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Info file elisp, produced by Makeinfo, -*- Text -*- from input file
elisp.texi.
This file documents GNU Emacs Lisp.
This is edition 1.03 of the GNU Emacs Lisp Reference Manual, for
Emacs Version 18.
Published by the Free Software Foundation, 675 Massachusetts
Avenue, Cambridge, MA 02139 USA
Copyright (C) 1990 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Foundation.
File: elisp, Node: Stream Type, Next: Keymap Type, Prev: Process Type, Up: Editing Types
Stream Type
-----------
A "stream" is an object that can be used as a source or sink for
characters--either to supply characters for input or to accept them
as output. Many different types can be used this way: markers,
buffers, strings, and functions. Most often, input streams
(character sources) obtain characters from the keyboard, a buffer, or
a file, and output streams (character sinks) send characters to a
buffer, such as a `*Help*' buffer, or to the echo area.
The object `nil', in addition to its other meanings, may be used
as a stream. It stands for the value of the variable
`standard-input' or `standard-output'. Also, the object `t' as a
stream specifies input using the minibuffer (*note Minibuffers::.) or
output in the echo area (*note The Echo Area::.).
Streams have no special printed representation or read syntax, and
print as whatever primitive type they are.
*Note Streams::, for a description of various functions related to
streams, including various parsing and printing functions.
File: elisp, Node: Keymap Type, Next: Syntax Table Type, Prev: Stream Type, Up: Editing Types
Keymap Type
-----------
A "keymap" maps keys typed by the user to functions. This mapping
controls how the user's command input is executed. Emacs defines two
kinds of keymaps: "full keymaps", which are vectors of 128 elements,
and "sparse keymaps", which are association lists whose first element
is the symbol `keymap'.
*Note Keymaps::, for information about creating keymaps, handling
prefix keys, local as well as global keymaps, and changing key
bindings.
File: elisp, Node: Syntax Table Type, Prev: Keymap Type, Up: Editing Types
Syntax Table Type
-----------------
A "syntax table" is a vector of 256 integers. Each element of the
vector defines how one character is interpreted when it appears in a
buffer. For example, in C mode (*note Major Modes::.), the `+'
character is punctuation, but in Lisp mode it is a valid character in
a symbol. These different interpretations are effected by changing
the syntax table entry for `+', i.e., at index 43.
Syntax tables are only used for scanning text in buffers, not for
reading Lisp expressions. The table the Lisp interpreter uses to
read expressions is built into the Emacs source code and cannot be
changed; thus, to change the list delimiters to be `{' and `}'
instead of `(' and `)' would be impossible.
*Note Syntax Tables::, for details about syntax classes and how to
make and modify syntax tables.
File: elisp, Node: Type Predicates, Next: Equality Predicates, Prev: Editing Types, Up: Types of Lisp Object
Type Predicates
===============
The Emacs Lisp interpreter itself does not perform type checking
on the actual arguments passed to functions when they are called. It
could not do otherwise, since variables in Lisp are not declared to
be of a certain type, as they are in other programming languages. It
is therefore up to the individual function to test whether each
actual argument belongs to a type that can be used by the function.
All built-in functions do check the types of their actual
arguments when appropriate and signal a `wrong-type-argument' error
if an argument is of the wrong type. For example, here is what
happens if you pass an argument to `+' which it cannot handle:
(+ 2 'a)
error--> Wrong type argument: integer-or-marker-p, a
Many functions, called "type predicates", are provided to test
whether an object is a member of a given type. (Following a
convention of long standing, the names of most Emacs Lisp predicates
end in `p'.)
Here is a table of predefined type predicates, in alphabetical
order, with references to further information.
`atom'
*note atom: List-related Predicates.
`arrayp'
*note arrayp: Array Functions.
`bufferp'
*note bufferp: Buffer Basics.
`char-or-string-p'
*note char-or-string-p: Predicates for Strings.
`consp'
*note consp: List-related Predicates.
`integer-or-marker-p'
*note integer-or-marker-p: Predicates on Markers.
`integerp'
*note integerp: Predicates on Numbers.
`keymapp'
*note keymapp: Creating Keymaps.
`listp'
*note listp: List-related Predicates.
`markerp'
*note markerp: Predicates on Markers.
`natnump'
*note natnump: Predicates on Numbers.
`nlistp'
*note nlistp: List-related Predicates.
`processp'
*note processp: Processes.
`sequencep'
*note sequencep: Sequence Functions.
`stringp'
*note stringp: Predicates for Strings.
`subrp'
*note subrp: Function Cells.
`symbolp'
*note symbolp: Symbols.
`syntax-table-p'
*note syntax-table-p: Syntax Tables.
`user-variable-p'
*note user-variable-p: Defining Variables.
`vectorp'
*note vectorp: Vectors.
`windowp'
*note windowp: Basic Windows.
File: elisp, Node: Equality Predicates, Prev: Type Predicates, Up: Types of Lisp Object
Equality Predicates
===================
Here we describe two functions that test for equality between any
two objects. Other functions test equality between objects of
specific types, e.g., strings. See the appropriate chapter
describing the data type for these predicates.
* Function: eq OBJECT1 OBJECT2
This function returns `t' if OBJECT1 and OBJECT2 are the same
object, `nil' otherwise. The "same object" means that a change
in one will be reflected by the same change in the other.
`eq' will be true if OBJECT1 and OBJECT2 are numbers with the
same value. Also, since symbol names are normally unique, if
the arguments are symbols with the same name, they are `eq'.
For other types (e.g., lists, vectors, strings), two arguments
with the same contents or elements are not necessarily `eq' to
each other: they are `eq' only if they are the same object.
(The `make-symbol' function returns an uninterned symbol that is
not interned in the standard `obarray'. When uninterned symbols
are in use, symbol names are no longer unique. Distinct symbols
with the same name are not `eq'. *Note Creating Symbols::.)
(eq 'foo 'foo)
=> t
(eq 456 456)
=> t
(eq "asdf" "asdf")
=> nil
(eq '(1 (2 (3))) '(1 (2 (3))))
=> nil
(eq [(1 2) 3] [(1 2) 3])
=> nil
(eq (point-marker) (point-marker))
=> nil
* Function: equal OBJECT1 OBJECT2
This function returns `t' if OBJECT1 and OBJECT2 have equal
components, `nil' otherwise. Whereas `eq' tests if its
arguments are the same object, `equal' looks inside nonidentical
arguments to see if their elements are the same. So, if two
objects are `eq', they are `equal', but the converse is not
always true.
(equal 'foo 'foo)
=> t
(equal 456 456)
=> t
(equal "asdf" "asdf")
=> t
(eq "asdf" "asdf")
=> nil
(equal '(1 (2 (3))) '(1 (2 (3))))
=> t
(eq '(1 (2 (3))) '(1 (2 (3))))
=> nil
(equal [(1 2) 3] [(1 2) 3])
=> t
(eq [(1 2) 3] [(1 2) 3])
=> nil
(equal (point-marker) (point-marker))
=> t
(eq (point-marker) (point-marker))
=> nil
Comparison of strings is case-sensitive.
(equal "asdf" "ASDF")
=> nil
The test for equality is implemented recursively, and circular
lists may therefore cause infinite recursion (leading to an error).
File: elisp, Node: Numbers, Next: Strings and Characters, Prev: Types of Lisp Object, Up: Top
Numbers
*******
Integers are the only kind of number in version 18 Emacs Lisp.
These are whole numbers such as -3, 0, 7, 13, and 511.
In version 19, there is a compile time option to support floating
point numbers, which are represented internally as the C type `double'.
A floating point number is a number with a fractional part, such as
-4.5, 0.0, or 2.71828. A floating point number can be expressed in
an exponential notation as well: thus, 1.5e2 equals 150; in this
example, `e2' stands for ten to the second power, and is multiplied
by 1.5.
* Menu:
* Number Basics:: Representation and range of numbers.
* Predicates on Numbers:: Testing for numbers.
* Comparison of Numbers:: Equality and inequality predicates.
* Arithmetic Operations:: How to add, subtract, multiply and divide.
* Bitwise Operations:: Logical and, or, not, shifting.
* Random Numbers:: Obtaining random integers, predictable or not.
File: elisp, Node: Number Basics, Next: Predicates on Numbers, Prev: Numbers, Up: Numbers
Number Basics
=============
The range of values for an integer depends on the machine. The
range is -8388608 to 8388607 (24 bits; i.e.,
-2**23
to
2**23 - 1
) on most machines, but on others it is -16777216 to 16777215 (25
bits), or -33554432 to 33554431 (26 bits). All of the examples shown
below assume an integer has 24 bits.
The Lisp reader reads numbers as a sequence of digits with an
optional sign.
1 ; The integer 1.
+1 ; Also the integer 1.
-1 ; The integer -1.
16777217 ; Also the integer 1, due to overflow.
0 ; The number 0.
-0 ; The number 0.
1. ; Invalid syntax.
To understand how various functions work on integers, especially
the bitwise operators (*note Bitwise Operations::.), it is often
helpful to view the numbers in their binary form.
In 24 bit binary, the decimal integer 5 looks like this:
0000 0000 0000 0000 0000 0101
(We have inserted spaces between groups of 4 bits, and two spaces
between groups of 8 bits, to make the binary integer easier to read.)
The integer -1 looks like this:
1111 1111 1111 1111 1111 1111
-1 is represented as 24 ones. (This is called "two's complement"
notation.)
The negative integer, -5, is creating by subtracting 4 from -1.
In binary, the decimal integer 4 is 100. Consequently, -5 looks like
this:
1111 1111 1111 1111 1111 1011
In this implementation, the largest 24 bit binary integer is the
decimal integer 8,388,607. In binary, this number looks like this:
0111 1111 1111 1111 1111 1111
Since the arithmetic functions do not check whether integers go
outside their range, when you add 1 to 8,388,607, the value is
negative integer -8,388,608:
(+ 1 8388607)
=> -8388608
=> 1000 0000 0000 0000 0000 0000
Many of the following functions accept markers for arguments as
well as integers. (*Note Markers::.) More precisely, the actual
parameters to such functions may be either integers or markers, which
is why we often give these parameters the name MARKER-OR-INT. When
the actual parameter is a marker, the position value of the marker is
used and the buffer of the marker is ignored.
File: elisp, Node: Predicates on Numbers, Next: Comparison of Numbers, Prev: Number Basics, Up: Numbers
Type Predicates for Numbers
===========================
The functions in this section test whether the argument is a
number or whether it is a certain sort of number. `integerp' and
`natnump' can take any type of Lisp object as argument (the
predicates would not be of much use otherwise); but the `zerop'
predicate requires an integer as its argument.
* Function: integerp OBJECT
This predicate tests whether its argument is an integer (a whole
number) and returns `t' if so, `nil' otherwise.
* Function: natnump OBJECT
The `natnump' predicate (whose name comes from the phrase
"natural-number-p") tests to see whether its argument is a
nonnegative integer, and returns `t' if so, `nil' otherwise. 0
is considered non-negative.
Markers are not converted to integers, hence `natnump' of a
marker is always `nil'.
People have pointed out that this function is misnamed, because
the term "natural number" is usually understood as excluding
zero. We are open to suggestions for a better name to use in
version 19.
* Function: zerop INTEGER
This predicate tests whether its argument is zero, and returns
`t' if so, `nil' otherwise. These two forms are equivalent:
`(zerop x) == (= x 0)'.
File: elisp, Node: Comparison of Numbers, Next: Arithmetic Operations, Prev: Predicates on Numbers, Up: Numbers
Comparison of Numbers
=====================
The integer type is implemented by storing the value in the
"pointer part" of a Lisp object (which, on typical target machines,
has 24 bits of pointer, 7 bits of type and 1 bit for the garbage
collector). Because of this, the function `eq' will return `t' for
two integers with the same value. *Note Equality Predicates::.
Common Lisp note: because of the way numbers are implemented in
Common Lisp, you generally need to use ``='' to test for
equality between numbers. However, GNU Emacs Lisp does not need
very large integers; as a consequence, it is possible to
restrict them to the size of a single word, allowing `eq' to be
used.
* Function: = MARKER-OR-INT1 MARKER-OR-INT2
This function tests whether its arguments are the same number,
and returns `t' if so, `nil' otherwise.
* Function: /= MARKER-OR-INT1 MARKER-OR-INT2
This function tests whether its arguments are not the same
number, and returns `t' if so, `nil' otherwise.
* Function: < MARKER-OR-INT1 MARKER-OR-INT2
This function tests whether its first argument is strictly less
than its second argument. It returns `t' if so, `nil' otherwise.
* Function: <= MARKER-OR-INT1 MARKER-OR-INT2
This function tests whether its first argument is less than or
equal to its second argument. It returns `t' if so, `nil'
otherwise.
* Function: > MARKER-OR-INT1 MARKER-OR-INT2
This function tests whether its first argument is strictly
greater than its second argument. It returns `t' if so, `nil'
otherwise.
* Function: >= MARKER-OR-INT1 MARKER-OR-INT2
This function tests whether its first argument is greater than
or equal to its second argument. It returns `t' if so, `nil'
otherwise.
* Function: max MARKER-OR-INT &rest MARKERS-OR-INTS
This function returns the largest of its arguments.
(max 20)
=> 20
(max 1 2)
=> 2
(max 1 3 2)
=> 3
* Function: min MARKER-OR-INT &rest MARKERS-OR-INTS
This function returns the smallest of its arguments.
File: elisp, Node: Arithmetic Operations, Next: Bitwise Operations, Prev: Comparison of Numbers, Up: Numbers
Arithmetic Operations
=====================
Emacs Lisp provides the traditional four arithmetic operations:
addition, subtraction, multiplication, and division. A remainder
function supplements the (integer) division function. In addition,
as a convenience, incrementing and decrementing functions are provided.
It is important to note that in GNU Emacs Lisp, arithmetic
functions do not check for overflow. Thus `(1+ 8388607)' may equal
-8388608, depending on your hardware.
* Function: 1+ MARKER-OR-INT
This function adds one to MARKER-OR-INT.
* Function: 1- MARKER-OR-INT
This function subtracts one from MARKER-OR-INT.
* Function: + &rest MARKERS-OR-INTS
This function adds its arguments together. When given no
arguments, `+' returns 0. It does not check for overflow.
(+)
=> 0
(+ 1)
=> 1
(+ 1 2 3 4)
=> 10
* Function: - &optional MARKER-OR-INT &rest OTHER-MARKERS-OR-INTS
The `-' function serves two purposes: negation and subtraction.
When `-' has a single argument, the value is the negative of the
argument. When there are multiple arguments, each of the
OTHER-MARKERS-OR-INTS is subtracted from MARKER-OR-INT,
cumulatively. If there are no arguments, the result is 0. This
function does not check for overflow.
(- 10 1 2 3 4)
=> 0
(- 10)
=> -10
(-)
=> 0
* Function: * &rest MARKERS-OR-INTEGERS
This function multiplies its arguments together, and returns the
product. When given no arguments, `*' returns 1. It does not
check for overflow.
(*)
=> 1
(* 1)
=> 1
(* 1 2 3 4)
=> 24
* Function: / DIVIDEND DIVISOR &rest DIVISORS
This function divides DIVIDEND by DIVISORS and returns the
quotient. If there are additional arguments DIVISORS, then
DIVIDEND is divided by each divisor in turn. Each argument
may be an integer or a marker.
The result is normally rounded towars zero after each division,
but some machines may round differently with negative arguments.
This is because the Lisp function `/' is implemented using the C
division operator, which has the same possibility for
machine-dependent rounding. In practice, all known machines
round in the standard fashion.
If you divide by 0, an `arith-error' error is signaled. (*Note
Errors::.)
(/ 6 2)
=> 3
(/ 5 2)
=> 2
(/ 25 3 2)
=> 4
(/ -17 6)
=> -2 ; (Could be -3 on some machines.)
* Function: % DIVIDEND DIVISOR
This function returns the value of DIVIDEND modulo DIVISOR; in
other words, the integer remainder after division of DIVIDEND by
DIVISOR. The sign of the result is the sign of DIVIDEND. The
sign of DIVISOR is ignored. The arguments must be integers.
For negative arguments, the value is in principle
machine-dependent since the quotient is; but in practice, all
known machines behave alike.
An `arith-error' results if DIVISOR is 0.
(% 9 4)
=> 1
(% -9 4)
=> -1
(% 9 -4)
=> 1
(% -9 -4)
=> -1
For any two numbers DIVIDEND and DIVISOR,
(+ (% DIVIDEND DIVISOR)
(* (/ DIVIDEND DIVISOR) DIVISOR))
always equals DIVIDEND.
File: elisp, Node: Bitwise Operations, Next: Random Numbers, Prev: Arithmetic Operations, Up: Numbers
Bitwise Operations on Integers
==============================
In a computer, an integer is represented as a binary number, a
sequence of "bits" (digits which are either zero or one). A bitwise
operation acts on the individual bits of such a sequence. For
example, "shifting" moves the whole sequence left or right one or
more places, reproducing the same pattern "moved over".
The bitwise operations in Emacs Lisp apply only to integers.
* Function: lsh INTEGER1 COUNT
`lsh', which is an abbreviation for "logical shift", shifts the
bits in INTEGER1 to the left COUNT places, or to the right if
COUNT is negative. If COUNT is negative, `lsh' shifts zeros
into the most-significant bit, producing a positive result even
if INTEGER1 is negative. Contrast this with `ash', below.
Thus, the decimal number 5 is the binary number 00000101.
Shifted once to the left, with a zero put in the one's place,
the number becomes 00001010, decimal 10.
Here are two examples of shifting the pattern of bits one place
to the left. Since the contents of the rightmost place has been
moved one place to the left, a value has to be inserted into the
rightmost place. With `lsh', a zero is placed into the
rightmost place. (These examples show only the low-order eight
bits of the binary pattern; the rest are all zero.)
(lsh 5 1)
=> 10
00000101 => 00001010 ; Decimal 5 becomes decimal 10.
(lsh 7 1)
=> 14
00000111 => 00001110 ; Decimal 7 becomes decimal 14.
As the examples illustrate, shifting the pattern of bits one
place to the left produces a number that is twice the value of
the previous number.
Note, however that functions do not check for overflow, and a
returned value may be negative (and in any case, no more than a
24 bit value) when an integer is sufficiently left shifted. For
example:
(lsh 8388607 1) ; left shift
=> -2
In binary, in the 24 bit implementation,
0111 1111 1111 1111 1111 1111 ; Decimal 8,388,607
becomes
1111 1111 1111 1111 1111 1110 ; Decimal -2
Shifting the pattern of bits two places to the left produces
results like this (with 8-bit binary numbers):
(lsh 3 2)
=> 12
00000011 => 00001100 ; Decimal 3 becomes decimal 12.
On the other hand, shifting the pattern of bits one place to the
right looks like this:
(lsh 6 -1)
=> 3
00000110 => 00000011 ; Decimal 6 becomes decimal 3.
(lsh 5 -1)
=> 2
00000101 => 00000010 ; Decimal 5 becomes decimal 2.
As the example illustrates, shifting the pattern of bits one
place to the right divides the value of the binary number by
two, rounding downward.
* Function: ash INTEGER1 COUNT
`ash' ("arithmetic shift") shifts the bits in INTEGER1 to the
left COUNT places, or to the right if COUNT is negative.
`ash' gives the same results as `lsh' except when INTEGER1 and
COUNT are both negative. In that case, `ash' puts a one in the
leftmost position, while `lsh' puts a zero in the leftmost
position.
Thus, with `ash', shifting the pattern of bits one place to the
right looks like this:
(ash -6 -1)
=> -3 ; Decimal -6 becomes decimal -3.
1111 1111 1111 1111 1111 1010
=>
1111 1111 1111 1111 1111 1101
In contrast, shifting the pattern of bits one place to the right
with `lsh' looks like this:
(lsh -6 -1)
=> 8388605 ; Decimal -6 becomes decimal 8,388,605.
1111 1111 1111 1111 1111 1010
=>
0111 1111 1111 1111 1111 1101
In this case, the 1 in the leftmost position is shifted one
place to the right, and a zero is shifted into the leftmost
position.
Here are other examples:
; 24-bit binary values
(lsh 5 2) ; 5 = 0000 0000 0000 0000 0000 0101
=> 20 ; 20 = 0000 0000 0000 0000 0001 0100
(ash 5 2)
=> 20
(lsh -5 2) ; -5 = 1111 1111 1111 1111 1111 1011
=> -20 ; -20 = 1111 1111 1111 1111 1110 1100
(ash -5 2)
=> -20
(lsh 5 -2) ; 5 = 0000 0000 0000 0000 0000 0101
=> 1 ; 1 = 0000 0000 0000 0000 0000 0001
(ash 5 -2)
=> 1
(lsh -5 -2) ; -5 = 1111 1111 1111 1111 1111 1011
=> 4194302 ; 0011 1111 1111 1111 1111 1110
(ash -5 -2) ; -5 = 1111 1111 1111 1111 1111 1011
=> -2 ; -2 = 1111 1111 1111 1111 1111 1110
* Function: logand &rest MARKERS-OR-INTS
This function returns the "logical and" of the arguments: the
Nth bit is set in the result if, and only if, the Nth bit is set
in all the arguments. ("Set" means that the value of the bit is
1 rather than 0.)
For example, using 4-bit binary numbers, the "logical and" of 13
and 12 is 12: 1101 combined with 1100 produces 1100.
In both the binary numbers, the leftmost two bits are set (i.e.,
they are 1's), so the leftmost two bits of the returned value
are set. However, for the rightmost two bits, each is zero in
at least one of the arguments, so the rightmost two bits of the
returned value are 0's.
Therefore,
(logand 13 12)
=> 12
If `logand' is not passed any argument, it returns a value of
-1. This number is an identity element for `logand' because its
binary representation consists entirely of ones. If `logand' is
passed just one argument, it returns that argument.
; 24-bit binary values
(logand 14 13) ; 14 = 0000 0000 0000 0000 0000 1110
; 13 = 0000 0000 0000 0000 0000 1101
=> 12 ; 12 = 0000 0000 0000 0000 0000 1100
(logand 14 13 4) ; 14 = 0000 0000 0000 0000 0000 1110
; 13 = 0000 0000 0000 0000 0000 1101
; 4 = 0000 0000 0000 0000 0000 0100
=> 4 ; 4 = 0000 0000 0000 0000 0000 0100
(logand)
=> -1 ; -1 = 1111 1111 1111 1111 1111 1111
* Function: logior &rest MARKERS-OR-INTS
This function returns the "inclusive or" of its arguments: the
Nth bit is set in the result if, and only if, the Nth bit is set
in at least one of the arguments. If there are no arguments,
the result is zero, which is an identity element for this
operation. If `logior' is passed just one argument, it returns
that argument.
; 24-bit binary values
(logior 12 5) ; 12 = 0000 0000 0000 0000 0000 1100
; 5 = 0000 0000 0000 0000 0000 0101
=> 13 ; 13 = 0000 0000 0000 0000 0000 1101
(logior 12 5 7) ; 12 = 0000 0000 0000 0000 0000 1100
; 5 = 0000 0000 0000 0000 0000 0101
; 7 = 0000 0000 0000 0000 0000 0111
=> 15 ; 15 = 0000 0000 0000 0000 0000 1111
* Function: logxor &rest MARKERS-OR-INTS
This function returns the "exclusive or" of its arguments: the
Nth bit is set in the result if, and only if, the Nth bit is set
in an odd number of the arguments. If there are no arguments,
the result is 0. If `logxor' is passed just one argument, it
returns that argument.
; 24-bit binary values
(logxor 12 5) ; 12 = 0000 0000 0000 0000 0000 1100
; 5 = 0000 0000 0000 0000 0000 0101
=> 9 ; 9 = 0000 0000 0000 0000 0000 1001
(logxor 12 5 7) ; 12 = 0000 0000 0000 0000 0000 1100
; 5 = 0000 0000 0000 0000 0000 0101
; 7 = 0000 0000 0000 0000 0000 0111
=> 14 ; 14 = 0000 0000 0000 0000 0000 1110
* Function: lognot INTEGER
This function returns the logical complement of its argument:
the Nth bit is one in the result if, and only if, the Nth bit is
zero in INTEGER, and vice-versa.
(lognot 5) ; 5 = 0000 0000 0000 0000 0000 0101
=> -6 ; -6 = 1111 1111 1111 1111 1111 1010
File: elisp, Node: Random Numbers, Prev: Bitwise Operations, Up: Numbers
Random Numbers
==============
* Function: random &optional FLAG
This function returns a pseudo-random number of type integer.
When called more than once, this function returns a series of
pseudo-random numbers.
In a computer, a series of a pseudo-random numbers is generated
in a deterministic fashion. The numbers are not truly random,
but they have certain properties that mimic a random series.
For example, all possible values occur equally often in a
pseudo-random series.
In Emacs, pseudo-random numbers are generated from a "seed"
number. If the `random' function starts with the same seed, it
generates the same sequence of numbers. Emacs always starts
with the same seed value, so the sequence of values of `random'
is actually the same in each Emacs run! For example, in one
operating system, the first call to `(random)' after you start
Emacs always returns -1457731, and the second one always returns
-7692030. This is helpful for debugging.
If you want different random numbers, execute `(random t)'.
This chooses a new seed based on the current time of day and on
Emacs' process ID number.
On some machines, any integer representable in Lisp may be the
result of `random'. On other machines, the result can never be
larger than a certain maximum or less than a certain (negative)
minimum.
File: elisp, Node: Strings and Characters, Next: Lists, Prev: Numbers, Up: Top
Strings and Characters
**********************
A string in Emacs Lisp is an array that contains an ordered
sequence of characters. Strings are used as names of symbols,
buffers, and files, to send messages to users, to hold text being
copied between buffers, and for many other purposes. Because strings
are so important, many functions are provided expressly for
manipulating them. Emacs Lisp programs use strings more often than
individual characters.
* Menu:
* Intro to Strings:: Basic properties of strings and characters.
* Predicates for Strings:: Testing whether an object is a string or char.
* Creating Strings:: Functions to allocate new strings.
* Text Comparison:: Comparing characters or strings.
* String Conversion:: Converting characters or strings and vice versa.
* Formatting Strings:: `format': Emacs's analog of `printf'.
* Character Case:: Case conversion functions.
File: elisp, Node: Intro to Strings, Next: Predicates for Strings, Prev: Strings and Characters, Up: Strings and Characters
Introduction to Strings and Characters
======================================
Characters are represented in Emacs Lisp as integers; whether an
integer was intended as a character or not is determined only by how
it is used. Strings in Emacs Lisp are arrays that contain an ordered
sequence of characters.
The length of a string (like any array) is fixed and independent
of the string contents, and cannot be altered. Strings in Lisp are
*not* terminated by a distinguished character code. (By contrast,
strings in C are terminated by a character with ASCII code 0.) This
means that any character, including the null character (ASCII code
0), is a valid element of a string.
Since strings are considered arrays, you can operate on them with
the general array functions. (*Note Sequences Arrays Vectors::.)
For example, you can access or change individual characters in a
string using the functions `aref' and `aset' (*note Array
Functions::.).
Each character in a string is stored in a single byte. Therefore,
numbers not in the range 0 to 255 are truncated when stored into a
string. This means that a string takes up much less memory than a
vector of the same length.
*Note Text::, for information about functions that display strings
or copy them into buffers. *Note Character Type::, and *Note String
Type::, for information about the syntax of characters and strings.
File: elisp, Node: Predicates for Strings, Next: Creating Strings, Prev: Intro to Strings, Up: Strings and Characters
The Predicates for Strings
==========================
For more information about general sequence and array predicates,
see *Note Sequences Arrays Vectors::, and *Note Arrays::.
* Function: stringp OBJECT
This function returns `t' if OBJECT is a string, `nil' otherwise.
* Function: char-or-string-p OBJECT
This function returns `t' if OBJECT is a string or a character
(i.e., an integer), `nil' otherwise.
File: elisp, Node: Creating Strings, Next: Text Comparison, Prev: Predicates for Strings, Up: Strings and Characters
Creating Strings
================
The following functions create strings, either from scratch, or by
putting strings together, or by taking them apart.
* Function: make-string COUNT CHARACTER
This function returns a string made up of COUNT repetitions of
CHARACTER. If COUNT is negative, an error is signaled.
(make-string 5 ?x)
=> "xxxxx"
(make-string 0 ?x)
=> ""
Other functions to compare with this one include
`char-to-string' (*note String Conversion::.), `make-vector'
(*note Vectors::.), and `make-list' (*note Building Lists::.).
* Function: substring STRING START &optional END
This function returns a new string which consists of those
characters from STRING in the range from (and including) the
character at the index START up to (but excluding) the character
at the index END. The first character is at index zero.
(substring "abcdefg" 0 3)
=> "abc"
Here the index for `a' is 0, the index for `b' is 1, and the
index for `c' is 2. Thus, three letters, `abc', are copied from
the full string. The index 3 marks the character position up to
which the substring is copied. The character whose index is 3
is actually the fourth character in the string.
A negative number counts from the end of the string, so that -1
signifies the index of the last character of the string. For
example:
(substring "abcdefg" -3 -1)
=> "ef"
In this example, the index for `e' is -3, the index for `f' is
-2, and the index for `g' is -1. Therefore, `e' and `f' are
included, and `g' is excluded.
When `nil' is used as an index, it falls after the last
character in the string. Thus:
(substring "abcdefg" -3 nil)
=> "efg"
Omitting the argument END is equivalent to specifying `nil'. It
follows that `(substring STRING 0)' returns a copy of all of
STRING.
(substring "abcdefg" 0)
=> "abcdefg"
But we recommend `copy-sequence' for this purpose.
A `wrong-type-argument' error is signaled if either START or END
are non-integers. An `args-out-of-range' error is signaled if
START indicates a character following END, or if either integer
is out of range for STRING.
Contrast this function with `buffer-substring' (*note Buffer
Contents::.), which returns a string containing a portion of the
text in the current buffer. The beginning of a string is at
index 0, but the beginning of a buffer is at index 1.
* Function: concat &rest SEQUENCES
This function returns a new string consisting of the characters
in the arguments passed to it. The arguments may be strings,
lists of numbers, or vectors of numbers; they are not themselves
changed. If no arguments are passed to `concat', it returns an
empty string.
(concat "abc" "-def")
=> "abc-def"
(concat "abc" (list 120 (+ 256 121)) [122])
=> "abcxyz"
(concat "The " "quick brown " "fox.")
=> "The quick brown fox."
(concat)
=> ""
The second example above shows how characters stored in strings
are taken modulo 256. In other words, each character in the
string is stored in one byte.
The `concat' function always constructs a new string that is not
`eq' to any existing string.
When an argument is an integer (not a sequence of integers), it
is converted to a string of digits making up the decimal printed
representation of the integer. This special case exists for
compatibility with Mocklisp, and we don't recommend you take
advantage of it. If you want to convert an integer in this way,
use `format' (*note Formatting Strings::.) or `int-to-string'
(*note String Conversion::.).
(concat 137)
=> "137"
(concat 54 321)
=> "54321"
For information about other concatenation functions, see
`mapconcat' in *Note Mapping Functions::, `vconcat' in *Note
Vectors::, and `append' in *Note Building Lists::.
File: elisp, Node: Text Comparison, Next: String Conversion, Prev: Creating Strings, Up: Strings and Characters
Comparison of Characters and Strings
====================================
* Function: char-equal CHARACTER1 CHARACTER2
This function returns `t' if the arguments represent the same
character, `nil' otherwise. This is done by comparing two
integers modulo 256.
(char-equal ?x ?x)
=> t
(char-to-string (+ 256 ?x))
=> "x"
(char-equal ?x (+ 256 ?x))
=> t
* Function: string= STRING1 STRING2
This function returns `t' if the characters of the two strings
match exactly; case is significant.
(string= "abc" "abc")
=> t
(string= "abc" "ABC")
=> nil
(string= "ab" "ABC")
=> nil
* Function: string-equal STRING1 STRING2
`string-equal' is another name for `string='.
* Function: string< STRING1 STRING2
This function compares two strings a character at a time. First
it scans both the strings at once to find the first pair of
corresponding characters that do not match. If the lesser
character of those two is the character from STRING1, then
STRING1 is less, and this function returns `t'. If the lesser
character is the one from STRING2, then STRING1 is greater, and
this function returns `nil'. If the two strings match entirely,
the value is `nil'.
Pairs of characters are compared by their ASCII codes. Keep in
mind that lower case letters have higher numeric values in the
ASCII character set than their upper case counterparts; numbers
and many punctuation characters have a lower numeric value than
upper case letters.
(string< "abc" "abd")
=> t
(string< "abd" "abc")
=> nil
(string< "123" "abc")
=> t
When the strings have different lengths, and they match up to
the length of STRING1, then the result is `t'. If they match up
to the length of STRING2, the result is `nil'. A string without
any characters in it is the smallest possible string.
(string< "" "abc")
=> t
(string< "ab" "abc")
=> t
(string< "abc" "")
=> nil
(string< "abc" "ab")
=> nil
(string< "" "")
=> nil
* Function: string-lessp STRING1 STRING2
`string-lessp' is another name for `string<'.
File: elisp, Node: String Conversion, Next: Formatting Strings, Prev: Text Comparison, Up: Strings and Characters
Conversion of Characters and Strings
====================================
Characters and strings may be converted into each other and into
integers. `format' and `prin1-to-string' (*note Output Functions::.)
may also be used to convert Lisp objects into strings.
`read-from-string' (*note Input Functions::.) may be used to
"convert" a string representation of a Lisp object into an object.
*Note Documentation::, for a description of the functions
`single-key-description' and `text-char-description', which return a
string representing the Emacs standard notation of the argument
character. These functions are used primarily for printing help
messages.
* Function: char-to-string CHARACTER
This function returns a new string with a length of one character.
The value of CHARACTER, modulo 256, is used to initialize the
element of the string.
This function is similar to `make-string' with an integer
argument of 1. (*Note Creating Strings::.) This conversion can
also be done with `format' using the `%c' format specification.
(*Note Formatting Strings::.)
(char-to-string ?x)
=> "x"
(char-to-string (+ 256 ?x))
=> "x"
(make-string 1 ?x)
=> "x"
* Function: string-to-char STRING
This function returns the first character in STRING. If the
string is empty, the function returns 0. The value is also 0
when the first character of STRING is the null character, ASCII
code 0.
(string-to-char "ABC")
=> 65
(string-to-char "xyz")
=> 120
(string-to-char "")
=> 0
(string-to-char "\000")
=> 0
This function may be eliminated in version 19 if it does not
seem useful enough to retain.
* Function: int-to-string INTEGER
This function returns a string consisting of the digits of
INTEGER, base ten. When passed a positive integer as an
argument, this function returns an unsigned string. When passed
a negative integer, the function returns a string with a leading
minus sign.
(int-to-string 256)
=> "256"
(int-to-string -23)
=> "-23"
See also the function `format' in *Note Formatting Strings::.
* Function: string-to-int STRING
This function returns the integer value of the characters in
STRING, read as a number in base ten.
The string is read starting from (and including) the first
character, and it is read until a non-digit is encountered. If
the first character is not a digit or a minus sign, this
function returns 0.
(string-to-int "256")
=> 256
(string-to-int "25 is a perfect square.")
=> 25
(string-to-int "X256")
=> 0
(string-to-int "-4")
=> -4
File: elisp, Node: Formatting Strings, Next: Character Case, Prev: String Conversion, Up: Strings and Characters
Formatting Strings
==================
"Formatting" means constructing a string by substitution of
computed values at various places in a constant string. This string
controls how the other values are printed as well as where they
appear; it is called a "format string".
Formatting is often useful for computing messages to be displayed.
In fact, the functions `message' and `error' provide the same
formatting feature described here; they differ from `format' only in
how they use the result of formatting.
* Function: format STRING &rest OBJECTS
This function returns a new string that is made by copying
STRING and then replacing any format specification in the copy
with encodings of the corresponding OBJECTS. The arguments
OBJECTS are the computed values to be formatted.
A format specification is a sequence of characters beginning with
a `%'. Thus, if there is a `%d' in STRING, the `format' function
replaces it with the printed representation of one of the values to
be formatted (one of the arguments OBJECTS). For example:
(format "The value of fill-column is %d." fill-column)
=> "The value of fill-column is 72."
If STRING contains more than one format specification, the format
specifications are matched in order with successive values from
OBJECTS. Thus, the first format specification in STRING is matched
with the first such value, the second format specification is matched
with the second such value, and so on. Any extra format
specifications (those for which there are no corresponding values)
cause unpredictable behavior. Any extra values to be formatted will
be ignored.
Certain format specifications require values of particular types.
However, no error is signaled if the value actually supplied fails to
have the expected type. Instead, meaningless text is likely to be
output.
Here is a table of the characters that can follow `%' to make up a
format specification:
`s'
Replace the specification with the printed representation of the
object. If there is no corresponding object, the empty string
is used.
`o'
Replace the specification with the base-eight representation of
an integer.
`d'
Replace the specification with the base-ten representation of an
integer.
`x'
Replace the specification with the base-sixteen representation
of an integer.
`c'
Replace the specification with the character which is the value
given.
`%'
A single `%' is placed in the string. This format specification
is unusual in that it does not use a value. For example,
`(format "%% %d" 30)' returns `"% 30"'.
Any other format character results in an `Invalid format
operation' error.
Here are several examples:
(format "The name of this buffer is %s." (buffer-name))
=> "The name of this buffer is strings.texi."
(format "The buffer object prints as %s." (current-buffer))
=> "The buffer object prints as #<buffer strings.texi>."
(format "The octal value of 18 is %o, and the hex value is %x."
18 18)
=> "The octal value of 18 is 22, and the hex value is 12."
All the specification characters allow an optional numeric prefix
between the `%' and the character. The optional numeric prefix
defines the minimum width for the object. If the printed
representation of the object contains fewer characters than this,
then it is padded. The padding is on the left if the prefix is
positive (or starts with zero) and on the right if the prefix is
negative. The padding character is normally a space, but if the
numeric prefix starts with a zero, zeros are used for padding.
(format "%06d will be padded on the left with zeros" 123)
=> "000123 will be padded on the left with zeros"
(format "%-6d will be padded on the right" 123)
=> "123 will be padded on the right"
No matter what the prefix, nothing in the printed representation
will be truncated. This allows the programmer to specify minimum
spacing without knowing how many characters there are in the object's
printed representation.
In the following three examples, `%7s' specifies a minimum width
of 7. In the first case, the string inserted in place of `%7s' has
only 3 letters, so 4 blank spaces are inserted for padding. In the
second case, the string `"specification"' is 13 letters wide but is
not truncated. In the third case, the padding is on the right.
(This does not work in version 18, but does work in version 19.)
(format "The word `%7s' actually has %d letters in it." "foo"
(length "foo"))
=> "The word ` foo' actually has 3 letters in it."
(format "The word `%7s' actually has %d letters in it."
"specification"
(length "specification"))
=> "The word `specification' actually has 13 letters in it."
(format "The word `%-7s' actually has %d letters in it." "foo"
(length "foo"))
=> "The word `foo ' actually has 3 letters in it."
;; `%-7s' fails to work in version 18, but does work in version 19.
;; In version 18, padding is not inserted.
