is
usually C-x X, i.e. CTL-x
followed by an upper case X; use this prefix for
commands made outside of the Edebug debugging buffer.)
In the Edebug debugging buffer, use the p
(edebug-bounce-point) command to see where in the
region the count-words-region is working.
Move point to some spot further down function and then type the
h (edebug-goto-here) command to jump to
that location.
Use the t (edebug-trace-mode) command to
cause Edebug to walk through the function on its own; use an
upper case T for edebug-Trace-fast-mode.
Set a breakpoint, then run Edebug in Trace mode until it reaches the stopping point.
We have now reached the end of this Introduction. You have now learned enough about programming in Emacs Lisp to set values, to write simple `.emacs' files for yourself and your friends, and write simple customizations and extensions to Emacs.
This is a place to stop. Or, if you wish, you can now go onward, and teach yourself.
You have learned some of the basic nuts and bolts of programming. But only some. There are a great many more brackets and hinges that are easy to use that we have not touched.
A path you can follow right now lies among the sources to GNU Emacs and in The GNU Emacs Lisp Reference Manual.
The Emacs Lisp sources are an adventure. When you read the sources and come across a function or expression that is unfamiliar, you need to figure out or find out what it does.
Go to the Reference Manual. It is a through, complete, and fairly easy-to-read description of Emacs Lisp. It is written not only for experts, but for people who know what you know. (The Reference Manual comes with the standard GNU Emacs distribution. Like this introduction, it comes as a Texinfo source file, so you can read it on-line and as a typeset, printed book.)
Go to the other on-line help that is part of GNU Emacs: the
on-line documentation for all functions, and
find-tags, the program that takes you to sources.
Here is an example of how I explore the sources. Because of its
name, `simple.el' is the file I looked at first, a long
time ago. As it happens some of the functions in
`simple.el' are complicated, or at least look
complicated at first sight. The first function, for example,
looks complicated. This is the open-line function.
You may want to walk through this function slowly, as we did with
the forward-sentence function. (See section 12.3
forward-sentence.) Or you may want to skip that
function and look at another, such as split-line.
You don't need to read all the functions. According to
count-words-in-defun, the split-line
function contains 27 words and symbols.
Even though it is short, split-line contains four
expressions we have not studied: skip-chars-forward,
indent-to, insert, and
`?\n'.
Consider the insert function. (It is mentioned in
passing in the review section in section 12 Regular Expression
Searches.) In Emacs, you can find out more about
insert by typing C-h f
(describe-function) and the name of the function.
This gives you the function documentation. You can look at its
source using find-tag, which is bound to
M-. (this is not so helpful in this case; the function
is a primitive written in C rather than Lisp). Finally, you can
find out what the Reference Manual has to say by visiting the
manual in Info, and typing i (Info-index)
and the name of the function, or by looking up
insert in the index to a printed copy of the manual.
Similarly, you can find out what is meant by `?\n'.
You can try using Info-index with
`?\n'. It turns out that this action won't help; but
don't give up. If you search the index for `\n'
without the `?', you will be taken directly to the
relevant section of the manual. (See section `Character Type' in
The GNU Emacs Lisp Reference Manual.
`?\n' stands for the newline character.)
You may be able to guess what is done by
skip-chars-forward and indent-to; or
you can look them up, too. (Incidentally, the
describe-function function itself is in
`help.el'; it is one of those long, but decipherable
functions. Its definition illustrates how to customize the
interactive expression without using the standard
character codes; and it shows how to create a temporary buffer.)
Other interesting source files include `paragraphs.el', `loaddefs.el', and `loadup.el'. The `paragraphs.el' file includes short, easily understood functions as well as longer ones. The `loaddefs.el' file contains the many standard autoloads and many keymaps. I have never looked at it all; only at parts. `loadup.el' is the file that loads the standard parts of Emacs; it tells you a great deal about how Emacs is built. (See section `Building Emacs' in The GNU Emacs Lisp Reference Manual, for more about building.)
As I said, you have learned some nuts and bolts; however, and
very importantly, we have hardly touched major aspects of
programming; I have said nothing about how to sort information,
except to use the predefined sort function; I have
nothing about how to store information, except to use variables
and lists; I have said nothing about how to write programs that
write programs. These are topics for another, and different kind
of book, a different kind of learning.
What you have done is learn enough for much practical work with GNU Emacs. What you have done is get started. This is the end of a beginning.
the-the Function
Sometimes when you you write text, you duplicate words--as with
"you you" near the beginning of this sentence. I find that most
frequently, I duplicate "the'; hence, I call the function for
detecting duplicated words, the-the.
As a first step, you could use the following regular expression to search for duplicates:
\\(\\w+[ \t\n]+\\)\\1
This regexp matches one or more word-constituent characters followed by one or more spaces, tabs, or newlines. However, it does not detect duplicated words on different lines, since the ending of the first word, the end of the line, is different from the ending of the second word, a space. (For more information about regular expressions, see section 12 Regular Expression Searches, as well as section `Syntax of Regular Expressions' in The GNU Emacs Manual, and section `Regular Expressions' in The GNU Emacs Lisp Reference Manual.)
You might try searching just for duplicated word-constituent characters but that does not work since the pattern detects doubles such as the two occurrences of `th' in `with the'.
Another possible regular expression is for word-constituent characters that are followed by non-word-constituent characters. Here, `\\w+' matches one or more word-constituent characters and `\\W*' matches zero or more non-word-constituent characters.
\\(\\(\\w+\\)\\W*\\)\\1
Again, not useful.
Here is the pattern that I use. It is not perfect, but good enough. `\\b' matches the empty string, provided it is at the beginning or end of a word; `[^@ \n\t]+' matches one or more occurrences of any characters that are not an @-sign, space, newline, or tab.
\\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b
One can write more complicated expressions, but I found that this expression is good enough, so I use it.
Here is the the-the function, as I include it in my
`.emacs' file, along with a handy global key binding:
(defun the-the ()
"Search forward for for a duplicated word."
(interactive)
(message "Searching for for duplicated words ...")
(push-mark)
;; This regexp is not perfect
;; but is fairly good over all:
(if (re-search-forward
"\\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b" nil 'move)
(message "Found duplicated word.")
(message "End of buffer")))
;; Bind `the-the' to C-c \
(global-set-key "\C-c\\" 'the-the)
Here is a test list:
one two two three four five five six seven
You can substitute the other regular expressions shown above in the function definition and try each of them on this list.
The kill ring is a list that is transformed into a ring by the
workings of the rotate-yank-pointer function. The
yank and yank-pop commands use the
rotate-yank-pointer function. This appendix
describes the rotate-yank-pointer function as well
as both the yank and the yank-pop
commands.
rotate-yank-pointer
Function
The rotate-yank-pointer function changes the element
in the kill ring to which kill-ring-yank-pointer
points. For example, it can change
kill-ring-yank-pointer from pointing to the second
element to point to the third element.
Here is the code for rotate-yank-pointer:
(defun rotate-yank-pointer (arg)
"Rotate the yanking point in the kill ring."
(interactive "p")
(let ((length (length kill-ring)))
(if (zerop length)
;; then-part
(error "Kill ring is empty")
;; else-part
(setq kill-ring-yank-pointer
(nthcdr (% (+ arg
(- length
(length
kill-ring-yank-pointer)))
length)
kill-ring)))))
The function looks complex, but as usual, it can be understood by taking it apart piece by piece. First look at it in skeletal form:
(defun rotate-yank-pointer (arg)
"Rotate the yanking point in the kill ring."
(interactive "p")
(let varlist
body...)
This function takes one argument, called arg. It has
a brief documentation string; and it is interactive with a small
`p', which means that the argument must be a
processed prefix passed to the function as a number.
The body of the function definition is a let
expression, which itself has a body as well as a
varlist.
The let expression declares a variable that will be
only usable within the bounds of this function. This variable is
called length and is bound to a value that is equal
to the number of items in the kill ring. This is done by using
the function called length. (Note that this function
has the same name as the variable called length; but
one use of the word is to name the function and the other is to
name the variable. The two are quite distinct. Similarly, an
English speaker will distinguish between the meanings of the word
`ship' when he says: "I must ship this package
immediately." and "I must get aboard the ship immediately.")
The function length tells the number of items there
are in a list, so (length kill-ring) returns the
number of items there are in the kill ring.
rotate-yank-pointer
The body of rotate-yank-pointer is a
let expression and the body of the let
expression is an if expression.
The purpose of the if expression is to find out
whether there is anything in the kill ring. If the kill ring is
empty, the error function stops evaluation of the
function and prints a message in the echo area. On the other
hand, if the kill ring has something in it, the work of the
function is done.
Here is the if-part and then-part of the if
expression:
(if (zerop length) ; if-part
(error "Kill ring is empty") ; then-part
...
If there is not anything in the kill ring, its length must be
zero and an error message sent to the user: `Kill ring is
empty'. The if expression uses the function
zerop which returns true if the value it is testing
is zero. When zerop tests true, the then-part of the
if is evaluated. The then-part is a list starting
with the function error, which is a function that is
similar to the message function (see section
1.8.5 The
message Function), in that it prints a one-line
message in the echo area. However, in addition to printing a
message, error also stops evaluation of the function
within which it is embedded. In this case, this means that the
rest of the function will not be evaluated if the length of the
kill ring is zero.
(In my opinion, it is slightly misleading, at least to humans, to use the term `error' as the name of this function. A better term would be `cancel'. Strictly speaking, of course, you cannot point to, much less rotate a pointer to a list that has no length, so from the point of view of the computer, the word `error' is correct. But a human expects to attempt this sort of thing, if only to find out whether the kill ring is full or empty. This is an act of exploration.
(From the human point of view, the act of exploration and discovery is not necessarily an error, and therefore should not be labeled as one, even in the bowels of a computer. As it is, the code in Emacs implies that a human who is acting virtuously, by exploring his or her environment, is making an error. This is bad. Even though the computer takes the same steps as it does when there is an `error', a term such as `cancel' would have a clearer connotation.)
if
expression
The else-part of the if expression is dedicated to
setting the value of kill-ring-yank-pointer when the
kill ring has something in it. The code looks like this:
(setq kill-ring-yank-pointer
(nthcdr (% (+ arg
(- length
(length kill-ring-yank-pointer)))
length)
kill-ring)))))
This needs some examination. Clearly,
kill-ring-yank-pointer is being set to be equal to
some CDR of the kill ring, using the nthcdr function
that is described in an earlier section. (See section 8.5
copy-region-as-kill.) But exactly how does it do
this?
Before looking at the details of the code let's first consider
the purpose of the rotate-yank-pointer function.
The rotate-yank-pointer function changes what
kill-ring-yank-pointer points to. If
kill-ring-yank-pointer starts by pointing to the
first element of a list, a call to
rotate-yank-pointer causes it to point to the second
element; and if kill-ring-yank-pointer points to the
second element, a call to rotate-yank-pointer causes
it to point to the third element. (And if
rotate-yank-pointer is given an argument greater
than 1, it jumps the pointer that many elements.)
The rotate-yank-pointer function uses
setq to reset what the
kill-ring-yank-pointer points to. If
kill-ring-yank-pointer points to the first element
of the kill ring, then, in the simplest case, the
rotate-yank-pointer function must cause it to point
to the second element. Put another way,
kill-ring-yank-pointer must be reset to have a value
equal to the CDR of the kill ring.
That is, under these circumstances,
(setq kill-ring-yank-pointer
("some text" "a different piece of text" "yet more text"))
(setq kill-ring
("some text" "a different piece of text" "yet more text"))
the code should do this:
(setq kill-ring-yank-pointer (cdr kill-ring))
As a result, the kill-ring-yank-pointer will look
like this:
kill-ring-yank-pointer
=> ("a different piece of text" "yet more text"))
The actual setq expression uses the
nthcdr function to do the job.
As we have seen before (see section 7.3 nthcdr), the
nthcdr function works by repeatedly taking the CDR
of a list--it takes the CDR of the CDR of the CDR ...
The two following expressions produce the same result:
(setq kill-ring-yank-pointer (cdr kill-ring)) (setq kill-ring-yank-pointer (nthcdr 1 kill-ring))
In the rotate-yank-pointer function, however, the
first argument to nthcdr is a rather complex looking
expression with lots of arithmetic inside of it:
(% (+ arg
(- length
(length kill-ring-yank-pointer)))
length)
As usual, we need to look at the most deeply embedded expression first and then work our way towards the light.
The most deeply embedded expression is (length
kill-ring-yank-pointer). This finds the length of the
current value of the kill-ring-yank-pointer.
(Remember that the kill-ring-yank-pointer is the
name of a variable whose value is a list.)
The measurement of the length is inside the expression:
(- length (length kill-ring-yank-pointer))
In this expression, the first length is the variable
that was assigned the length of the kill ring in the
let statement at the beginning of the function. (One
might think this function would be clearer if the variable
length were named length-of-kill-ring
instead; but if you look at the text of the whole function, you
will see that it is so short that naming this variable
length is not a bother, unless you are pulling the
function apart into very tiny pieces as we are doing here.)
So the line (- length (length
kill-ring-yank-pointer)) tells the difference between the
length of the kill ring and the length of the list whose name is
kill-ring-yank-pointer.
To see how all this fits into the
rotate-yank-pointer function, let's begin by
analyzing the case where kill-ring-yank-pointer
points to the first element of the kill ring, just as
kill-ring does, and see what happens when
rotate-yank-pointer is called with an argument of 1.
In this case, the variable length and the value of
the expression (length kill-ring-yank-pointer will
be the same since the variable length is the length
of the kill ring and the kill-ring-yank-pointer is
pointing to the whole kill ring. Consequently, the value of
(- length (length kill-ring-yank-pointer))
will be zero. Since the value of arg will be 1, this
will mean that the value of the whole expression
(+ arg (- length (length kill-ring-yank-pointer)))
will be 1.
Consequently, the argument to nthcdr will be found
as the result of the expression
(% 1 length)
% remainder function
To understand (% 1 length), we need to understand
%. According to its documentation (which I just
found by typing C-h f % RET), the
% function returns the remainder of its first
argument divided by its second argument. For example, the
remainder of 5 divided by 2 is 1. (2 goes into 5 twice with a
remainder of 1.)
What surprises people who don't often do arithmetic is that a smaller number can be divided by a larger number and have a remainder. In the example we just used, 5 was divided by 2. We can reverse that and ask, what is the result of dividing 2 by 5? If you can use fractions, the answer is obviously 2/5 or .4; but if, as here, you can only use whole numbers, the result has to be something different. Clearly, 5 can go into 2 zero times, but what of the remainder? To see what the answer is, consider a case that has to be familiar from childhood:
By considering the cases as parallel, we can see that
and so on.
So, in this code, if the value of length is 5, then
the result of evaluating
(% 1 5)
is 1. (I just checked this by placing the cursor after the expression and typing C-x C-e. Indeed, 1 is printed in the echo area.)
% in
rotate-yank-pointer
When the kill-ring-yank-pointer points to the
beginning of the kill ring, and the argument passed to
rotate-yank-pointer is 1, the %
expression returns 1:
(- length (length kill-ring-yank-pointer))
=> 0
therefore,
(+ arg (- length (length kill-ring-yank-pointer)))
=> 1
and consequently:
(% (+ arg (- length (length kill-ring-yank-pointer)))
length)
=> 1
regardless of the value of length.
As a result of this, the setq kill-ring-yank-pointer
expression simplifies to:
(setq kill-ring-yank-pointer (nthcdr 1 kill-ring))
What it does is now easy to understand. Instead of pointing as it
did to the first element of the kill ring, the
kill-ring-yank-pointer is set to point to the second
element.
Clearly, if the argument passed to
rotate-yank-pointer is two, then the
kill-ring-yank-pointer is set to (nthcdr 2
kill-ring); and so on for different values of the
argument.
Similarly, if the kill-ring-yank-pointer starts out
pointing to the second element of the kill ring, it length is
shorter than the length of the kill ring by 1, so the computation
of the remainder is based on the expression (% (+ arg 1)
length). This means that the
kill-ring-yank-pointer is moved from the second
element of the kill ring to the third element if the argument
passed to rotate-yank-pointer is 1.
The final question is, what happens if the
kill-ring-yank-pointer is set to the last
element of the kill ring? Will a call to
rotate-yank-pointer mean that nothing more can be
taken from the kill ring? The answer is no. What happens is
different and useful. The kill-ring-yank-pointer is
set to point to the beginning of the kill ring instead.
Let's see how this works by looking at the code, assuming the
length of the kill ring is 5 and the argument passed to
rotate-yank-pointer is 1. When the
kill-ring-yank-pointer points to the last element of
the kill ring, its length is 1. The code looks like this:
(% (+ arg (- length (length kill-ring-yank-pointer))) length)
When the variables are replaced by their numeric values, the expression looks like this:
(% (+ 1 (- 5 1)) 5)
This expression can be evaluated by looking at the most embedded
inner expression first and working outwards: The value of
(- 5 1) is 4; the sum of (+ 1 4) is 5;
and the remainder of dividing 5 by 5 is zero. So what
rotate-yank-pointer will do is
(setq kill-ring-yank-pointer (nthcdr 0 kill-ring))
which will set the kill-ring-yank-pointer to point
to the beginning of the kill ring.
So what happens with successive calls to
rotate-yank-pointer is that it moves the
kill-ring-yank-pointer from element to element in
the kill ring until it reaches the end; then it jumps back to the
beginning. And this is why the kill ring is called a ring, since
by jumping back to the beginning, it is as if the list has no
end! (And what is a ring, but an entity with no end?)
yank
After learning about rotate-yank-pointer, the code
for the yank function is almost easy. It has only
one tricky part, which is the computation of the argument to be
passed to rotate-yank-pointer.
The code looks like this:
(defun yank (&optional arg)
"Reinsert the last stretch of killed text.
More precisely, reinsert the stretch of killed text most
recently killed OR yanked.
With just C-U as argument, same but put point in front
(and mark at end). With argument n, reinsert the nth
most recently killed stretch of killed text.
See also the command \\[yank-pop]."
(interactive "*P")
(rotate-yank-pointer (if (listp arg) 0
(if (eq arg '-) -1
(1- arg))))
(push-mark (point))
(insert (car kill-ring-yank-pointer))
(if (consp arg)
(exchange-point-and-mark)))
Glancing over this code, we can understand the last few lines
readily enough. The mark is pushed, that is, remembered; then the
first element (the CAR) of what the
kill-ring-yank-pointer points to is inserted; and
then, if the argument passed the function is a cons,
point and mark are exchanged so the point is put in the front of
the inserted text rather than at the end. This option is
explained in the documentation. The function itself is
interactive with "*P". This means it will not work
on a read-only buffer, and that the unprocessed prefix argument
is passed to the function.
The hard part of yank is understanding the
computation that determines the value of the argument passed to
rotate-yank-pointer. Fortunately, it is not so
difficult as it looks at first sight.
What happens is that the result of evaluating one or both of the
if expressions will be a number and that number will
be the argument passed to rotate-yank-pointer.
Laid out with comments, the code looks like this:
(if (listp arg) ; if-part
0 ; then-part
(if (eq arg '-) ; else-part, inner if
-1 ; inner if's then-part
(1- arg)))) ; inner if's else-part
This code consists of two if expression, one the
else-part of the other.
The first or outer if expression tests whether the
argument passed to yank is a list. Oddly enough,
this will be true if yank is called without an
argument--because then it will be passed the value of
nil for the optional argument and an evaluation of
(listp nil) returns true! So, if no argument is
passed to yank, the argument passed to
rotate-yank-pointer inside of yank is
zero. This means the pointer is not moved and the first element
to which kill-ring-yank-pointer points is inserted,
as we expect. Similarly, if the argument for yank is
C-u, this will be read as a list, so again, a zero
will be passed to rotate-yank-pointer.
(C-u produces an unprocessed prefix argument of
(4), which is a list of one element.) At the same
time, later in the function, this argument will be read as a
cons so point will be put in the front and mark at
the end of the insertion. (The P argument to
interactive is designed to provide these values for
the case when an optional argument is not provided or when it is
C-u.)
The then-part of the outer if expression handles the
case then there is no argument or when it is C-u. The
else-part handles the other situations. The else-part is itself
another if expression.
The inner if expression tests whether the argument
is a minus sign. (This is done by pressing the META
and - keys at the same time, or the ESC key
and then the - key). In this case, the
rotate-yank-pointer function is passed -1
as an argument. This moves the
kill-ring-yank-pointer backwards, which is what is
desired.
If the true-or-false-test of the inner if expression
is false (that is, if the argument is not a minus sign), the
else-part of the expression is evaluated. This is the expression
(1- arg). Because of the two if
expressions, it will only occur when the argument is a positive
number or when it is a negative number (not just a minus sign on
its own). What (1- arg) does is decrement the number
and return it. (The 1- function subtracts one from
its argument.) This means that if the argument to
rotate-yank-pointer is 1, it is reduced to zero,
which means the first element to which
kill-ring-yank-pointer points is yanked back, as you
would expect.
Finally, the question arises, what happens if either the
remainder function, %, or the nthcdr
function is passed a negative argument, as they quite well may?
The answers can be found by a quick test. When (% -1
5) is evaluated, a negative number is returned; and if
nthcdr is called with a negative number, it returns
the same value as if it were called with a first argument of
zero. This can be seen be evaluating the following code.
Here the `=>' points to the result of evaluating
the code preceding it. This was done by positioning the cursor
after the code and typing C-x C-e
(eval-last-sexp) in the usual fashion. You can do
this if you are reading this in Info inside of GNU Emacs.
(% -1 5)
=> -1
(setq animals '(cats dogs elephants))
=> (cats dogs elephants)
(nthcdr 1 animals)
=> (dogs elephants)
(nthcdr 0 animals)
=> (cats dogs elephants)
(nthcdr -1 animals)
=> (cats dogs elephants)
So, if a minus sign or a negative number is passed to
yank, the kill-ring-yank-point is
rotated backwards until it reaches the beginning of the list.
Then it stays there. Unlike the other case, when it jumps from
the end of the list to the beginning of the list, making a ring,
it stops. This makes sense. You often want to get back to the
most recently clipped out piece of text, but you don't usually
want to insert text from as many as thirty kill commands ago. So
you need to work through the ring to get to the end, but won't
cycle around it inadvertently if you are trying to come back to
the beginning.
Incidentally, any number passed to yank with a minus
sign preceding it will be treated as -1. This is evidently a
simplification for writing the program. You don't need to jump
back towards the beginning of the kill ring more than one place
at a time and doing this is easier than writing a function to
determine the magnitude of the number that follows the minus
sign.
yank-pop
After understanding yank, the yank-pop
function is easy. Leaving out the documentation to save space, it
looks like this:
(defun yank-pop (arg)
(interactive "*p")
(if (not (eq last-command 'yank))
(error "Previous command was not a yank"))
(setq this-command 'yank)
(let ((before (< (point) (mark))))
(delete-region (point) (mark))
(rotate-yank-pointer arg)
(set-mark (point))
(insert (car kill-ring-yank-pointer))
(if before (exchange-point-and-mark))))
The function is interactive with a small `p' so the
prefix argument is processed and passed to the function. The
command can only be used after a previous yank; otherwise an
error message is sent. This check uses the variable
last-command which is discussed elsewhere. (See
section 8.5
copy-region-as-kill.)
The let clause sets the variable before
to true or false depending whether point is before or after mark
and then the region between point and mark is deleted. This is
the region that was just inserted by the previous yank and it is
this text that will be replaced. Next the
kill-ring-yank-pointer is rotated so that the
previously inserted text is not reinserted yet again. Mark is set
at the beginning of the place the new text will be inserted and
then the first element to which
kill-ring-yank-pointer points is inserted. This
leaves point after the new text. If in the previous yank, point
was left before the inserted text, point and mark are now
exchanged so point is again left in front of the newly inserted
text. That is all there is to it!
Printed axes help you understand a graph. They convey scale. In an earlier chapter (see section 15 Readying a Graph), we wrote the code to print the body of a graph. Here we write the code for print and labelling vertical and horizontal axes, along with the body itself.
Since insertions fill a buffer to the right and below point, the new graph printing function should first print the Y or vertical axis, then the body of the graph, and finally the X or horizontal axis. This sequence lays out for us the contents of the function:
Here is an example of how a finished graph should look:
10 -
*
* *
* **
* ***
5 - * *******
* *** *******
*************
***************
1 - ****************
| | | |
1 5 10 15
In this graph, both the vertical and the horizontal axes are labeled with numbers. However, in some graphs, the horizontal axis is time and would be better labeled with months, like this:
5 - *
* ** *
*******
********** **
1 - **************
| ^ |
Jan June Jan
Indeed, with a little thought, we can easily come up with a variety of vertical and horizontal labelling schemes. Our task could become complicated. But complications breed confusion. Rather than permit this, it is better choose a simple labelling scheme for our first effort, and to modify or replace it later.
These considerations suggest the following outline for the
print-graph function:
(defun print-graph (numbers-list)
"documentation..."
(let ((height ...
...))
(print-Y-axis height ... )
(graph-body-print numbers-list)
(print-X-axis ... )))
We can work on each part of the print-graph function
definition in turn.
print-graph Varlist
In writing the print-graph function, the first task
is to write the varlist in the let expression. (We
will leave aside for the moment any thoughts about making the
function interactive or about the contents of its documentation
string.)
The varlist should set several values. Clearly, the top of the
label for the vertical axis must be at least the height of the
graph, which means that we must obtain this information here.
Note that the print-graph-body function also
requires this information. There is no reason to calculate the
height of the graph in two different places, so we should change
for print-graph-body from the way we defined it
earlier to take advantage of the calculation.
Similarly, both the function for printing the X axis labels and
the print-graph-body function need to learn the
value of the width of each symbol. We can perform the calculation
here and change the definition for print-graph-body
from the way we defined it in the previous chapter.
The length of the label for the horizontal axis must be at least as long as the graph. However, this information is used only in the function that prints the horizontal axis, so it does not need to be calculated here.
These thoughts lead us directly to the following form for the
varlist in the let for print-graph:
(let ((height (apply 'max numbers-list)) ; First version.
(symbol-width (length graph-blank)))
As we shall see, this expression is not quite right.
print-Y-axis Function
The job of the print-Y-axis function is to print a
label for the vertical axis that looks like this:
10 -
5 -
1 -
The function should be passed the height of the graph, and then should construct and insert the appropriate numbers and marks.
It is easy enough to see in the figure what the Y axis label should look like; but to say in words, and then to write a function definition to do the job is another matter. It is not quite true to say that we want a number and a tick every five lines: there are only three lines between the `1' and the `5' (lines 2, 3, and 4), but four lines between the `5' and the `10' (lines 6, 7, 8, and 9). It is better to say that we want a number and a tick mark on the base line (number 1) and then that we want a number and a tick on the fifth line from the bottom and on every line that is a multiple of five.
The next issue is what height the label should be. Suppose the
maximum height of tallest column of the graph is seven. Should
the highest label on the Y axis be `5 -', and should
the graph stick up above the label? Or should the highest label
be `7 -', and mark the peak of the graph? Or should
the highest label be 10 -, which is a multiple of
five, and be higher than the topmost value of the graph?
The latter form is preferred. Most graphs are drawn within
rectangles whose sides are an integral number of steps long--5,
10, 15, and so on for a step distance of five. But as soon as we
decide to use a step height for the vertical axis, we discover
that the simple expression in the varlist for computing the
height is wrong. The expression is (apply 'max
numbers-list). This returns the precise height, not the
maximum height plus whatever is necessary to round up to the
nearest multiple of five. A more complex expression is required.
As usual in cases like this, a complex problem becomes simpler if it is divided into several smaller problems.
First, consider the case when the highest value of the graph is an integral multiple of five--when it is 5, 10, 15 ,or some higher multiple of five. In this case, we can use this value as the Y axis height.
A fairly simply way to determine whether a number is a multiple of five is to divide it by five and see if the division results in a remainder. If there is no remainder, the number is a multiple of five. Thus, seven divided by five has a remainder of two, and seven is not an integral multiple of five. Put in slightly different language, more reminiscent of the classroom, five goes into seven once, with a remainder of two. However, five goes into ten twice, with no remainder: ten is an integral multiple of five.
In Lisp, the function for computing a
remainder is %. The function returns the
remainder of its first argument divided by its second
argument. As it happens, % is a function in Emacs
Lisp that you cannot discover using apropos: you
find nothing if you type M-x apropos RET remainder
RET. The only way to learn of the existence of
% is to read about it in a book such as this or
in the Emacs Lisp sources. The % function is used
in the code for rotate-yank-pointer, which is
described in an appendix. (See section B.1.1 The Body of
rotate-yank-pointer.)
You can try the % function by evaluating the
following two expressions:
(% 7 5) (% 10 5)
The first expression returns 2 and the second expression returns 0.
To test whether the returned value is zero or some other number,
we can use the zerop function. This function returns
t if its argument, which must be a number, is zero.
(zerop (% 7 5))
=> nil
(zerop (% 10 5))
=> t
Thus, the following expression will return t if the
height of the graph is evenly divisible by five:
(zerop (% height 5))
(The value of height, of course, can be found from
(apply 'max numbers-list).)
On the other hand, if the value of height is not a
multiple of five, we want to reset the value to the next higher
multiple of five. This is straightforward arithmetic using
functions with which we are already familiar. First, we divide
the value of height by five to determine how many
times five goes into the number. Thus, five goes into twelve
twice. If we add one to this quotient and multiply by five, we
will obtain the value of the next multiple of five that is larger
than the height. Five goes into twelve twice. Add one to two, and
multiply by five; the result is fifteen, with is the next
multiple of five that is higher than twelve. The Lisp expression
for this is:
(* (1+ (/ height 5)) 5)
For example, if you evaluate the following, the result is 15:
(* (1+ (/ 12 5)) 5)
All through this discussion, we have been using `five' as the
value for spacing labels on the Y axis; but we may want to use
some other value. For generality, we should replace `five' with a
variable to which we can assign a value. The best name I can
think of for this variable is Y-axis-label-spacing.
Using this term, and an if expression, we produce
the following:
(if (zerop (% height Y-axis-label-spacing))
height
;; else
(* (1+ (/ height Y-axis-label-spacing))
Y-axis-label-spacing))
This expression returns the value of height itself
if the height is an even multiple of the value of the
Y-axis-label-spacing or else it computes and returns
a value of height that is equal to the next higher
multiple of the value of the Y-axis-label-spacing.
We can now include this expression in the let
expression of the print-graph function (after first
setting the value of Y-axis-label-spacing):
(defvar Y-axis-label-spacing 5
"Number of lines from one Y axis label to next.")
...
(let* ((height (apply 'max numbers-list))
(height-of-top-line
(if (zerop (% height Y-axis-label-spacing))
height
;; else
(* (1+ (/ height Y-axis-label-spacing))
Y-axis-label-spacing)))
(symbol-width (length graph-blank))))
...
(Note use of the let* function: the initial value of
height is computed once by the (apply 'max
numbers-list) expression and then the resulting value of
height is used to compute its final value. See
section The
let* expression, for more about
let*.)
When we print the vertical axis, we want to insert strings such as `5 -' and `10 - ' every five lines. Moreover, we want the numbers and dashes to line up, so shorter numbers must be padded with leading spaces. If some of the strings use two digit numbers, the strings with single digit numbers must include a leading blank space before the number.
To figure out the length of the number, the
length function is used. But the
length function works only with a string, not
with a number. So the number has to be converted from being a
number to being a string. This is done with the
int-to-string function. For example,
(length (int-to-string 35))
=> 2
(length (int-to-string 100))
=> 3
In addition, in each label, each number is followed by a string
such as ` - ', which we will call the
Y-axis-tic marker. This variable is defined with
defvar:
(defvar Y-axis-tic " - " "String that follows number in a Y axis label.")
The length of the Y label is the sum of the length of the Y axis tick mark and the length of the number of the top of the graph.
(length (concat (int-to-string height) Y-axis-tic)))
This value will be calculated by the print-graph
function in its varlist as full-Y-label-width and
passed on. (Note that we did not think to include this in the
varlist when we first proposed it.)
To make a complete vertical axis label, a tick mark is
concatinated with a number; and the two together may be preceded
by one or more spaces depending on how long the number is. The
label consists of three parts: the (optional) leading spaces, the
number, and the tic mark. The function is passed the value of the
number for the specific row, and the value of the width of the
top line, which is calculated (just once) by
print-graph.
(defun Y-axis-element (number full-Y-label-width)
"Construct a NUMBERed label element.
A numbered element looks like this ` 5 - ',
and is padded as needed so all line up with
the element for the largest number."
(let* ((leading-spaces
(- full-Y-label-width
(length
(concat (int-to-string number)
Y-axis-tic)))))
(concat
(make-string leading-spaces ? )
(int-to-string number)
Y-axis-tic)))
The Y-axis-element function concatinates together
the leading spaces, if any; the number, as a string; and the tick
mark.
To figure out how many leading spaces the label will need, the function subtracts the actual length of the label--the length of the number plus the length of the tic mark--from the desired label width.
Blank spaces are inserted using the
make-string function. This function takes two
arguments: the first tells it how long the string will be and
the second is a symbol for the character to insert, in a
special format. In this case, the format is a question mark
followed by a blank space, like this, `? '. See
section `Character Type' in The GNU Emacs Lisp Reference
Manual, for a description of the syntax for characters.
The int-to-string function is used in the
concatination expression, to convert the number to a string that
is concatinated with the leading spaces and the tic mark.
The preceding functions provide all the tools needed to construct a function that generates a list of numbered and blank strings to insert as the label for the vertical axis:
(defun Y-axis-column (height width-of-label)
"Construct list of Y axis labels and blank strings.
For HEIGHT of line above base and WIDTH-OF-LABEL."
(let (Y-axis)
(while (> height 1)
(if (zerop (% height Y-axis-label-spacing))
;; Insert label.
(setq Y-axis
(cons
(Y-axis-element height width-of-label)
Y-axis))
;; Else, insert blanks.
(setq Y-axis
(cons
(make-string width-of-label ? )
Y-axis)))
(setq height (1- height)))
;; Insert base line.
(setq Y-axis
(cons (Y-axis-element 1 width-of-label) Y-axis))
(nreverse Y-axis)))
In this function, we start with the value of height
and repetitively subtract one from its value. After each
subtraction, we test to see whether the value is an integral
multiple of the Y-axis-label-spacing. If it is, we
construct a numbered label using the Y-axis-element
function; if not, we construct a blank label using the
make-string function. The base line consists of the
number one followed by a tic mark.
print-Y-axis
The list constructed by the Y-axis-column function
is passed to the print-Y-axis function, which
inserts the list as a column.
(defun print-Y-axis
(height full-Y-label-width &optional vertical-step)
"Insert Y axis using HEIGHT and FULL-Y-LABEL-WIDTH.
Height must be the maximum height of the graph.
Full width is the width of the highest label element.
Optionally, print according to VERTICAL-STEP."
;; Value of height and full-Y-label-width
;; are passed by `print-graph'.
(let ((start (point)))
(insert-rectangle
(Y-axis-column height full-Y-label-width vertical-step))
;; Place point ready for inserting graph.
(goto-char start)
;; Move point forward by value of full-Y-label-width
(forward-char full-Y-label-width)))
The print-Y-axis uses the
insert-rectangle function to insert the Y axis
labels created by the Y-axis-column function. In
addition, it places point at the correct position for printing
the body of the graph.
You can test print-Y-axis:
Y-axis-label-spacing Y-axis-tic Y-axis-element Y-axis-columnprint-Y-axis
(print-Y-axis 12 5)
eval-expression).
graph-body-print expression into the
minibuffer with C-y (yank).
Emacs will print labels vertically, the top one being `10 -
'. (The print-graph function will pass the
value of height-of-top-line, which in this case
would be 15.)
print-X-axis Function
X axis labels are much like Y axis labels, except that the tics are on a line above the numbers. Labels should look like this:
| | | |
1 5 10 15
The first tic is under the first column of the graph and is
preceded by several blank spaces. These spaces provide room in
rows above for the Y axis labels. The second, third, fourth, and
subsequent tics are all spaced equally, according to the value of
X-axis-label-spacing.
The second row of the X axis consists of numbers, preceded by
several blank spaces and also separated according to the value of
the variable X-axis-label-spacing.
The value of the variable X-axis-label-spacing
should itself be measured in units of symbol-width,
since you may want to change the width of the symbols that you
are using to print the body of the graph without changing the
ways the graph is labeled.
The print-X-axis function is constructed in more or
less the same fashion as the print-Y-axis function
except that it has two lines: the line of tic marks and the
numbers. We will write a separate function to print each line and
then combine them within the print-X-axis function.
This is a three step process:
print-X-axis-tic-line.
print-X-axis-numbered-line.
print-X-axis function, using
print-X-axis-tic-line and
print-X-axis-numbered-line.
The first function should print the X axis tic marks. We must specify the tic marks themselves and their spacing:
(defvar X-axis-label-spacing
(if (boundp 'graph-blank)
(* 5 (length graph-blank)) 5)
"Number of units from one X axis label to next.")
(Note that the value of graph-blank is set by
another defvar. The boundp predicate
checks whether it has already been set; boundp
returns nil if it has not. If
graph-blank were unbound and we did not use this
conditional construction, we would receive an error message
saying `Symbol's value as variable is void'.)
(defvar X-axis-tic-symbol "|" "String to insert to point to a column in X axis.")
The goal is to make a line that looks like this:
| | | |
The first tic is indented so that it is under the first column, which is indented to provide space for the Y axis labels.
A tic element consists of the blank spaces that stretch from one
tic to the next plus a tic symbol. The number of blanks is
determined by the width of the tic symbol and the
X-axis-label-spacing.
The code looks like this:
;;; X-axis-tic-element
...
(concat
(make-string
;; Make a string of blanks.
(- (* symbol-width X-axis-label-spacing)
(length X-axis-tic-symbol))
? )
;; Concatinate blanks with tic symbol.
X-axis-tic-symbol)
...
Next, we determine how many blanks are needed to indent the first
tic mark to the first column of the graph. This uses the value of
full-Y-label-width passed it by the
print-graph function.
The code to make X-axis-leading-spaces looks like
this:
;; X-axis-leading-spaces ... (make-string full-Y-label-width ? ) ...
We also need to determine the length of the horizontal axis, which is the length of the numbers list, and the number of tics in the horizontal axis:
;; X-length
...
(length numbers-list)
;; tic-width
...
(* symbol-width X-axis-label-spacing)
;; number-of-X-tics
(if (zerop (% (X-length tic-width)))
(/ (X-length tic-width))
(1+ (/ (X-length tic-width))))
All this leads us directly to the function for printing the X axis tic line:
(defun print-X-axis-tic-line
(number-of-X-tics X-axis-leading-spaces X-axis-tic-element)
"Print tics for X axis."
(insert X-axis-leading-spaces)
(insert X-axis-tic-symbol) ; Under first column.
;; Insert second tic in the right spot.
(insert (concat
(make-string
(- (* symbol-width X-axis-label-spacing)
;; Insert white space up to second tic symbol.
(* 2 (length X-axis-tic-symbol)))
? )
X-axis-tic-symbol))
;; Insert remaining tics.
(while (> number-of-X-tics 1)
(insert X-axis-tic-element)
(setq number-of-X-tics (1- number-of-X-tics))))
The line of numbers is equally straightforward:
First, we create a numbered element with blank spaces before each number:
(defun X-axis-element (number)
"Construct a numbered X axis element."
(let ((leading-spaces
(- (* symbol-width X-axis-label-spacing)
(length (int-to-string number)))))
(concat (make-string leading-spaces ? )
(int-to-string number))))
Next, we create the function to print the numbered line, starting with the number "1" under the first column:
(defun print-X-axis-numbered-line
(number-of-X-tics X-axis-leading-spaces)
"Print line of X-axis numbers"
(let ((number X-axis-label-spacing))
(insert X-axis-leading-spaces)
(insert "1")
(insert (concat
(make-string
;; Insert white space up to next number.
(- (* symbol-width X-axis-label-spacing) 2)
? )
(int-to-string number)))
;; Insert remaining numbers.
(setq number (+ number X-axis-label-spacing))
(while (> number-of-X-tics 1)
(insert (X-axis-element number))
(setq number (+ number X-axis-label-spacing))
(setq number-of-X-tics (1- number-of-X-tics)))))
Finally, we need to write the print-X-axis that uses
print-X-axis-tic-line and
print-X-axis-numbered-line.
The function must determine the local values of the variables
used by both print-X-axis-tic-line and
print-X-axis-numbered-line, and then it must call
them. Also, it must print the carriage return that separates the
two lines.
The function consists of a varlist that specifies five local variables, and calls to each of the two line printing functions:
(defun print-X-axis (numbers-list)
"Print X axis labels to length of NUMBERS-LIST."
(let* ((leading-spaces
(make-string full-Y-label-width ? ))
;; symbol-width is provided by graph-body-print
(tic-width (* symbol-width X-axis-label-spacing))
(X-length (length numbers-list))
(X-tic
(concat
(make-string
;; Make a string of blanks.
(- (* symbol-width X-axis-label-spacing)
(length X-axis-tic-symbol))
? )
;; Concatinate blanks with tic symbol.
X-axis-tic-symbol))
(tic-number
(if (zerop (% X-length tic-width))
(/ X-length tic-width)
(1+ (/ X-length tic-width)))))
(print-X-axis-tic-line tic-number leading-spaces X-tic)
(insert "\n")
(print-X-axis-numbered-line tic-number leading-spaces)))
You can test print-X-axis:
X-axis-tic-symbol,
X-axis-label-spacing,
print-X-axis-tic-line, as well as
X-axis-element,
print-X-axis-numbered-line, and
print-X-axis.
(progn
(let ((full-Y-label-width 5)
(symbol-width 1))
(print-X-axis
'(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16))))
eval-expression).
yank).
Emacs will print the horizontal axis like this:
| | | | |
1 5 10 15 20
Now we are nearly ready to print the whole graph.
The function to print the graph with the proper labels follows the outline we created earlier (see section C A Graph with Labelled Axes), but with additions.
Here is the outline:
(defun print-graph (numbers-list)
"documentation..."
(let ((height ...
...))
(print-Y-axis height ... )
(graph-body-print numbers-list)
(print-X-axis ... )))
The final version is different from what we planned in two ways: first, it contains additional values calculated once in the varlist; second, it carries an option to specify the labels' increment per row. This latter feature turns out to be essential; otherwise a graph may have more rows than fit on a display or on a sheet of paper.
This new feature requires a change to the
Y-axis-column function, to add
vertical-step to it. The function looks like this:
;;; Final version.
(defun Y-axis-column
(height width-of-label &optional vertical-step)
"Construct list of labels for Y axis.
HEIGHT is maximum height of graph.
WIDTH-OF-LABEL is maximum width of label.
VERTICAL-STEP, an option, is a positive integer
that specifies how much a Y axis label increments
for each line. For example, a step of 5 means
that each line is five units of the graph."
(let (Y-axis
(number-per-line (or vertical-step 1)))
(while (> height 1)
(if (zerop (% height Y-axis-label-spacing))
;; Insert label.
(setq Y-axis
(cons
(Y-axis-element
(* height number-per-line)
width-of-label)
Y-axis))
;; Else, insert blanks.
(setq Y-axis
(cons
(make-string width-of-label ? )
Y-axis)))
(setq height (1- height)))
;; Insert base line.
(setq Y-axis (cons (Y-axis-element
(or vertical-step 1)
width-of-label)
Y-axis))
(nreverse Y-axis)))
The values for the maximum height of graph and the width of a
symbol are computed by print-graph in its
let expression; so graph-body-print
must be changed to accept them.
;;; Final version.
(defun graph-body-print (numbers-list height symbol-width)
"Print a bar graph of the NUMBERS-LIST.
The numbers-list consists of the Y-axis values.
HEIGHT is maximum height of graph.
SYMBOL-WIDTH is number of each column."
(let (from-position)
(while numbers-list
(setq from-position (point))
(insert-rectangle
(column-of-graph height (car numbers-list)))
(goto-char from-position)
(forward-char symbol-width)
;; Draw graph column by column.
(sit-for 0)
(setq numbers-list (cdr numbers-list)))
;; Place point for X axis labels.
(forward-line height)
(insert "\n")))
Finally, the code for the print-graph function:
;;; Final version.
(defun print-graph
(numbers-list &optional vertical-step)
"Print labelled bar graph of the NUMBERS-LIST.
The numbers-list consists of the Y-axis values.
Optionally, VERTICAL-STEP, a positive integer,
specifies how much a Y axis label increments for
each line. For example, a step of 5 means that
each row is five units."
(let* ((symbol-width (length graph-blank))
;; height is both the largest number
;; and the number with the most digits.
(height (apply 'max numbers-list))
(height-of-top-line
(if (zerop (% height Y-axis-label-spacing))
height
;; else
(* (1+ (/ height Y-axis-label-spacing))
Y-axis-label-spacing)))
(vertical-step (or vertical-step 1))
(full-Y-label-width
(length
(concat
(int-to-string
(* height-of-top-line vertical-step))
Y-axis-tic))))
(print-Y-axis
height-of-top-line full-Y-label-width vertical-step)
(graph-body-print
numbers-list height-of-top-line symbol-width)
(print-X-axis numbers-list)))
print-graph
We can test the print-graph function with a short
list of numbers
Y-axis-column,
graph-body-print, and print-graph (in
addition to the rest of the code.)
(print-graph '(3 2 5 6 7 5 3 4 6 4 3 2 1))
eval-expression).
yank).
Emacs will print a graph that looks like this:
10 -
*
** *
5 - **** *
**** ***
* *********
************
1 - *************
| | | |
1 5 10 15
On the other hand, if you pass print-graph a
vertical-step value of 2, by evaluating this
expression:
(print-graph '(3 2 5 6 7 5 3 4 6 4 3 2 1) 2)
The graph looks like this:
20 -
*
** *
10 - **** *
**** ***
* *********
************
2 - *************
| | | |
1 5 10 15
(A question: is the `2' on the bottom of the vertical axis a bug or a feature? If you think it is a bug, and should be a `1' instead, (or even a `0'), you can modify the sources.)
Now for the graph for which all this code was written: a graph that shows how many function definitions contain fewer than 10 words and symbols, how many contain between 10 and 19 words and symbols, how many contain between 20 and 29 words and symbols, and so on.
This is a multi-step process. First make sure you have loaded all the requisit code.
It is a good idea to reset the value of
top-of-ranges in case you have sent it to some
different value. You can evaluate the following:
(setq top-of-ranges '(10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300)
Next create a list of the number of words and symbols in each range.
Evaluate the following:
(setq list-for-graph
(defuns-per-range
(sort
(recursive-lengths-list-many-files
(directory-files "/usr/local/emacs/lisp"
t ".+el$"))
'<)
top-of-ranges))
On my machine, this takes about an hour. It looks though 303 Lisp
files in my copy of Emacs version 19.23. After all that
computing, the list-for-graph has this value:
(537 1027 955 785 594 483 349 292 224 199 166 120 116 99 90 80 67 48 52 45 41 33 28 26 25 20 12 28 11 13 220)
This means that my copy of Emacs has 537 function definitions with fewer than 10 words or symbols in them, 1,027 function definitions with 10 to 19 words or symbols in them, 955 function definitions with 20 to 29 words or symbols in them, and so on.
Clearly, just by looking at this list we can see that most function definitions contain ten to thirty words and symbols.
Now for printing. We do not want to print a graph that is 1,030 lines high ... Instead, we should print a graph that is fewer than twenty-five lines high. A graph that height can be displayed on almost any monitor, and easily printed on a sheet of paper.
This means that each value in list-for-graph must be
reduced to one-fiftieth it present value.
Here is a short function to do just that, using two functions we
have not yet seen, mapcar and lambda.
(defun one-fiftieth (full-range) "Return list, each number one-fiftieth of previous." (mapcar '(lambda (arg) (/ arg 50)) full-range))
lambda Expression
lambda is the symbol for an anonymous function, a
function without a name. Every time you use an anonymous
function, you need to include its whole body.
Thus,
(lambda (arg) (/ arg 50))
is a function definition that says `return the value resulting
from dividing whatever is passed to me as arg by
50'.
Earlier, for example, we had a function
multiply-by-seven; it multiplied its argument by 7.
This function is similar, except it divides its argument by 50;
and, it has no name. The anonymous equivalent of
multiply-by-seven is:
(lambda (number) (* 7 number))
(See section 3.1 The
defun Special Form.)
If we want to multiply 3 by 7, we can write:
(multiply-by-seven 3)
\_______________/ ^
| |
function argument
This expression returns 21.
Similarly, we can write:
((lambda (number) (* 7 number)) 3)
\____________________________/ ^
| |
anonymous function argument
If we want to divide 100 by 50, we can write:
((lambda (arg) (/ arg 50)) 100)
\______________________/ \_/
| |
anonymous function argument
This expression returns 2. The 100 is passed to the function, which divides that number by 50.
See section `Lambda Expressions' in The GNU Emacs Lisp
Reference Manual, for more about lambda. Lisp
and lambda expressions derive from the Lambda Calculus.
mapcar Function
mapcar is a function that calls its first argument
with each element of its second argument, in turn. The second
argument must be a sequence.
For example,
(mapcar '1+ '(2 4 6))
=> (3 5 7)
The function 1+ which adds one to its argument, is
executed on each element of the list, and a new list is
returned.
Contrast this with apply, which applies its first
argument to all the remaining. (See section 15 Readying a Graph, for a
explanation of apply.)
In the definition of one-fiftieth, the first
argument is the anonymous function:
(lambda (arg) (/ arg 50))
and the second argument is full-range, which will be
bound to list-for-graph.
The whole expression looks like this:
(mapcar '(lambda (arg) (/ arg 50)) full-range))
See section `Mapping Functions' in The GNU Emacs Lisp
Reference Manual, for more about mapcar.
Using the one-fiftieth function, we can generate a
list in which each element is one-fiftieth the size of the
corresponding element in list-for-graph.
(setq fiftieth-list-for-graph
(one-fiftieth list-for-graph))
The resulting list looks like this:
(10 20 19 15 11 9 6 5 4 3 3 2 2 1 1 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 4)
This we are almost ready to print! (We also notice the loss of information: many of the higher ranges are 0, meaning that fewer than 50 defuns had that many words or symbols--but not necessarily meaning that none had that many words or symbols.)
I said `almost ready to print'! Of course, there is a bug in the
print-graph function ... It has a
vertical-step option, but not a
horizontal-step option. The
top-of-range scale goes from 10 to 300 by tens. But
the print-graph function will print only by ones.
This is a classic example of what some consider the most insidious type of bug, the bug of omission. This is not the kind of bug you can find by studying the code, for it is not in the code; it is an omitted feature. Your best actions are to try your program early and often; and try to arrange, as much as you can, to write code that is easy to understand and easy to change. Try to be aware, whenever you can, that whatever you have written, will be rewritten, if not soon, eventually. A hard maxim to follow.
It is the print-X-axis-numbered-line function that
needs the work; and then the print-X-axis and the
print-graph functions need to be adapted. Not much
needs to be done; there is one nicety: the numbers ought to line
up under the tic marks. This takes a little thought.
Here is the corrected print-X-axis-numbered-line:
(defun print-X-axis-numbered-line
(number-of-X-tics X-axis-leading-spaces
&optional horizontal-step)
"Print line of X-axis numbers"
(let ((number X-axis-label-spacing)
(horizontal-step (or horizontal-step 1)))
(insert X-axis-leading-spaces)
;; Delete extra leading spaces.
(delete-char
(- (1-
(length (int-to-string horizontal-step)))))
(insert (concat
(make-string
;; Insert white space.
(- (* symbol-width
X-axis-label-spacing)
(1-
(length
(int-to-string horizontal-step)))
2)
? )
(int-to-string
(* number horizontal-step))))
;; Insert remaining numbers.
(setq number (+ number X-axis-label-spacing))
(while (> number-of-X-tics 1)
(insert (X-axis-element
(* number horizontal-step)))
(setq number (+ number X-axis-label-spacing))
(setq number-of-X-tics (1- number-of-X-tics)))))
If you are reading this in Info, you can see the new versions of
print-X-axis print-graph and evaluate
them. If you are reading this in a printed book, you can see the
changed lines here (the full text is too much to print).
(defun print-X-axis (numbers-list horizontal-step)
...
(print-X-axis-numbered-line
tic-number leading-spaces horizontal-step))
(defun print-graph
(numbers-list
&optional vertical-step horizontal-step)
...
(print-X-axis numbers-list horizontal-step))
When made and installed, you can call the
print-graph command like this:
(print-graph fiftieth-list-for-graph 50 10)
Here is the graph:
1000 - *
**
**
**
**
750 - ***
***
***
***
****
500 - *****
******
******
******
*******
250 - ********
********* *
*********** *
************* *
50 - ***************** * *
| | | | | | | |
10 50 100 150 200 250 300 350
The largest group of functions contain 10 -- 19 words and symbols each.
if
defun, Counting words in a
defun
let
expression sample
let
expression, parts of
let
variables uninitialized
interactive
let expression
print-graph
varlist
%
let
expression
defun
defun
About the Author
Robert J. Chassell has worked with GNU Emacs since 1985. He writes and edits, teaches Emacs and Emacs Lisp, and is a director and the Secretary/Treasurer of the Free Software Foundation, Inc. He has an abiding interest in social and economic history and flies his own airplane.
It is curious to track the path by which the word `argument' came to have two different meanings, one in mathematics and the other in everyday English. According to the Oxford English Dictionary, the word derives from the Latin for `to make clear, prove'; thus it came to mean, by one thread of derivation, `the evidence offered as proof', which is to say, `the information offered', which led to its meaning in Lisp. But in the other thread of derivation, it came to mean `to assert in a manner against which others may make counter assertions', which led to the meaning of the word as a disputation. (Note here that the English word has two different definitions attached to it at the same time. By contrast, in Emacs Lisp, a symbol cannot have two different function definitions at the same time.)
Actually, you can cons an element to an atom to
produce a dotted pair. Dotted pairs are not discussed here; see
section `Dotted Pair Notation' in The GNU Emacs Lisp
Reference Manual.
This document was generated on 3 June 2000 using the texi2html translator version 1.51a.