@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 Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999
+@c Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/lists
@node Lists, Sequences Arrays Vectors, Strings and Characters, Top
Lists in Lisp are not a primitive data type; they are built up from
@dfn{cons cells}. A cons cell is a data object that represents an
-ordered pair. It records two Lisp objects, one labeled as the @sc{car},
-and the other labeled as the @sc{cdr}. These names are traditional; see
-@ref{Cons Cell Type}. @sc{cdr} is pronounced ``could-er.''
-
- A list is a series of cons cells chained together, one cons cell per
-element of the list. By convention, the @sc{car}s of the cons cells are
-the elements of the list, and the @sc{cdr}s are used to chain the list:
-the @sc{cdr} of each cons cell is the following cons cell. The @sc{cdr}
-of the last cons cell is @code{nil}. This asymmetry between the
-@sc{car} and the @sc{cdr} is entirely a matter of convention; at the
+ordered pair. That is, it has two slots, and each slot @dfn{holds}, or
+@dfn{refers to}, some Lisp object. One slot is known as the @sc{car},
+and the other is known as the @sc{cdr}. (These names are traditional;
+see @ref{Cons Cell Type}.) @sc{cdr} is pronounced ``could-er.''
+
+ We say that ``the @sc{car} of this cons cell is'' whatever object
+its @sc{car} slot currently holds, and likewise for the @sc{cdr}.
+
+ A list is a series of cons cells ``chained together,'' so that each
+cell refers to the next one. There is one cons cell for each element of
+the list. By convention, the @sc{car}s of the cons cells hold the
+elements of the list, and the @sc{cdr}s are used to chain the list: the
+@sc{cdr} slot of each cons cell refers to the following cons cell. The
+@sc{cdr} of the last cons cell is @code{nil}. This asymmetry between
+the @sc{car} and the @sc{cdr} is entirely a matter of convention; at the
level of cons cells, the @sc{car} and @sc{cdr} slots have the same
characteristics.
@end example
Each pair of boxes represents a cons cell. Each box ``refers to'',
-``points to'' or ``contains'' a Lisp object. (These terms are
-synonymous.) The first box, which is the @sc{car} of the first cons
-cell, contains the symbol @code{tulip}. The arrow from the @sc{cdr} of
-the first cons cell to the second cons cell indicates that the @sc{cdr}
-of the first cons cell points to the second cons cell.
+``points to'' or ``holds'' a Lisp object. (These terms are
+synonymous.) The first box, which describes the @sc{car} of the first
+cons cell, contains the symbol @code{tulip}. The arrow from the
+@sc{cdr} box of the first cons cell to the second cons cell indicates
+that the @sc{cdr} of the first cons cell is the second cons cell.
The same list can be illustrated in a different sort of box notation
like this:
@example
@group
- ___ ___ ___ ___
- |___|___|--> |___|___|--> nil
+ --- --- --- ---
+ | | |--> | | |--> nil
+ --- --- --- ---
| |
| |
--> tulip --> lily
@example
@group
- ___ ___ ___ ___ ___ ___
- |___|___|--> |___|___|--> |___|___|--> nil
+ --- --- --- --- --- ---
+ | | |--> | | |--> | | |--> nil
+ --- --- --- --- --- ---
| | |
| | |
| --> oak --> maple
|
- | ___ ___ ___ ___
- --> |___|___|--> |___|___|--> nil
+ | --- --- --- ---
+ --> | | |--> | | |--> nil
+ --- --- --- ---
| |
| |
--> pine --> needles
@cindex list elements
@defun car cons-cell
-This function returns the value pointed to by the first pointer of the
+This function returns the value referred to by the first slot of the
cons cell @var{cons-cell}. Expressed another way, this function
returns the @sc{car} of @var{cons-cell}.
@end defun
@defun cdr cons-cell
-This function returns the value pointed to by the second pointer of
+This function returns the value referred to by the second slot of
the cons cell @var{cons-cell}. Expressed another way, this function
returns the @sc{cdr} of @var{cons-cell}.
@end example
@end defun
+@tindex pop
+@defmac pop listname
+This macro is a way of examining the @sc{car} of a list,
+and taking it off the list, all at once. It is new in Emacs 21.
+
+It operates on the list which is stored in the symbol @var{listname}.
+It removes this element from the list by setting @var{listname}
+to the @sc{cdr} of its old value---but it also returns the @sc{car}
+of that list, which is the element being removed.
+
+@example
+x
+ @result{} (a b c)
+(pop x)
+ @result{} a
+x
+ @result{} (b c)
+@end example
+@end defmac
+
@defun nth n list
This function returns the @var{n}th element of @var{list}. Elements
are numbered starting with zero, so the @sc{car} of @var{list} is
(nth n x) @equiv{} (car (nthcdr n x))
@end group
@end example
+
+The function @code{elt} is similar, but applies to any kind of sequence.
+For historical reasons, it takes its arguments in the opposite order.
+@xref{Sequence Functions}.
@end defun
@defun nthcdr n list
This function returns the @var{n}th @sc{cdr} of @var{list}. In other
-words, it removes the first @var{n} links of @var{list} and returns
+words, it skips past the first @var{n} links of @var{list} and returns
what follows.
If @var{n} is zero or negative, @code{nthcdr} returns all of
@end example
@end defun
+@defun last list &optional n
+This function reruns the last link of the given @var{list}. The
+@code{car} of this link is the list's last element. If @var{list} is
+null, @code{nil} is returned. If @var{n} is non-nil the
+@var{n}-th-to-last link is returned instead, or the whole @var{list} if
+@var{n} is bigger than @var{list}'s length.
+@end defun
+
+@defun safe-length list
+This function returns the length of @var{list}, with no risk
+of either an error or an infinite loop.
+
+If @var{list} is not really a list, @code{safe-length} returns 0. If
+@var{list} is circular, it returns a finite value which is at least the
+number of distinct elements.
+@end defun
+
+ The most common way to compute the length of a list, when you are not
+worried that it may be circular, is with @code{length}. @xref{Sequence
+Functions}.
+
+@defun caar cons-cell
+This is the same as @code{(car (car @var{cons-cell}))}.
+@end defun
+
+@defun cadr cons-cell
+This is the same as @code{(car (cdr @var{cons-cell}))}
+or @code{(nth 1 @var{cons-cell})}.
+@end defun
+
+@defun cdar cons-cell
+This is the same as @code{(cdr (car @var{cons-cell}))}.
+@end defun
+
+@defun cddr cons-cell
+This is the same as @code{(cdr (cdr @var{cons-cell}))}
+or @code{(nthcdr 2 @var{cons-cell})}.
+@end defun
+
@node Building Lists
@comment node-name, next, previous, up
@section Building Cons Cells and Lists
@cindex consing
@code{cons} is often used to add a single element to the front of a
-list. This is called @dfn{consing the element onto the list}. For
-example:
+list. This is called @dfn{consing the element onto the list}.
+@footnote{There is no strictly equivalent way to add an element to
+the end of a list. You can use @code{(append @var{listname} (list
+@var{newelt}))}, which creates a whole new list by copying @var{listname}
+and adding @var{newelt} to its end. Or you can use @code{(nconc
+@var{listname} (list @var{newelt}))}, which modifies @var{listname}
+by following all the @sc{cdr}s and then replacing the terminating
+@code{nil}. Compare this to adding an element to the beginning of a
+list with @code{cons}, which neither copies nor modifies the list.}
+For example:
@example
(setq list (cons newelt list))
any symbol can serve both purposes.
@end defun
+@tindex push
+@defmac push newelt listname
+This macro provides an alternative way to write
+@code{(setq @var{listname} (cons @var{newelt} @var{listname}))}.
+It is new in Emacs 21.
+@end defmac
+
@defun list &rest objects
This function creates a list with @var{objects} as its elements. The
resulting list is always @code{nil}-terminated. If no @var{objects}
@defun append &rest sequences
@cindex copying lists
This function returns a list containing all the elements of
-@var{sequences}. The @var{sequences} may be lists, vectors, or strings,
-but the last one should be a list. All arguments except the last one
-are copied, so none of them are altered.
+@var{sequences}. The @var{sequences} may be lists, vectors,
+bool-vectors, or strings, but the last one should usually be a list.
+All arguments except the last one are copied, so none of the arguments
+is altered. (See @code{nconc} in @ref{Rearrangement}, for a way to join
+lists with no copying.)
More generally, the final argument to @code{append} may be any Lisp
object. The final argument is not copied or converted; it becomes the
``dotted list'' since its final @sc{cdr} is not @code{nil} as required
in a true list.
-See @code{nconc} in @ref{Rearrangement}, for a way to join lists with no
-copying.
+The @code{append} function also allows integers as arguments. It
+converts them to strings of digits, making up the decimal print
+representation of the integer, and then uses the strings instead of the
+original integers. @strong{Don't use this feature; we plan to eliminate
+it. If you already use this feature, change your programs now!} 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}).
+@end defun
-Here is an example of using @code{append}:
+ Here is an example of using @code{append}:
@example
@group
@end group
@end example
-You can see how @code{append} works by looking at a box diagram. The
+ You can see how @code{append} works by looking at a box diagram. The
variable @code{trees} is set to the list @code{(pine oak)} and then the
variable @code{more-trees} is set to the list @code{(maple birch pine
oak)}. However, the variable @code{trees} continues to refer to the
@group
more-trees trees
| |
-| ___ ___ ___ ___ -> ___ ___ ___ ___
- --> |___|___|--> |___|___|--> |___|___|--> |___|___|--> nil
+| --- --- --- --- -> --- --- --- ---
+ --> | | |--> | | |--> | | |--> | | |--> nil
+ --- --- --- --- --- --- --- ---
| | | |
| | | |
--> maple -->birch --> pine --> oak
@end group
@end smallexample
-An empty sequence contributes nothing to the value returned by
+ An empty sequence contributes nothing to the value returned by
@code{append}. As a consequence of this, a final @code{nil} argument
-forces a copy of the previous argument.
+forces a copy of the previous argument:
@example
@group
@result{} (pine oak)
@end group
@group
-(setq wood (append trees ()))
+(setq wood (append trees nil))
@result{} (pine oak)
@end group
@group
This once was the usual way to copy a list, before the function
@code{copy-sequence} was invented. @xref{Sequences Arrays Vectors}.
-With the help of @code{apply}, we can append all the lists in a list of
-lists:
+ Here we show the use of vectors and strings as arguments to @code{append}:
+
+@example
+@group
+(append [a b] "cd" nil)
+ @result{} (a b 99 100)
+@end group
+@end example
+
+ With the help of @code{apply} (@pxref{Calling Functions}), we can append
+all the lists in a list of lists:
@example
@group
@end group
@end example
-If no @var{sequences} are given, @code{nil} is returned:
+ If no @var{sequences} are given, @code{nil} is returned:
@example
@group
@end group
@end example
-Here are some examples where the final argument is not a list:
+ Here are some examples where the final argument is not a list:
@example
(append '(x y) 'z)
- @result{} (x y z)
+ @result{} (x y . z)
(append '(x y) [z])
- @result{} (x y [z])
+ @result{} (x y . [z])
@end example
@noindent
resulting list. Instead, the sequence becomes the final @sc{cdr}, like
any other non-list final argument.
-The @code{append} function also allows integers as arguments. It
-converts them to strings of digits, making up the decimal print
-representation of the integer, and then uses the strings instead of the
-original integers. @strong{Don't use this feature; we plan to eliminate
-it. If you already use this feature, change your programs now!} 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}).
-@end defun
-
@defun reverse list
This function creates a new list whose elements are the elements of
@var{list}, but in reverse order. The original argument @var{list} is
@end example
@end defun
+@defun remq object list
+This function returns a copy of @var{list}, with all elements removed
+which are @code{eq} to @var{object}. The letter @samp{q} in @code{remq}
+says that it uses @code{eq} to compare @var{object} against the elements
+of @code{list}.
+
+@example
+@group
+(setq sample-list '(a b c a b c))
+ @result{} (a b c a b c)
+@end group
+@group
+(remq 'a sample-list)
+ @result{} (b c b c)
+@end group
+@group
+sample-list
+ @result{} (a b c a b c)
+@end group
+@end example
+@noindent
+The function @code{delq} offers a way to perform this operation
+destructively. See @ref{Sets And Lists}.
+@end defun
+
@node Modifying Lists
@section Modifying Existing List Structure
+@cindex destructive list operations
You can modify the @sc{car} and @sc{cdr} contents of a cons cell with the
-primitives @code{setcar} and @code{setcdr}.
+primitives @code{setcar} and @code{setcdr}. We call these ``destructive''
+operations because they change existing list structure.
@cindex CL note---@code{rplaca} vrs @code{setcar}
@quotation
@defun setcar cons object
This function stores @var{object} as the new @sc{car} of @var{cons},
-replacing its previous @sc{car}. It returns the value @var{object}.
-For example:
+replacing its previous @sc{car}. In other words, it changes the
+@sc{car} slot of @var{cons} to refer to @var{object}. It returns the
+value @var{object}. For example:
@example
@group
@example
@group
- ___ ___ ___ ___ ___ ___
-x1---> |___|___|----> |___|___|--> |___|___|--> nil
+ --- --- --- --- --- ---
+x1---> | | |----> | | |--> | | |--> nil
+ --- --- --- --- --- ---
| --> | |
| | | |
--> a | --> b --> c
|
- ___ ___ |
-x2--> |___|___|--
+ --- --- |
+x2--> | | |--
+ --- ---
|
|
--> z
@defun setcdr cons object
This function stores @var{object} as the new @sc{cdr} of @var{cons},
-replacing its previous @sc{cdr}. It returns the value @var{object}.
+replacing its previous @sc{cdr}. In other words, it changes the
+@sc{cdr} slot of @var{cons} to refer to @var{object}. It returns the
+value @var{object}.
@end defun
Here is an example of replacing the @sc{cdr} of a list with a
You can delete elements from the middle of a list by altering the
@sc{cdr}s of the cons cells in the list. For example, here we delete
the second element, @code{b}, from the list @code{(a b c)}, by changing
-the @sc{cdr} of the first cell:
+the @sc{cdr} of the first cons cell:
@example
@group
@end group
@end example
+@need 4000
Here is the result in box notation:
@example
Here are some functions that rearrange lists ``destructively'' by
modifying the @sc{cdr}s of their component cons cells. We call these
functions ``destructive'' because they chew up the original lists passed
-to them as arguments, to produce a new list that is the returned value.
+to them as arguments, relinking their cons cells to form a new list that
+is the returned value.
-@ifinfo
+@ifnottex
See @code{delq}, in @ref{Sets And Lists}, for another function
that modifies cons cells.
-@end ifinfo
+@end ifnottex
@iftex
The function @code{delq} in the following section is another example
of destructive list manipulation.
@end group
@end example
+However, the other arguments (all but the last) must be lists.
+
A common pitfall is to use a quoted constant list as a non-last
argument to @code{nconc}. If you do this, your program will change
each time you run it! Here is what happens:
This function reverses the order of the elements of @var{list}.
Unlike @code{reverse}, @code{nreverse} alters its argument by reversing
the @sc{cdr}s in the cons cells forming the list. The cons cell that
-used to be the last one in @var{list} becomes the first cell of the
+used to be the last one in @var{list} becomes the first cons cell of the
value.
For example:
@result{} (4 3 2 1)
@end group
@group
-;; @r{The cell that was first is now last.}
+;; @r{The cons cell that was first is now last.}
x
@result{} (1)
@end group
increasing order sort, the @var{predicate} should return @code{t} 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} is that it rearranges the cons
cells forming @var{list} 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{list};
-each cons cell in the result contains the same element that it contained
-before. The result differs from the argument @var{list} because the
-cells themselves have been reordered.
-
Sorting does not change the @sc{car}s of the cons cells in @var{list};
the cons cell that originally contained the element @code{a} in
@var{list} still has @code{a} in its @sc{car} after sorting, but it now
@end example
@noindent
-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:
+@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 '<))
functions for sets include @code{memq} and @code{delq}, and their
@code{equal} versions, @code{member} and @code{delete}.
-@cindex CL note---lack @code{union}, @code{set}
+@cindex CL note---lack @code{union}, @code{intersection}
@quotation
@b{Common Lisp note:} Common Lisp has functions @code{union} (which
avoids duplicate elements) and @code{intersection} for set operations,
This function destructively removes all elements @code{eq} to
@var{object} from @var{list}. The letter @samp{q} in @code{delq} says
that it uses @code{eq} to compare @var{object} against the elements of
-the list, like @code{memq}.
+the list, like @code{memq} and @code{remq}.
@end defun
When @code{delq} deletes elements from the front of the list, it does so
@end group
@end example
-Note that @code{(delq 'b sample-list)} modifies @code{sample-list} to
-splice out the second element, but @code{(delq 'a sample-list)} does not
+Note that @code{(delq 'c sample-list)} modifies @code{sample-list} to
+splice out the third element, but @code{(delq 'a sample-list)} does not
splice anything---it just returns a shorter list. Don't assume that a
variable which formerly held the argument @var{list} now has fewer
elements, or that it still holds the original list! Instead, save the
@end example
The following two functions are like @code{memq} and @code{delq} but use
-@code{equal} rather than @code{eq} to compare elements. They are new in
-Emacs 19.
+@code{equal} rather than @code{eq} to compare elements. @xref{Equality
+Predicates}.
@defun member object list
The function @code{member} tests to see whether @var{object} is a member
@end example
@end defun
-@defun delete object list
-This function destructively removes all elements @code{equal} to
-@var{object} from @var{list}. It is to @code{delq} as @code{member} is
-to @code{memq}: it uses @code{equal} to compare elements with
-@var{object}, like @code{member}; when it finds an element that matches,
-it removes the element just as @code{delq} would. For example:
+@defun delete object sequence
+If @code{sequence} is a list, this function destructively removes all
+elements @code{equal} to @var{object} from @var{sequence}. For lists,
+@code{delete} is to @code{delq} as @code{member} is to @code{memq}: it
+uses @code{equal} to compare elements with @var{object}, like
+@code{member}; when it finds an element that matches, it removes the
+element just as @code{delq} would.
+
+If @code{sequence} is a vector or string, @code{delete} returns a copy
+of @code{sequence} with all elements @code{equal} to @code{object}
+removed.
+
+For example:
@example
@group
(delete '(2) '((2) (1) (2)))
- @result{} '((1))
+ @result{} ((1))
+@end group
+@group
+(delete '(2) [(2) (1) (2)])
+ @result{} [(1)]
+@end group
+@end example
+@end defun
+
+@defun remove object sequence
+This function is the non-destructive counterpart of @code{delete}. If
+returns a copy of @code{sequence}, a list, vector, or string, with
+elements @code{equal} to @code{object} removed. For example:
+
+@example
+@group
+(remove '(2) '((2) (1) (2)))
+ @result{} ((1))
+@end group
+@group
+(remove '(2) [(2) (1) (2)])
+ @result{} [(1)]
@end group
@end example
@end defun
@quotation
-@b{Common Lisp note:} The functions @code{member} and @code{delete} in
-GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common
-Lisp versions do not use @code{equal} to compare elements.
+@b{Common Lisp note:} The functions @code{member}, @code{delete} and
+@code{remove} in GNU Emacs Lisp are derived from Maclisp, not Common
+Lisp. The Common Lisp versions do not use @code{equal} to compare
+elements.
@end quotation
+ See also the function @code{add-to-list}, in @ref{Setting Variables},
+for another way to add an element to a list stored in a variable.
+
@node Association Lists
@section Association Lists
@cindex association list
An @dfn{association list}, or @dfn{alist} for short, records a mapping
from keys to values. It is a list of cons cells called
-@dfn{associations}: the @sc{car} of each cell is the @dfn{key}, and the
+@dfn{associations}: the @sc{car} of each cons cell is the @dfn{key}, and the
@sc{cdr} is the @dfn{associated value}.@footnote{This usage of ``key''
is not related to the term ``key sequence''; it means a value used to
look up an item in a table. In this case, the table is the alist, and
@noindent
Here we regard @code{red} as the value associated with @code{rose}. One
-advantage of this method is that you can store other related
+advantage of this kind of alist is that you can store other related
information---even a list of other items---in the @sc{cdr} of the
@sc{cdr}. One disadvantage is that you cannot use @code{rassq} (see
below) to find the element containing a given value. When neither of
@end smallexample
@end defun
+ The functions @code{assoc-ignore-representation} and
+@code{assoc-ignore-case} are much like @code{assoc} except using
+@code{compare-strings} to do the comparison. @xref{Text Comparison}.
+
@defun rassoc value alist
This function returns the first association with value @var{value} in
@var{alist}. It returns @code{nil} if no association in @var{alist} has
@end smallexample
@end defun
+@defun assoc-default key alist test default
+This function searches @var{alist} for a match for @var{key}. For each
+element of @var{alist}, it compares the element (if it is an atom) or
+the element's @sc{car} (if it is a cons) against @var{key}, by calling
+@var{test} with two arguments: the element or its @sc{car}, and
+@var{key}. The arguments are passed in that order so that you can get
+useful results using @code{string-match} with an alist that contains
+regular expressions (@pxref{Regexp Search}). If @var{test} is omitted
+or @code{nil}, @code{equal} is used for comparison.
+
+If an alist element matches @var{key} by this criterion,
+then @code{assoc-default} returns a value based on this element.
+If the element is a cons, then the value is the element's @sc{cdr}.
+Otherwise, the return value is @var{default}.
+
+If no alist element matches @var{key}, @code{assoc-default} returns
+@code{nil}.
+@end defun
+
@defun copy-alist alist
@cindex copying alists
This function returns a two-level deep copy of @var{alist}: it creates a
@end smallexample
@end defun
+@defun assoc-delete-all key alist
+@tindex assoc-delete-all
+This function deletes from @var{alist} all the elements whose @sc{car}
+is @var{key}. It returns the modified alist.
+@example
+(assoc-delete-all 'foo
+ '((foo 1) (bar 2) (foo 3) (lose 4)))
+ @result{} ((bar 2) (lose 4))
+@end example
+@end defun
HCoop / bpt / emacs.git / blobdiff