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sequences.texi
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sequences.texi
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@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990--1995, 1998--1999, 2001--2021 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Sequences Arrays Vectors
@chapter Sequences, Arrays, and Vectors
@cindex sequence
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 fixed-length object with a slot for each of its
elements. All the elements are accessible in constant time. The four
types of arrays are strings, vectors, char-tables and bool-vectors.
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
@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.
* Rings:: Managing a fixed-size ring of objects.
@end menu
@node Sequence Functions
@section Sequences
This section describes functions that accept any kind of sequence.
@defun sequencep object
This function returns @code{t} if @var{object} is a list, vector,
string, bool-vector, or char-table, @code{nil} otherwise. See also
@code{seqp} below.
@end defun
@defun length sequence
@cindex string length
@cindex list length
@cindex vector length
@cindex sequence length
@cindex bool-vector length
@cindex char-table length
@anchor{Definition of length}
This function returns the number of elements in @var{sequence}. The
function signals the @code{wrong-type-argument} error if the argument
is not a sequence or is a dotted list; it signals the
@code{circular-list} error if the argument is a circular list. 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}.
If you need to compute the width of a string on display, you should use
@code{string-width} (@pxref{Size of Displayed Text}), not @code{length},
since @code{length} only counts the number of characters, but does not
account for the display width of each character.
@defun length< sequence length
Return non-@code{nil} if @var{sequence} is shorter than @var{length}.
This may be more efficient than computing the length of @var{sequence}
if @var{sequence} is a long list.
@end defun
@defun length> sequence length
Return non-@code{nil} if @var{sequence} is longer than @var{length}.
@end defun
@defun length= sequence length
Return non-@code{nil} if the length of @var{sequence} is equal to
@var{length}.
@end defun
@defun elt sequence index
@anchor{Definition of elt}
@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 seqr
@cindex copying sequences
This function returns a copy of @var{seqr}, which should be either a
sequence or a record. The copy is the same type of object as the
original, and it has the same elements in the same order. However, if
@var{seqr} is empty, like a string or a vector of zero length, the
value returned by this function might not be a copy, but an empty
object of the same type and identical to @var{seqr}.
Storing a new element into the copy does not affect the original
@var{seqr}, and vice versa. However, the elements of the copy
are not copies; they are identical (@code{eq}) to the elements
of the original. Therefore, changes made within these elements, as
found via the copy, are also visible in the original.
If the argument 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 (list 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 reverse sequence
@cindex string reverse
@cindex list reverse
@cindex vector reverse
@cindex sequence reverse
This function creates a new sequence whose elements are the elements
of @var{sequence}, but in reverse order. The original argument @var{sequence}
is @emph{not} altered. Note that char-tables cannot be reversed.
@example
@group
(setq x '(1 2 3 4))
@result{} (1 2 3 4)
@end group
@group
(reverse x)
@result{} (4 3 2 1)
x
@result{} (1 2 3 4)
@end group
@group
(setq x [1 2 3 4])
@result{} [1 2 3 4]
@end group
@group
(reverse x)
@result{} [4 3 2 1]
x
@result{} [1 2 3 4]
@end group
@group
(setq x "xyzzy")
@result{} "xyzzy"
@end group
@group
(reverse x)
@result{} "yzzyx"
x
@result{} "xyzzy"
@end group
@end example
@end defun
@defun nreverse sequence
@cindex reversing a string
@cindex reversing a list
@cindex reversing a vector
This function reverses the order of the elements of @var{sequence}.
Unlike @code{reverse} the original @var{sequence} may be modified.
For example:
@example
@group
(setq x (list 'a 'b 'c))
@result{} (a b c)
@end group
@group
x
@result{} (a b c)
(nreverse x)
@result{} (c b a)
@end group
@group
;; @r{The cons cell that was first is now last.}
x
@result{} (a)
@end group
@end example
To avoid confusion, we usually store the result of @code{nreverse}
back in the same variable which held the original list:
@example
(setq x (nreverse x))
@end example
Here is the @code{nreverse} of our favorite example, @code{(a b c)},
presented graphically:
@smallexample
@group
@r{Original list head:} @r{Reversed list:}
------------- ------------- ------------
| car | cdr | | car | cdr | | car | cdr |
| a | nil |<-- | b | o |<-- | c | o |
| | | | | | | | | | | | |
------------- | --------- | - | -------- | -
| | | |
------------- ------------
@end group
@end smallexample
For the vector, it is even simpler because you don't need setq:
@example
(setq x (copy-sequence [1 2 3 4]))
@result{} [1 2 3 4]
(nreverse x)
@result{} [4 3 2 1]
x
@result{} [4 3 2 1]
@end example
Note that unlike @code{reverse}, this function doesn't work with strings.
Although you can alter string data by using @code{aset}, it is strongly
encouraged to treat strings as immutable even when they are mutable.
@xref{Mutability}.
@end defun
@defun sort sequence predicate
@cindex stable sort
@cindex sorting lists
@cindex sorting vectors
This function sorts @var{sequence} stably. Note that this function doesn't work
for all sequences; it may be used only for lists and vectors. If @var{sequence}
is a list, it is modified destructively. This functions returns the sorted
@var{sequence} and compares elements using @var{predicate}. A stable sort is
one in which elements with equal sort keys maintain their relative order before
and after the sort. Stability is important when successive sorts are used to
order elements according to different criteria.
The argument @var{predicate} must be a function that accepts two
arguments. It is called with two elements of @var{sequence}. To get an
increasing order sort, the @var{predicate} should return non-@code{nil} if the
first element is ``less'' than the second, or @code{nil} if not.
The comparison function @var{predicate} must give reliable results for
any given pair of arguments, at least within a single call to
@code{sort}. It must be @dfn{antisymmetric}; that is, if @var{a} is
less than @var{b}, @var{b} must not be less than @var{a}. It must be
@dfn{transitive}---that is, if @var{a} is less than @var{b}, and @var{b}
is less than @var{c}, then @var{a} must be less than @var{c}. If you
use a comparison function which does not meet these requirements, the
result of @code{sort} is unpredictable.
The destructive aspect of @code{sort} for lists is that it rearranges the
cons cells forming @var{sequence} by changing @sc{cdr}s. A nondestructive
sort function would create new cons cells to store the elements in their
sorted order. If you wish to make a sorted copy without destroying the
original, copy it first with @code{copy-sequence} and then sort.
Sorting does not change the @sc{car}s of the cons cells in @var{sequence};
the cons cell that originally contained the element @code{a} in
@var{sequence} still has @code{a} in its @sc{car} after sorting, but it now
appears in a different position in the list due to the change of
@sc{cdr}s. For example:
@example
@group
(setq nums (list 1 3 2 6 5 4 0))
@result{} (1 3 2 6 5 4 0)
@end group
@group
(sort nums #'<)
@result{} (0 1 2 3 4 5 6)
@end group
@group
nums
@result{} (1 2 3 4 5 6)
@end group
@end example
@noindent
@strong{Warning}: Note that the list in @code{nums} no longer contains
0; this is the same cons cell that it was before, but it is no longer
the first one in the list. Don't assume a variable that formerly held
the argument now holds the entire sorted list! Instead, save the result
of @code{sort} and use that. Most often we store the result back into
the variable that held the original list:
@example
(setq nums (sort nums #'<))
@end example
For the better understanding of what stable sort is, consider the following
vector example. After sorting, all items whose @code{car} is 8 are grouped
at the beginning of @code{vector}, but their relative order is preserved.
All items whose @code{car} is 9 are grouped at the end of @code{vector},
but their relative order is also preserved:
@example
@group
(setq
vector
(vector '(8 . "xxx") '(9 . "aaa") '(8 . "bbb") '(9 . "zzz")
'(9 . "ppp") '(8 . "ttt") '(8 . "eee") '(9 . "fff")))
@result{} [(8 . "xxx") (9 . "aaa") (8 . "bbb") (9 . "zzz")
(9 . "ppp") (8 . "ttt") (8 . "eee") (9 . "fff")]
@end group
@group
(sort vector (lambda (x y) (< (car x) (car y))))
@result{} [(8 . "xxx") (8 . "bbb") (8 . "ttt") (8 . "eee")
(9 . "aaa") (9 . "zzz") (9 . "ppp") (9 . "fff")]
@end group
@end example
@xref{Sorting}, for more functions that perform sorting.
See @code{documentation} in @ref{Accessing Documentation}, for a
useful example of @code{sort}.
@end defun
@cindex sequence functions in seq
@cindex seq library
@cindex sequences, generalized
The @file{seq.el} library provides the following additional sequence
manipulation macros and functions, prefixed with @code{seq-}. To use
them, you must first load the @file{seq} library.
All functions defined in this library are free of side-effects;
i.e., they do not modify any sequence (list, vector, or string) that
you pass as an argument. Unless otherwise stated, the result is a
sequence of the same type as the input. For those functions that take
a predicate, this should be a function of one argument.
The @file{seq.el} library can be extended to work with additional
types of sequential data-structures. For that purpose, all functions
are defined using @code{cl-defgeneric}. @xref{Generic Functions}, for
more details about using @code{cl-defgeneric} for adding extensions.
@defun seq-elt sequence index
This function returns the element of @var{sequence} at the specified
@var{index}, which is an integer whose valid value range is zero to
one less than the length of @var{sequence}. For out-of-range values
on built-in sequence types, @code{seq-elt} behaves like @code{elt}.
For the details, see @ref{Definition of elt}.
@example
@group
(seq-elt [1 2 3 4] 2)
@result{} 3
@end group
@end example
@code{seq-elt} returns places settable using @code{setf}
(@pxref{Setting Generalized Variables}).
@example
@group
(setq vec [1 2 3 4])
(setf (seq-elt vec 2) 5)
vec
@result{} [1 2 5 4]
@end group
@end example
@end defun
@defun seq-length sequence
This function returns the number of elements in @var{sequence}. For
built-in sequence types, @code{seq-length} behaves like @code{length}.
@xref{Definition of length}.
@end defun
@defun seqp object
This function returns non-@code{nil} if @var{object} is a sequence
(a list or array), or any additional type of sequence defined via
@file{seq.el} generic functions. This is an extensible variant of
@code{sequencep}.
@example
@group
(seqp [1 2])
@result{} t
@end group
@group
(seqp 2)
@result{} nil
@end group
@end example
@end defun
@defun seq-drop sequence n
This function returns all but the first @var{n} (an integer)
elements of @var{sequence}. If @var{n} is negative or zero,
the result is @var{sequence}.
@example
@group
(seq-drop [1 2 3 4 5 6] 3)
@result{} [4 5 6]
@end group
@group
(seq-drop "hello world" -4)
@result{} "hello world"
@end group
@end example
@end defun
@defun seq-take sequence n
This function returns the first @var{n} (an integer) elements of
@var{sequence}. If @var{n} is negative or zero, the result
is @code{nil}.
@example
@group
(seq-take '(1 2 3 4) 3)
@result{} (1 2 3)
@end group
@group
(seq-take [1 2 3 4] 0)
@result{} []
@end group
@end example
@end defun
@defun seq-take-while predicate sequence
This function returns the members of @var{sequence} in order,
stopping before the first one for which @var{predicate} returns @code{nil}.
@example
@group
(seq-take-while (lambda (elt) (> elt 0)) '(1 2 3 -1 -2))
@result{} (1 2 3)
@end group
@group
(seq-take-while (lambda (elt) (> elt 0)) [-1 4 6])
@result{} []
@end group
@end example
@end defun
@defun seq-drop-while predicate sequence
This function returns the members of @var{sequence} in order,
starting from the first one for which @var{predicate} returns @code{nil}.
@example
@group
(seq-drop-while (lambda (elt) (> elt 0)) '(1 2 3 -1 -2))
@result{} (-1 -2)
@end group
@group
(seq-drop-while (lambda (elt) (< elt 0)) [1 4 6])
@result{} [1 4 6]
@end group
@end example
@end defun
@defun seq-do function sequence
This function applies @var{function} to each element of
@var{sequence} in turn (presumably for side effects), and returns
@var{sequence}.
@end defun
@defun seq-map function sequence
This function returns the result of applying @var{function} to each
element of @var{sequence}. The returned value is a list.
@example
@group
(seq-map #'1+ '(2 4 6))
@result{} (3 5 7)
@end group
@group
(seq-map #'symbol-name [foo bar])
@result{} ("foo" "bar")
@end group
@end example
@end defun
@defun seq-map-indexed function sequence
This function returns the result of applying @var{function} to each
element of @var{sequence} and its index within @var{seq}. The
returned value is a list.
@example
@group
(seq-map-indexed (lambda (elt idx)
(list idx elt))
'(a b c))
@result{} ((0 a) (b 1) (c 2))
@end group
@end example
@end defun
@defun seq-mapn function &rest sequences
This function returns the result of applying @var{function} to each
element of @var{sequences}. The arity (@pxref{What Is a Function,
subr-arity}) of @var{function} must match the number of sequences.
Mapping stops at the end of the shortest sequence, and the returned
value is a list.
@example
@group
(seq-mapn #'+ '(2 4 6) '(20 40 60))
@result{} (22 44 66)
@end group
@group
(seq-mapn #'concat '("moskito" "bite") ["bee" "sting"])
@result{} ("moskitobee" "bitesting")
@end group
@end example
@end defun
@defun seq-filter predicate sequence
@cindex filtering sequences
This function returns a list of all the elements in @var{sequence}
for which @var{predicate} returns non-@code{nil}.
@example
@group
(seq-filter (lambda (elt) (> elt 0)) [1 -1 3 -3 5])
@result{} (1 3 5)
@end group
@group
(seq-filter (lambda (elt) (> elt 0)) '(-1 -3 -5))
@result{} nil
@end group
@end example
@end defun
@defun seq-remove predicate sequence
@cindex removing from sequences
This function returns a list of all the elements in @var{sequence}
for which @var{predicate} returns @code{nil}.
@example
@group
(seq-remove (lambda (elt) (> elt 0)) [1 -1 3 -3 5])
@result{} (-1 -3)
@end group
@group
(seq-remove (lambda (elt) (< elt 0)) '(-1 -3 -5))
@result{} nil
@end group
@end example
@end defun
@defun seq-reduce function sequence initial-value
@cindex reducing sequences
This function returns the result of calling @var{function} with
@var{initial-value} and the first element of @var{sequence}, then calling
@var{function} with that result and the second element of @var{sequence},
then with that result and the third element of @var{sequence}, etc.
@var{function} should be a function of two arguments.
@var{function} is called with two arguments. @var{intial-value}
(and then the accumulated value) is used as the first argument, and
the elements in @var{sequence} are used for the second argument.
If @var{sequence} is empty, this returns @var{initial-value} without
calling @var{function}.
@example
@group
(seq-reduce #'+ [1 2 3 4] 0)
@result{} 10
@end group
@group
(seq-reduce #'+ '(1 2 3 4) 5)
@result{} 15
@end group
@group
(seq-reduce #'+ '() 3)
@result{} 3
@end group
@end example
@end defun
@defun seq-some predicate sequence
This function returns the first non-@code{nil} value returned by
applying @var{predicate} to each element of @var{sequence} in turn.
@example
@group
(seq-some #'numberp ["abc" 1 nil])
@result{} t
@end group
@group
(seq-some #'numberp ["abc" "def"])
@result{} nil
@end group
@group
(seq-some #'null ["abc" 1 nil])
@result{} t
@end group
@group
(seq-some #'1+ [2 4 6])
@result{} 3
@end group
@end example
@end defun
@defun seq-find predicate sequence &optional default
This function returns the first element in @var{sequence} for which
@var{predicate} returns non-@code{nil}. If no element matches
@var{predicate}, the function returns @var{default}.
Note that this function has an ambiguity if the found element is
identical to @var{default}, as in that case it cannot be known whether
an element was found or not.
@example
@group
(seq-find #'numberp ["abc" 1 nil])
@result{} 1
@end group
@group
(seq-find #'numberp ["abc" "def"])
@result{} nil
@end group
@end example
@end defun
@defun seq-every-p predicate sequence
This function returns non-@code{nil} if applying @var{predicate}
to every element of @var{sequence} returns non-@code{nil}.
@example
@group
(seq-every-p #'numberp [2 4 6])
@result{} t
@end group
@group
(seq-every-p #'numberp [2 4 "6"])
@result{} nil
@end group
@end example
@end defun
@defun seq-empty-p sequence
This function returns non-@code{nil} if @var{sequence} is empty.
@example
@group
(seq-empty-p "not empty")
@result{} nil
@end group
@group
(seq-empty-p "")
@result{} t
@end group
@end example
@end defun
@defun seq-count predicate sequence
This function returns the number of elements in @var{sequence} for which
@var{predicate} returns non-@code{nil}.
@example
(seq-count (lambda (elt) (> elt 0)) [-1 2 0 3 -2])
@result{} 2
@end example
@end defun
@cindex sorting sequences
@defun seq-sort function sequence
This function returns a copy of @var{sequence} that is sorted
according to @var{function}, a function of two arguments that returns
non-@code{nil} if the first argument should sort before the second.
@end defun
@defun seq-sort-by function predicate sequence
This function is similar to @code{seq-sort}, but the elements of
@var{sequence} are transformed by applying @var{function} on them
before being sorted. @var{function} is a function of one argument.
@example
(seq-sort-by #'seq-length #'> ["a" "ab" "abc"])
@result{} ["abc" "ab" "a"]
@end example
@end defun
@defun seq-contains-p sequence elt &optional function
This function returns non-@code{nil} if at least one element in
@var{sequence} is equal to @var{elt}. If the optional argument
@var{function} is non-@code{nil}, it is a function of two arguments to
use instead of the default @code{equal}.
@example
@group
(seq-contains-p '(symbol1 symbol2) 'symbol1)
@result{} t
@end group
@group
(seq-contains-p '(symbol1 symbol2) 'symbol3)
@result{} nil
@end group
@end example
@end defun
@defun seq-set-equal-p sequence1 sequence2 &optional testfn
This function checks whether @var{sequence1} and @var{sequence2}
contain the same elements, regardless of the order. If the optional
argument @var{testfn} is non-@code{nil}, it is a function of two
arguments to use instead of the default @code{equal}.
@example
@group
(seq-set-equal-p '(a b c) '(c b a))
@result{} t
@end group
@group
(seq-set-equal-p '(a b c) '(c b))
@result{} nil
@end group
@group
(seq-set-equal-p '("a" "b" "c") '("c" "b" "a"))
@result{} t
@end group
@group
(seq-set-equal-p '("a" "b" "c") '("c" "b" "a") #'eq)
@result{} nil
@end group
@end example
@end defun
@defun seq-position sequence elt &optional function
This function returns the index of the first element in
@var{sequence} that is equal to @var{elt}. If the optional argument
@var{function} is non-@code{nil}, it is a function of two arguments to
use instead of the default @code{equal}.
@example
@group
(seq-position '(a b c) 'b)
@result{} 1
@end group
@group
(seq-position '(a b c) 'd)
@result{} nil
@end group
@end example
@end defun
@defun seq-uniq sequence &optional function
This function returns a list of the elements of @var{sequence} with
duplicates removed. If the optional argument @var{function} is non-@code{nil},
it is a function of two arguments to use instead of the default @code{equal}.
@example
@group
(seq-uniq '(1 2 2 1 3))
@result{} (1 2 3)
@end group
@group
(seq-uniq '(1 2 2.0 1.0) #'=)
@result{} (1 2)
@end group
@end example
@end defun
@defun seq-subseq sequence start &optional end
@cindex sub-sequence
This function returns a subset of @var{sequence} from @var{start}
to @var{end}, both integers (@var{end} defaults to the last element).
If @var{start} or @var{end} is negative, it counts from the end of
@var{sequence}.
@example
@group
(seq-subseq '(1 2 3 4 5) 1)
@result{} (2 3 4 5)
@end group
@group
(seq-subseq '[1 2 3 4 5] 1 3)
@result{} [2 3]
@end group
@group
(seq-subseq '[1 2 3 4 5] -3 -1)
@result{} [3 4]
@end group
@end example
@end defun
@defun seq-concatenate type &rest sequences
This function returns a sequence of type @var{type} made of the
concatenation of @var{sequences}. @var{type} may be: @code{vector},
@code{list} or @code{string}.
@example
@group
(seq-concatenate 'list '(1 2) '(3 4) [5 6])
@result{} (1 2 3 4 5 6)
@end group
@group
(seq-concatenate 'string "Hello " "world")
@result{} "Hello world"
@end group
@end example
@end defun
@defun seq-mapcat function sequence &optional type
This function returns the result of applying @code{seq-concatenate}
to the result of applying @var{function} to each element of
@var{sequence}. The result is a sequence of type @var{type}, or a
list if @var{type} is @code{nil}.
@example
@group
(seq-mapcat #'seq-reverse '((3 2 1) (6 5 4)))
@result{} (1 2 3 4 5 6)
@end group
@end example
@end defun
@defun seq-partition sequence n
This function returns a list of the elements of @var{sequence}
grouped into sub-sequences of length @var{n}. The last sequence may
contain less elements than @var{n}. @var{n} must be an integer. If
@var{n} is a negative integer or 0, the return value is @code{nil}.
@example
@group
(seq-partition '(0 1 2 3 4 5 6 7) 3)
@result{} ((0 1 2) (3 4 5) (6 7))
@end group
@end example
@end defun
@defun seq-intersection sequence1 sequence2 &optional function
@cindex sequences, intersection of
@cindex intersection of sequences
This function returns a list of the elements that appear both in
@var{sequence1} and @var{sequence2}. If the optional argument
@var{function} is non-@code{nil}, it is a function of two arguments to
use to compare elements instead of the default @code{equal}.
@example
@group
(seq-intersection [2 3 4 5] [1 3 5 6 7])
@result{} (3 5)
@end group
@end example
@end defun
@defun seq-difference sequence1 sequence2 &optional function
This function returns a list of the elements that appear in
@var{sequence1} but not in @var{sequence2}. If the optional argument
@var{function} is non-@code{nil}, it is a function of two arguments to
use to compare elements instead of the default @code{equal}.
@example
@group
(seq-difference '(2 3 4 5) [1 3 5 6 7])
@result{} (2 4)
@end group
@end example
@end defun
@defun seq-group-by function sequence
This function separates the elements of @var{sequence} into an alist
whose keys are the result of applying @var{function} to each element
of @var{sequence}. Keys are compared using @code{equal}.
@example
@group
(seq-group-by #'integerp '(1 2.1 3 2 3.2))
@result{} ((t 1 3 2) (nil 2.1 3.2))
@end group
@group
(seq-group-by #'car '((a 1) (b 2) (a 3) (c 4)))
@result{} ((b (b 2)) (a (a 1) (a 3)) (c (c 4)))
@end group