emacs/lispref/sequences.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/sequences
@node Sequences Arrays Vectors, Symbols, Lists, Top
@chapter Sequences, Arrays, and Vectors
@cindex sequence

  Recall that the @dfn{sequence} type is the union of three other Lisp
types: lists, vectors, and strings.  In other words, any list is a
sequence, any vector is a sequence, and any string 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 directly containing all
its elements.  Therefore, all the elements are accessible in constant
time.  The length of an existing array cannot be changed.  Both strings
and vectors are 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.  Therefore, 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        | |
           | |      | |  ________   _______  | |   
           | |______| | |        | |       | | |
           |          | | String | | Vector| | |
           |          | |________| |_______| | |
           |          |______________________| |
           |___________________________________|

@center @r{The relationship between sequences, arrays, and vectors}
@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::               Functions specifically for vectors.
@end menu

@node Sequence Functions
@section Sequences

  In Emacs Lisp, a @dfn{sequence} is either a list, a vector or a
string.  The common property that all sequences have is that each is an
ordered collection 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, or
string, @code{nil} otherwise.
@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}.

See also @code{append} in @ref{Building Lists}, @code{concat} in
@ref{Creating Strings}, and @code{vconcat} in @ref{Vectors}, for others
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

@defun length sequence
@cindex string length
@cindex list length
@cindex vector length
@cindex sequence length
Returns the number of elements in @var{sequence}.  If @var{sequence} is
a cons cell that is not a list (because the final @sc{cdr} is not
@code{nil}), a @code{wrong-type-argument} error is signaled.

@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
@end example
@end defun

@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, then out-of-range values of @var{index} return @code{nil};
otherwise, they 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
(char-to-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 duplicates @code{aref} (@pxref{Array Functions}) and
@code{nth} (@pxref{List Elements}), except that it works for any kind of
sequence.
@end defun

@node Arrays
@section Arrays
@cindex array

  An @dfn{array} object refers directly to 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.

  When you create an array, you must specify how many elements it has.
The amount of space allocated depends on the number of elements.
Therefore, it is impossible to change the size of an array once it is
created.  You cannot add or remove elements.  However, you can replace
an element with a different value.

  Emacs defines two types of array, both of which are one-dimensional:
@dfn{strings} and @dfn{vectors}.  A vector is a general array; its
elements can be any Lisp objects.  A string is a specialized array; its
elements must be characters (i.e., integers between 0 and 255).  Each
type of array has its own read syntax.  @xref{String Type}, and
@ref{Vector Type}.

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

  In principle, if you wish to have an array of 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 characters.

@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

@node Array Functions
@section Functions that Operate on Arrays

  In this section, we describe the functions that accept both strings
and vectors.

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

@example
@group
(arrayp [a])
@result{} t
(arrayp "asdf")
@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
(elt primes 4)
     @result{} 11
@end group

@group
(aref "abcdefg" 1)
     @result{} 98           ; @r{@samp{b} is @sc{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.
@end defun

@defun fillarray array object
This function fills the array @var{array} with pointers to @var{object},
replacing any previous values.  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

  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; its elements can be any Lisp objects.  (The other
kind of array in Emacs Lisp is the @dfn{string}, whose elements must be
characters.)  Vectors in Emacs serve as syntax tables (vectors of
integers), as obarrays (vectors of symbols), and in 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
in their order.  Thus, a vector containing the symbols @code{a},
@code{b} and @code{c} is printed as @code{[a b c]}.  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 of 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

  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 lists, vectors,
or strings.  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

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 to digits in this way, use
@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 provides a way to convert a vector into a
list with the same elements (@pxref{Building Lists}):

@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