Improve the explanation of version control to include concepts

[emacs.git] / lispref / sequences.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001,
@c   2002, 2003, 2004, 2005, 2006, 2007  Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/sequences
@node Sequences Arrays Vectors, Hash Tables, Lists, Top
@chapter Sequences, Arrays, and Vectors
@cindex sequence

  Recall that the @dfn{sequence} type is the union of two other Lisp
types: lists and arrays.  In other words, any list is a sequence, and
any array is a sequence.  The common property that all sequences have is
that each is an ordered collection of elements.

  An @dfn{array} is a single primitive object that has a slot for each
of its elements.  All the elements are accessible in constant time, but
the length of an existing array cannot be changed.  Strings, vectors,
char-tables and bool-vectors are the four types of arrays.

  A list is a sequence of elements, but it is not a single primitive
object; it is made of cons cells, one cell per element.  Finding the
@var{n}th element requires looking through @var{n} cons cells, so
elements farther from the beginning of the list take longer to access.
But it is possible to add elements to the list, or remove elements.

  The following diagram shows the relationship between these types:

@example
@group
          _____________________________________________
         |                                             |
         |          Sequence                           |
         |  ______   ________________________________  |
         | |      | |                                | |
         | | List | |             Array              | |
         | |      | |    ________       ________     | |
         | |______| |   |        |     |        |    | |
         |          |   | Vector |     | String |    | |
         |          |   |________|     |________|    | |
         |          |  ____________   _____________  | |
         |          | |            | |             | | |
         |          | | Char-table | | Bool-vector | | |
         |          | |____________| |_____________| | |
         |          |________________________________| |
         |_____________________________________________|
@end group
@end example

  The elements of vectors and lists may be any Lisp objects.  The
elements of strings are all characters.

@menu
* Sequence Functions::    Functions that accept any kind of sequence.
* Arrays::                Characteristics of arrays in Emacs Lisp.
* Array Functions::       Functions specifically for arrays.
* Vectors::               Special characteristics of Emacs Lisp vectors.
* Vector Functions::      Functions specifically for vectors.
* Char-Tables::           How to work with char-tables.
* Bool-Vectors::          How to work with bool-vectors.
@end menu

@node Sequence Functions
@section Sequences

  In Emacs Lisp, a @dfn{sequence} is either a list or an array.  The
common property of all sequences is that they are ordered collections of
elements.  This section describes functions that accept any kind of
sequence.

@defun sequencep object
Returns @code{t} if @var{object} is a list, vector, string,
bool-vector, or char-table, @code{nil} otherwise.
@end defun

@defun length sequence
@cindex string length
@cindex list length
@cindex vector length
@cindex sequence length
@cindex char-table length
This function returns the number of elements in @var{sequence}.  If
@var{sequence} is a dotted list, a @code{wrong-type-argument} error is
signaled.  Circular lists may cause an infinite loop.  For a
char-table, the value returned is always one more than the maximum
Emacs character code.

@xref{Definition of safe-length}, for the related function @code{safe-length}.

@example
@group
(length '(1 2 3))
    @result{} 3
@end group
@group
(length ())
    @result{} 0
@end group
@group
(length "foobar")
    @result{} 6
@end group
@group
(length [1 2 3])
    @result{} 3
@end group
@group
(length (make-bool-vector 5 nil))
    @result{} 5
@end group
@end example
@end defun

@noindent
See also @code{string-bytes}, in @ref{Text Representations}.

@defun elt sequence index
@cindex elements of sequences
This function returns the element of @var{sequence} indexed by
@var{index}.  Legitimate values of @var{index} are integers ranging
from 0 up to one less than the length of @var{sequence}.  If
@var{sequence} is a list, out-of-range values behave as for
@code{nth}.  @xref{Definition of nth}.  Otherwise, out-of-range values
trigger an @code{args-out-of-range} error.

@example
@group
(elt [1 2 3 4] 2)
     @result{} 3
@end group
@group
(elt '(1 2 3 4) 2)
     @result{} 3
@end group
@group
;; @r{We use @code{string} to show clearly which character @code{elt} returns.}
(string (elt "1234" 2))
     @result{} "3"
@end group
@group
(elt [1 2 3 4] 4)
     @error{} Args out of range: [1 2 3 4], 4
@end group
@group
(elt [1 2 3 4] -1)
     @error{} Args out of range: [1 2 3 4], -1
@end group
@end example

This function generalizes @code{aref} (@pxref{Array Functions}) and
@code{nth} (@pxref{Definition of nth}).
@end defun

@defun copy-sequence sequence
@cindex copying sequences
Returns a copy of @var{sequence}.  The copy is the same type of object
as the original sequence, and it has the same elements in the same order.

Storing a new element into the copy does not affect the original
@var{sequence}, and vice versa.  However, the elements of the new
sequence are not copies; they are identical (@code{eq}) to the elements
of the original.  Therefore, changes made within these elements, as
found via the copied sequence, are also visible in the original
sequence.

If the sequence is a string with text properties, the property list in
the copy is itself a copy, not shared with the original's property
list.  However, the actual values of the properties are shared.
@xref{Text Properties}.

This function does not work for dotted lists.  Trying to copy a
circular list may cause an infinite loop.

See also @code{append} in @ref{Building Lists}, @code{concat} in
@ref{Creating Strings}, and @code{vconcat} in @ref{Vector Functions},
for other ways to copy sequences.

@example
@group
(setq bar '(1 2))
     @result{} (1 2)
@end group
@group
(setq x (vector 'foo bar))
     @result{} [foo (1 2)]
@end group
@group
(setq y (copy-sequence x))
     @result{} [foo (1 2)]
@end group

@group
(eq x y)
     @result{} nil
@end group
@group
(equal x y)
     @result{} t
@end group
@group
(eq (elt x 1) (elt y 1))
     @result{} t
@end group

@group
;; @r{Replacing an element of one sequence.}
(aset x 0 'quux)
x @result{} [quux (1 2)]
y @result{} [foo (1 2)]
@end group

@group
;; @r{Modifying the inside of a shared element.}
(setcar (aref x 1) 69)
x @result{} [quux (69 2)]
y @result{} [foo (69 2)]
@end group
@end example
@end defun

@node Arrays
@section Arrays
@cindex array

  An @dfn{array} object has slots that hold a number of other Lisp
objects, called the elements of the array.  Any element of an array may
be accessed in constant time.  In contrast, an element of a list
requires access time that is proportional to the position of the element
in the list.

  Emacs defines four types of array, all one-dimensional: @dfn{strings},
@dfn{vectors}, @dfn{bool-vectors} and @dfn{char-tables}.  A vector is a
general array; its elements can be any Lisp objects.  A string is a
specialized array; its elements must be characters.  Each type of array
has its own read syntax.
@xref{String Type}, and @ref{Vector Type}.

  All four kinds of array share these characteristics:

@itemize @bullet
@item
The first element of an array has index zero, the second element has
index 1, and so on.  This is called @dfn{zero-origin} indexing.  For
example, an array of four elements has indices 0, 1, 2, @w{and 3}.

@item
The length of the array is fixed once you create it; you cannot
change the length of an existing array.

@item
For purposes of evaluation, the array is a constant---in other words,
it evaluates to itself.

@item
The elements of an array may be referenced or changed with the functions
@code{aref} and @code{aset}, respectively (@pxref{Array Functions}).
@end itemize

    When you create an array, other than a char-table, you must specify
its length.  You cannot specify the length of a char-table, because that
is determined by the range of character codes.

  In principle, if you want an array of text characters, you could use
either a string or a vector.  In practice, we always choose strings for
such applications, for four reasons:

@itemize @bullet
@item
They occupy one-fourth the space of a vector of the same elements.

@item
Strings are printed in a way that shows the contents more clearly
as text.

@item
Strings can hold text properties.  @xref{Text Properties}.

@item
Many of the specialized editing and I/O facilities of Emacs accept only
strings.  For example, you cannot insert a vector of characters into a
buffer the way you can insert a string.  @xref{Strings and Characters}.
@end itemize

  By contrast, for an array of keyboard input characters (such as a key
sequence), a vector may be necessary, because many keyboard input
characters are outside the range that will fit in a string.  @xref{Key
Sequence Input}.

@node Array Functions
@section Functions that Operate on Arrays

  In this section, we describe the functions that accept all types of
arrays.

@defun arrayp object
This function returns @code{t} if @var{object} is an array (i.e., a
vector, a string, a bool-vector or a char-table).

@example
@group
(arrayp [a])
     @result{} t
(arrayp "asdf")
     @result{} t
(arrayp (syntax-table))    ;; @r{A char-table.}
     @result{} t
@end group
@end example
@end defun

@defun aref array index
@cindex array elements
This function returns the @var{index}th element of @var{array}.  The
first element is at index zero.

@example
@group
(setq primes [2 3 5 7 11 13])
     @result{} [2 3 5 7 11 13]
(aref primes 4)
     @result{} 11
@end group
@group
(aref "abcdefg" 1)
     @result{} 98           ; @r{@samp{b} is @acronym{ASCII} code 98.}
@end group
@end example

See also the function @code{elt}, in @ref{Sequence Functions}.
@end defun

@defun aset array index object
This function sets the @var{index}th element of @var{array} to be
@var{object}.  It returns @var{object}.

@example
@group
(setq w [foo bar baz])
     @result{} [foo bar baz]
(aset w 0 'fu)
     @result{} fu
w
     @result{} [fu bar baz]
@end group

@group
(setq x "asdfasfd")
     @result{} "asdfasfd"
(aset x 3 ?Z)
     @result{} 90
x
     @result{} "asdZasfd"
@end group
@end example

If @var{array} is a string and @var{object} is not a character, a
@code{wrong-type-argument} error results.  The function converts a
unibyte string to multibyte if necessary to insert a character.
@end defun

@defun fillarray array object
This function fills the array @var{array} with @var{object}, so that
each element of @var{array} is @var{object}.  It returns @var{array}.

@example
@group
(setq a [a b c d e f g])
     @result{} [a b c d e f g]
(fillarray a 0)
     @result{} [0 0 0 0 0 0 0]
a
     @result{} [0 0 0 0 0 0 0]
@end group
@group
(setq s "When in the course")
     @result{} "When in the course"
(fillarray s ?-)
     @result{} "------------------"
@end group
@end example

If @var{array} is a string and @var{object} is not a character, a
@code{wrong-type-argument} error results.
@end defun

The general sequence functions @code{copy-sequence} and @code{length}
are often useful for objects known to be arrays.  @xref{Sequence Functions}.

@node Vectors
@section Vectors
@cindex vector (type)

  Arrays in Lisp, like arrays in most languages, are blocks of memory
whose elements can be accessed in constant time.  A @dfn{vector} is a
general-purpose array of specified length; its elements can be any Lisp
objects.  (By contrast, a string can hold only characters as elements.)
Vectors in Emacs are used for obarrays (vectors of symbols), and as part
of keymaps (vectors of commands).  They are also used internally as part
of the representation of a byte-compiled function; if you print such a
function, you will see a vector in it.

  In Emacs Lisp, the indices of the elements of a vector start from zero
and count up from there.

  Vectors are printed with square brackets surrounding the elements.
Thus, a vector whose elements are the symbols @code{a}, @code{b} and
@code{a} is printed as @code{[a b a]}.  You can write vectors in the
same way in Lisp input.

  A vector, like a string or a number, is considered a constant for
evaluation: the result of evaluating it is the same vector.  This does
not evaluate or even examine the elements of the vector.
@xref{Self-Evaluating Forms}.

  Here are examples illustrating these principles:

@example
@group
(setq avector [1 two '(three) "four" [five]])
     @result{} [1 two (quote (three)) "four" [five]]
(eval avector)
     @result{} [1 two (quote (three)) "four" [five]]
(eq avector (eval avector))
     @result{} t
@end group
@end example

@node Vector Functions
@section Functions for Vectors

  Here are some functions that relate to vectors:

@defun vectorp object
This function returns @code{t} if @var{object} is a vector.

@example
@group
(vectorp [a])
     @result{} t
(vectorp "asdf")
     @result{} nil
@end group
@end example
@end defun

@defun vector &rest objects
This function creates and returns a vector whose elements are the
arguments, @var{objects}.

@example
@group
(vector 'foo 23 [bar baz] "rats")
     @result{} [foo 23 [bar baz] "rats"]
(vector)
     @result{} []
@end group
@end example
@end defun

@defun make-vector length object
This function returns a new vector consisting of @var{length} elements,
each initialized to @var{object}.

@example
@group
(setq sleepy (make-vector 9 'Z))
     @result{} [Z Z Z Z Z Z Z Z Z]
@end group
@end example
@end defun

@defun vconcat &rest sequences
@cindex copying vectors
This function returns a new vector containing all the elements of the
@var{sequences}.  The arguments @var{sequences} may be true lists,
vectors, strings or bool-vectors.  If no @var{sequences} are given, an
empty vector is returned.

The value is a newly constructed vector that is not @code{eq} to any
existing vector.

@example
@group
(setq a (vconcat '(A B C) '(D E F)))
     @result{} [A B C D E F]
(eq a (vconcat a))
     @result{} nil
@end group
@group
(vconcat)
     @result{} []
(vconcat [A B C] "aa" '(foo (6 7)))
     @result{} [A B C 97 97 foo (6 7)]
@end group
@end example

The @code{vconcat} function also allows byte-code function objects as
arguments.  This is a special feature to make it easy to access the entire
contents of a byte-code function object.  @xref{Byte-Code Objects}.

In Emacs versions before 21, the @code{vconcat} function allowed
integers as arguments, converting them to strings of digits, but that
feature has been eliminated.  The proper way to convert an integer to
a decimal number in this way is with @code{format} (@pxref{Formatting
Strings}) or @code{number-to-string} (@pxref{String Conversion}).

For other concatenation functions, see @code{mapconcat} in @ref{Mapping
Functions}, @code{concat} in @ref{Creating Strings}, and @code{append}
in @ref{Building Lists}.
@end defun

  The @code{append} function also provides a way to convert a vector into a
list with the same elements:

@example
@group
(setq avector [1 two (quote (three)) "four" [five]])
     @result{} [1 two (quote (three)) "four" [five]]
(append avector nil)
     @result{} (1 two (quote (three)) "four" [five])
@end group
@end example

@node Char-Tables
@section Char-Tables
@cindex char-tables
@cindex extra slots of char-table

  A char-table is much like a vector, except that it is indexed by
character codes.  Any valid character code, without modifiers, can be
used as an index in a char-table.  You can access a char-table's
elements with @code{aref} and @code{aset}, as with any array.  In
addition, a char-table can have @dfn{extra slots} to hold additional
data not associated with particular character codes.  Char-tables are
constants when evaluated.

@cindex subtype of char-table
  Each char-table has a @dfn{subtype} which is a symbol.  The subtype
has two purposes: to distinguish char-tables meant for different uses,
and to control the number of extra slots.  For example, display tables
are char-tables with @code{display-table} as the subtype, and syntax
tables are char-tables with @code{syntax-table} as the subtype.  A valid
subtype must have a @code{char-table-extra-slots} property which is an
integer between 0 and 10.  This integer specifies the number of
@dfn{extra slots} in the char-table.

@cindex parent of char-table
  A char-table can have a @dfn{parent}, which is another char-table.  If
it does, then whenever the char-table specifies @code{nil} for a
particular character @var{c}, it inherits the value specified in the
parent.  In other words, @code{(aref @var{char-table} @var{c})} returns
the value from the parent of @var{char-table} if @var{char-table} itself
specifies @code{nil}.

@cindex default value of char-table
  A char-table can also have a @dfn{default value}.  If so, then
@code{(aref @var{char-table} @var{c})} returns the default value
whenever the char-table does not specify any other non-@code{nil} value.

@defun make-char-table subtype &optional init
Return a newly created char-table, with subtype @var{subtype}.  Each
element is initialized to @var{init}, which defaults to @code{nil}.  You
cannot alter the subtype of a char-table after the char-table is
created.

There is no argument to specify the length of the char-table, because
all char-tables have room for any valid character code as an index.
@end defun

@defun char-table-p object
This function returns @code{t} if @var{object} is a char-table,
otherwise @code{nil}.
@end defun

@defun char-table-subtype char-table
This function returns the subtype symbol of @var{char-table}.
@end defun

@defun set-char-table-default char-table char new-default
This function sets the default value of generic character @var{char}
in @var{char-table} to @var{new-default}.

There is no special function to access default values in a char-table.
To do that, use @code{char-table-range} (see below).
@end defun

@defun char-table-parent char-table
This function returns the parent of @var{char-table}.  The parent is
always either @code{nil} or another char-table.
@end defun

@defun set-char-table-parent char-table new-parent
This function sets the parent of @var{char-table} to @var{new-parent}.
@end defun

@defun char-table-extra-slot char-table n
This function returns the contents of extra slot @var{n} of
@var{char-table}.  The number of extra slots in a char-table is
determined by its subtype.
@end defun

@defun set-char-table-extra-slot char-table n value
This function stores @var{value} in extra slot @var{n} of
@var{char-table}.
@end defun

  A char-table can specify an element value for a single character code;
it can also specify a value for an entire character set.

@defun char-table-range char-table range
This returns the value specified in @var{char-table} for a range of
characters @var{range}.  Here are the possibilities for @var{range}:

@table @asis
@item @code{nil}
Refers to the default value.

@item @var{char}
Refers to the element for character @var{char}
(supposing @var{char} is a valid character code).

@item @var{charset}
Refers to the value specified for the whole character set
@var{charset} (@pxref{Character Sets}).

@item @var{generic-char}
A generic character stands for a character set, or a row of a
character set; specifying the generic character as argument is
equivalent to specifying the character set name.  @xref{Splitting
Characters}, for a description of generic characters.
@end table
@end defun

@defun set-char-table-range char-table range value
This function sets the value in @var{char-table} for a range of
characters @var{range}.  Here are the possibilities for @var{range}:

@table @asis
@item @code{nil}
Refers to the default value.

@item @code{t}
Refers to the whole range of character codes.

@item @var{char}
Refers to the element for character @var{char}
(supposing @var{char} is a valid character code).

@item @var{charset}
Refers to the value specified for the whole character set
@var{charset} (@pxref{Character Sets}).

@item @var{generic-char}
A generic character stands for a character set; specifying the generic
character as argument is equivalent to specifying the character set
name.  @xref{Splitting Characters}, for a description of generic characters.
@end table
@end defun

@defun map-char-table function char-table
This function calls @var{function} for each element of @var{char-table}.
@var{function} is called with two arguments, a key and a value.  The key
is a possible @var{range} argument for @code{char-table-range}---either
a valid character or a generic character---and the value is
@code{(char-table-range @var{char-table} @var{key})}.

Overall, the key-value pairs passed to @var{function} describe all the
values stored in @var{char-table}.

The return value is always @code{nil}; to make this function useful,
@var{function} should have side effects.  For example,
here is how to examine each element of the syntax table:

@example
(let (accumulator)
  (map-char-table
   #'(lambda (key value)
       (setq accumulator
             (cons (list key value) accumulator)))
   (syntax-table))
  accumulator)
@result{}
((475008 nil) (474880 nil) (474752 nil) (474624 nil)
 ... (5 (3)) (4 (3)) (3 (3)) (2 (3)) (1 (3)) (0 (3)))
@end example
@end defun

@node Bool-Vectors
@section Bool-vectors
@cindex Bool-vectors

  A bool-vector is much like a vector, except that it stores only the
values @code{t} and @code{nil}.  If you try to store any non-@code{nil}
value into an element of the bool-vector, the effect is to store
@code{t} there.  As with all arrays, bool-vector indices start from 0,
and the length cannot be changed once the bool-vector is created.
Bool-vectors are constants when evaluated.

  There are two special functions for working with bool-vectors; aside
from that, you manipulate them with same functions used for other kinds
of arrays.

@defun make-bool-vector length initial
Return a new bool-vector of @var{length} elements,
each one initialized to @var{initial}.
@end defun

@defun bool-vector-p object
This returns @code{t} if @var{object} is a bool-vector,
and @code{nil} otherwise.
@end defun

  Here is an example of creating, examining, and updating a
bool-vector.  Note that the printed form represents up to 8 boolean
values as a single character.

@example
(setq bv (make-bool-vector 5 t))
     @result{} #&5"^_"
(aref bv 1)
     @result{} t
(aset bv 3 nil)
     @result{} nil
bv
     @result{} #&5"^W"
@end example

@noindent
These results make sense because the binary codes for control-_ and
control-W are 11111 and 10111, respectively.

@ignore
   arch-tag: fcf1084a-cd29-4adc-9f16-68586935b386
@end ignore