This is Info file ../../info/lispref.info, produced by Makeinfo version 1.68 from the input file lispref.texi. INFO-DIR-SECTION XEmacs Editor START-INFO-DIR-ENTRY * Lispref: (lispref). XEmacs Lisp Reference Manual. END-INFO-DIR-ENTRY Edition History: GNU Emacs Lisp Reference Manual Second Edition (v2.01), May 1993 GNU Emacs Lisp Reference Manual Further Revised (v2.02), August 1993 Lucid Emacs Lisp Reference Manual (for 19.10) First Edition, March 1994 XEmacs Lisp Programmer's Manual (for 19.12) Second Edition, April 1995 GNU Emacs Lisp Reference Manual v2.4, June 1995 XEmacs Lisp Programmer's Manual (for 19.13) Third Edition, July 1995 XEmacs Lisp Reference Manual (for 19.14 and 20.0) v3.1, March 1996 XEmacs Lisp Reference Manual (for 19.15 and 20.1, 20.2, 20.3) v3.2, April, May, November 1997 XEmacs Lisp Reference Manual (for 21.0) v3.3, April 1998 Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software Foundation, Inc. Copyright (C) 1994, 1995 Sun Microsystems, Inc. Copyright (C) 1995, 1996 Ben Wing. 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. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled "GNU General Public License" is included exactly as in the original, and 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 the section entitled "GNU General Public License" may be included in a translation approved by the Free Software Foundation instead of in the original English.  File: lispref.info, Node: List Elements, Next: Building Lists, Prev: List-related Predicates, Up: Lists Accessing Elements of Lists =========================== - Function: car CONS-CELL This function returns the value pointed to by the first pointer of the cons cell CONS-CELL. Expressed another way, this function returns the CAR of CONS-CELL. As a special case, if CONS-CELL is `nil', then `car' is defined to return `nil'; therefore, any list is a valid argument for `car'. An error is signaled if the argument is not a cons cell or `nil'. (car '(a b c)) => a (car '()) => nil - Function: cdr CONS-CELL This function returns the value pointed to by the second pointer of the cons cell CONS-CELL. Expressed another way, this function returns the CDR of CONS-CELL. As a special case, if CONS-CELL is `nil', then `cdr' is defined to return `nil'; therefore, any list is a valid argument for `cdr'. An error is signaled if the argument is not a cons cell or `nil'. (cdr '(a b c)) => (b c) (cdr '()) => nil - Function: car-safe OBJECT This function lets you take the CAR of a cons cell while avoiding errors for other data types. It returns the CAR of OBJECT if OBJECT is a cons cell, `nil' otherwise. This is in contrast to `car', which signals an error if OBJECT is not a list. (car-safe OBJECT) == (let ((x OBJECT)) (if (consp x) (car x) nil)) - Function: cdr-safe OBJECT This function lets you take the CDR of a cons cell while avoiding errors for other data types. It returns the CDR of OBJECT if OBJECT is a cons cell, `nil' otherwise. This is in contrast to `cdr', which signals an error if OBJECT is not a list. (cdr-safe OBJECT) == (let ((x OBJECT)) (if (consp x) (cdr x) nil)) - Function: nth N LIST This function returns the Nth element of LIST. Elements are numbered starting with zero, so the CAR of LIST is element number zero. If the length of LIST is N or less, the value is `nil'. If N is negative, `nth' returns the first element of LIST. (nth 2 '(1 2 3 4)) => 3 (nth 10 '(1 2 3 4)) => nil (nth -3 '(1 2 3 4)) => 1 (nth n x) == (car (nthcdr n x)) - Function: nthcdr N LIST This function returns the Nth CDR of LIST. In other words, it removes the first N links of LIST and returns what follows. If N is zero or negative, `nthcdr' returns all of LIST. If the length of LIST is N or less, `nthcdr' returns `nil'. (nthcdr 1 '(1 2 3 4)) => (2 3 4) (nthcdr 10 '(1 2 3 4)) => nil (nthcdr -3 '(1 2 3 4)) => (1 2 3 4) Many convenience functions are provided to make it easier for you to access particular elements in a nested list. All of these can be rewritten in terms of the functions just described. - Function: caar CONS-CELL - Function: cadr CONS-CELL - Function: cdar CONS-CELL - Function: cddr CONS-CELL - Function: caaar CONS-CELL - Function: caadr CONS-CELL - Function: cadar CONS-CELL - Function: caddr CONS-CELL - Function: cdaar CONS-CELL - Function: cdadr CONS-CELL - Function: cddar CONS-CELL - Function: cdddr CONS-CELL - Function: caaaar CONS-CELL - Function: caaadr CONS-CELL - Function: caadar CONS-CELL - Function: caaddr CONS-CELL - Function: cadaar CONS-CELL - Function: cadadr CONS-CELL - Function: caddar CONS-CELL - Function: cadddr CONS-CELL - Function: cdaaar CONS-CELL - Function: cdaadr CONS-CELL - Function: cdadar CONS-CELL - Function: cdaddr CONS-CELL - Function: cddaar CONS-CELL - Function: cddadr CONS-CELL - Function: cdddar CONS-CELL - Function: cddddr CONS-CELL Each of these functions is equivalent to one or more applications of `car' and/or `cdr'. For example, (cadr x) is equivalent to (car (cdr x)) and (cdaddr x) is equivalent to (cdr (car (cdr (cdr x)))) That is to say, read the a's and d's from right to left and apply a `car' or `cdr' for each a or d found, respectively. - Function: first LIST This is equivalent to `(nth 0 LIST)', i.e. the first element of LIST. (Note that this is also equivalent to `car'.) - Function: second LIST This is equivalent to `(nth 1 LIST)', i.e. the second element of LIST. - Function: third LIST - Function: fourth LIST - Function: fifth LIST - Function: sixth LIST - Function: seventh LIST - Function: eighth LIST - Function: ninth LIST - Function: tenth LIST These are equivalent to `(nth 2 LIST)' through `(nth 9 LIST)' respectively, i.e. the third through tenth elements of LIST.  File: lispref.info, Node: Building Lists, Next: Modifying Lists, Prev: List Elements, Up: Lists Building Cons Cells and Lists ============================= Many functions build lists, as lists reside at the very heart of Lisp. `cons' is the fundamental list-building function; however, it is interesting to note that `list' is used more times in the source code for Emacs than `cons'. - Function: cons OBJECT1 OBJECT2 This function is the fundamental function used to build new list structure. It creates a new cons cell, making OBJECT1 the CAR, and OBJECT2 the CDR. It then returns the new cons cell. The arguments OBJECT1 and OBJECT2 may be any Lisp objects, but most often OBJECT2 is a list. (cons 1 '(2)) => (1 2) (cons 1 '()) => (1) (cons 1 2) => (1 . 2) `cons' is often used to add a single element to the front of a list. This is called "consing the element onto the list". For example: (setq list (cons newelt list)) Note that there is no conflict between the variable named `list' used in this example and the function named `list' described below; any symbol can serve both purposes. - Function: list &rest OBJECTS This function creates a list with OBJECTS as its elements. The resulting list is always `nil'-terminated. If no OBJECTS are given, the empty list is returned. (list 1 2 3 4 5) => (1 2 3 4 5) (list 1 2 '(3 4 5) 'foo) => (1 2 (3 4 5) foo) (list) => nil - Function: make-list LENGTH OBJECT This function creates a list of length LENGTH, in which all the elements have the identical value OBJECT. Compare `make-list' with `make-string' (*note Creating Strings::.). (make-list 3 'pigs) => (pigs pigs pigs) (make-list 0 'pigs) => nil - Function: append &rest SEQUENCES This function returns a list containing all the elements of SEQUENCES. The 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. More generally, the final argument to `append' may be any Lisp object. The final argument is not copied or converted; it becomes the CDR of the last cons cell in the new list. If the final argument is itself a list, then its elements become in effect elements of the result list. If the final element is not a list, the result is a "dotted list" since its final CDR is not `nil' as required in a true list. See `nconc' in *Note Rearrangement::, for a way to join lists with no copying. Here is an example of using `append': (setq trees '(pine oak)) => (pine oak) (setq more-trees (append '(maple birch) trees)) => (maple birch pine oak) trees => (pine oak) more-trees => (maple birch pine oak) (eq trees (cdr (cdr more-trees))) => t You can see how `append' works by looking at a box diagram. The variable `trees' is set to the list `(pine oak)' and then the variable `more-trees' is set to the list `(maple birch pine oak)'. However, the variable `trees' continues to refer to the original list: more-trees trees | | | ___ ___ ___ ___ -> ___ ___ ___ ___ --> |___|___|--> |___|___|--> |___|___|--> |___|___|--> nil | | | | | | | | --> maple -->birch --> pine --> oak An empty sequence contributes nothing to the value returned by `append'. As a consequence of this, a final `nil' argument forces a copy of the previous argument. trees => (pine oak) (setq wood (append trees ())) => (pine oak) wood => (pine oak) (eq wood trees) => nil This once was the usual way to copy a list, before the function `copy-sequence' was invented. *Note Sequences Arrays Vectors::. With the help of `apply', we can append all the lists in a list of lists: (apply 'append '((a b c) nil (x y z) nil)) => (a b c x y z) If no SEQUENCES are given, `nil' is returned: (append) => nil Here are some examples where the final argument is not a list: (append '(x y) 'z) => (x y . z) (append '(x y) [z]) => (x y . [z]) The second example shows that when the final argument is a sequence but not a list, the sequence's elements do not become elements of the resulting list. Instead, the sequence becomes the final CDR, like any other non-list final argument. The `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. *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 `format' (*note Formatting Strings::.) or `number-to-string' (*note String Conversion::.). - Function: reverse LIST This function creates a new list whose elements are the elements of LIST, but in reverse order. The original argument LIST is *not* altered. (setq x '(1 2 3 4)) => (1 2 3 4) (reverse x) => (4 3 2 1) x => (1 2 3 4)  File: lispref.info, Node: Modifying Lists, Next: Sets And Lists, Prev: Building Lists, Up: Lists Modifying Existing List Structure ================================= You can modify the CAR and CDR contents of a cons cell with the primitives `setcar' and `setcdr'. Common Lisp note: Common Lisp uses functions `rplaca' and `rplacd' to alter list structure; they change structure the same way as `setcar' and `setcdr', but the Common Lisp functions return the cons cell while `setcar' and `setcdr' return the new CAR or CDR. * Menu: * Setcar:: Replacing an element in a list. * Setcdr:: Replacing part of the list backbone. This can be used to remove or add elements. * Rearrangement:: Reordering the elements in a list; combining lists.  File: lispref.info, Node: Setcar, Next: Setcdr, Up: Modifying Lists Altering List Elements with `setcar' ------------------------------------ Changing the CAR of a cons cell is done with `setcar'. When used on a list, `setcar' replaces one element of a list with a different element. - Function: setcar CONS OBJECT This function stores OBJECT as the new CAR of CONS, replacing its previous CAR. It returns the value OBJECT. For example: (setq x '(1 2)) => (1 2) (setcar x 4) => 4 x => (4 2) When a cons cell is part of the shared structure of several lists, storing a new CAR into the cons changes one element of each of these lists. Here is an example: ;; Create two lists that are partly shared. (setq x1 '(a b c)) => (a b c) (setq x2 (cons 'z (cdr x1))) => (z b c) ;; Replace the CAR of a shared link. (setcar (cdr x1) 'foo) => foo x1 ; Both lists are changed. => (a foo c) x2 => (z foo c) ;; Replace the CAR of a link that is not shared. (setcar x1 'baz) => baz x1 ; Only one list is changed. => (baz foo c) x2 => (z foo c) Here is a graphical depiction of the shared structure of the two lists in the variables `x1' and `x2', showing why replacing `b' changes them both: ___ ___ ___ ___ ___ ___ x1---> |___|___|----> |___|___|--> |___|___|--> nil | --> | | | | | | --> a | --> b --> c | ___ ___ | x2--> |___|___|-- | | --> z Here is an alternative form of box diagram, showing the same relationship: x1: -------------- -------------- -------------- | car | cdr | | car | cdr | | car | cdr | | a | o------->| b | o------->| c | nil | | | | -->| | | | | | -------------- | -------------- -------------- | x2: | -------------- | | car | cdr | | | z | o---- | | | --------------  File: lispref.info, Node: Setcdr, Next: Rearrangement, Prev: Setcar, Up: Modifying Lists Altering the CDR of a List -------------------------- The lowest-level primitive for modifying a CDR is `setcdr': - Function: setcdr CONS OBJECT This function stores OBJECT as the new CDR of CONS, replacing its previous CDR. It returns the value OBJECT. Here is an example of replacing the CDR of a list with a different list. All but the first element of the list are removed in favor of a different sequence of elements. The first element is unchanged, because it resides in the CAR of the list, and is not reached via the CDR. (setq x '(1 2 3)) => (1 2 3) (setcdr x '(4)) => (4) x => (1 4) You can delete elements from the middle of a list by altering the CDRs of the cons cells in the list. For example, here we delete the second element, `b', from the list `(a b c)', by changing the CDR of the first cell: (setq x1 '(a b c)) => (a b c) (setcdr x1 (cdr (cdr x1))) => (c) x1 => (a c) Here is the result in box notation: -------------------- | | -------------- | -------------- | -------------- | car | cdr | | | car | cdr | -->| car | cdr | | a | o----- | b | o-------->| c | nil | | | | | | | | | | -------------- -------------- -------------- The second cons cell, which previously held the element `b', still exists and its CAR is still `b', but it no longer forms part of this list. It is equally easy to insert a new element by changing CDRs: (setq x1 '(a b c)) => (a b c) (setcdr x1 (cons 'd (cdr x1))) => (d b c) x1 => (a d b c) Here is this result in box notation: -------------- ------------- ------------- | car | cdr | | car | cdr | | car | cdr | | a | o | -->| b | o------->| c | nil | | | | | | | | | | | | --------- | -- | ------------- ------------- | | ----- -------- | | | --------------- | | | car | cdr | | -->| d | o------ | | | ---------------  File: lispref.info, Node: Rearrangement, Prev: Setcdr, Up: Modifying Lists Functions that Rearrange Lists ------------------------------ Here are some functions that rearrange lists "destructively" by modifying the CDRs 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. See `delq', in *Note Sets And Lists::, for another function that modifies cons cells. - Function: nconc &rest LISTS This function returns a list containing all the elements of LISTS. Unlike `append' (*note Building Lists::.), the LISTS are *not* copied. Instead, the last CDR of each of the LISTS is changed to refer to the following list. The last of the LISTS is not altered. For example: (setq x '(1 2 3)) => (1 2 3) (nconc x '(4 5)) => (1 2 3 4 5) x => (1 2 3 4 5) Since the last argument of `nconc' is not itself modified, it is reasonable to use a constant list, such as `'(4 5)', as in the above example. For the same reason, the last argument need not be a list: (setq x '(1 2 3)) => (1 2 3) (nconc x 'z) => (1 2 3 . z) x => (1 2 3 . z) A common pitfall is to use a quoted constant list as a non-last argument to `nconc'. If you do this, your program will change each time you run it! Here is what happens: (defun add-foo (x) ; We want this function to add (nconc '(foo) x)) ; `foo' to the front of its arg. (symbol-function 'add-foo) => (lambda (x) (nconc (quote (foo)) x)) (setq xx (add-foo '(1 2))) ; It seems to work. => (foo 1 2) (setq xy (add-foo '(3 4))) ; What happened? => (foo 1 2 3 4) (eq xx xy) => t (symbol-function 'add-foo) => (lambda (x) (nconc (quote (foo 1 2 3 4) x))) - Function: nreverse LIST This function reverses the order of the elements of LIST. Unlike `reverse', `nreverse' alters its argument by reversing the CDRs in the cons cells forming the list. The cons cell that used to be the last one in LIST becomes the first cell of the value. For example: (setq x '(1 2 3 4)) => (1 2 3 4) x => (1 2 3 4) (nreverse x) => (4 3 2 1) ;; The cell that was first is now last. x => (1) To avoid confusion, we usually store the result of `nreverse' back in the same variable which held the original list: (setq x (nreverse x)) Here is the `nreverse' of our favorite example, `(a b c)', presented graphically: Original list head: Reversed list: ------------- ------------- ------------ | car | cdr | | car | cdr | | car | cdr | | a | nil |<-- | b | o |<-- | c | o | | | | | | | | | | | | | | ------------- | --------- | - | -------- | - | | | | ------------- ------------ - Function: sort LIST PREDICATE This function sorts LIST stably, though destructively, and returns the sorted list. It compares elements using 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 PREDICATE must be a function that accepts two arguments. It is called with two elements of LIST. To get an increasing order sort, the PREDICATE should return `t' if the first element is "less than" the second, or `nil' if not. The destructive aspect of `sort' is that it rearranges the cons cells forming LIST by changing CDRs. 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 `copy-sequence' and then sort. Sorting does not change the CARs of the cons cells in LIST; the cons cell that originally contained the element `a' in LIST still has `a' in its CAR after sorting, but it now appears in a different position in the list due to the change of CDRs. For example: (setq nums '(1 3 2 6 5 4 0)) => (1 3 2 6 5 4 0) (sort nums '<) => (0 1 2 3 4 5 6) nums => (1 2 3 4 5 6) Note that the list in `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 `sort' and use that. Most often we store the result back into the variable that held the original list: (setq nums (sort nums '<)) *Note Sorting::, for more functions that perform sorting. See `documentation' in *Note Accessing Documentation::, for a useful example of `sort'.  File: lispref.info, Node: Sets And Lists, Next: Association Lists, Prev: Modifying Lists, Up: Lists Using Lists as Sets =================== A list can represent an unordered mathematical set--simply consider a value an element of a set if it appears in the list, and ignore the order of the list. To form the union of two sets, use `append' (as long as you don't mind having duplicate elements). Other useful functions for sets include `memq' and `delq', and their `equal' versions, `member' and `delete'. Common Lisp note: Common Lisp has functions `union' (which avoids duplicate elements) and `intersection' for set operations, but XEmacs Lisp does not have them. You can write them in Lisp if you wish. - Function: memq OBJECT LIST This function tests to see whether OBJECT is a member of LIST. If it is, `memq' returns a list starting with the first occurrence of OBJECT. Otherwise, it returns `nil'. The letter `q' in `memq' says that it uses `eq' to compare OBJECT against the elements of the list. For example: (memq 'b '(a b c b a)) => (b c b a) (memq '(2) '((1) (2))) ; `(2)' and `(2)' are not `eq'. => nil - Function: delq OBJECT LIST This function destructively removes all elements `eq' to OBJECT from LIST. The letter `q' in `delq' says that it uses `eq' to compare OBJECT against the elements of the list, like `memq'. When `delq' deletes elements from the front of the list, it does so simply by advancing down the list and returning a sublist that starts after those elements: (delq 'a '(a b c)) == (cdr '(a b c)) When an element to be deleted appears in the middle of the list, removing it involves changing the CDRs (*note Setcdr::.). (setq sample-list '(a b c (4))) => (a b c (4)) (delq 'a sample-list) => (b c (4)) sample-list => (a b c (4)) (delq 'c sample-list) => (a b (4)) sample-list => (a b (4)) Note that `(delq 'c sample-list)' modifies `sample-list' to splice out the third element, but `(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 LIST now has fewer elements, or that it still holds the original list! Instead, save the result of `delq' and use that. Most often we store the result back into the variable that held the original list: (setq flowers (delq 'rose flowers)) In the following example, the `(4)' that `delq' attempts to match and the `(4)' in the `sample-list' are not `eq': (delq '(4) sample-list) => (a c (4)) The following two functions are like `memq' and `delq' but use `equal' rather than `eq' to compare elements. They are new in Emacs 19. - Function: member OBJECT LIST The function `member' tests to see whether OBJECT is a member of LIST, comparing members with OBJECT using `equal'. If OBJECT is a member, `member' returns a list starting with its first occurrence in LIST. Otherwise, it returns `nil'. Compare this with `memq': (member '(2) '((1) (2))) ; `(2)' and `(2)' are `equal'. => ((2)) (memq '(2) '((1) (2))) ; `(2)' and `(2)' are not `eq'. => nil ;; Two strings with the same contents are `equal'. (member "foo" '("foo" "bar")) => ("foo" "bar") - Function: delete OBJECT LIST This function destructively removes all elements `equal' to OBJECT from LIST. It is to `delq' as `member' is to `memq': it uses `equal' to compare elements with OBJECT, like `member'; when it finds an element that matches, it removes the element just as `delq' would. For example: (delete '(2) '((2) (1) (2))) => '((1)) Common Lisp note: The functions `member' and `delete' in XEmacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not use `equal' to compare elements. See also the function `add-to-list', in *Note Setting Variables::, for another way to add an element to a list stored in a variable.  File: lispref.info, Node: Association Lists, Next: Property Lists, Prev: Sets And Lists, Up: Lists Association Lists ================= An "association list", or "alist" for short, records a mapping from keys to values. It is a list of cons cells called "associations": the CAR of each cell is the "key", and the CDR is the "associated value".(1) Here is an example of an alist. The key `pine' is associated with the value `cones'; the key `oak' is associated with `acorns'; and the key `maple' is associated with `seeds'. '((pine . cones) (oak . acorns) (maple . seeds)) The associated values in an alist may be any Lisp objects; so may the keys. For example, in the following alist, the symbol `a' is associated with the number `1', and the string `"b"' is associated with the *list* `(2 3)', which is the CDR of the alist element: ((a . 1) ("b" 2 3)) Sometimes it is better to design an alist to store the associated value in the CAR of the CDR of the element. Here is an example: '((rose red) (lily white) (buttercup yellow)) Here we regard `red' as the value associated with `rose'. One advantage of this method is that you can store other related information--even a list of other items--in the CDR of the CDR. One disadvantage is that you cannot use `rassq' (see below) to find the element containing a given value. When neither of these considerations is important, the choice is a matter of taste, as long as you are consistent about it for any given alist. Note that the same alist shown above could be regarded as having the associated value in the CDR of the element; the value associated with `rose' would be the list `(red)'. Association lists are often used to record information that you might otherwise keep on a stack, since new associations may be added easily to the front of the list. When searching an association list for an association with a given key, the first one found is returned, if there is more than one. In XEmacs Lisp, it is *not* an error if an element of an association list is not a cons cell. The alist search functions simply ignore such elements. Many other versions of Lisp signal errors in such cases. Note that property lists are similar to association lists in several respects. A property list behaves like an association list in which each key can occur only once. *Note Property Lists::, for a comparison of property lists and association lists. - Function: assoc KEY ALIST This function returns the first association for KEY in ALIST. It compares KEY against the alist elements using `equal' (*note Equality Predicates::.). It returns `nil' if no association in ALIST has a CAR `equal' to KEY. For example: (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) => ((pine . cones) (oak . acorns) (maple . seeds)) (assoc 'oak trees) => (oak . acorns) (cdr (assoc 'oak trees)) => acorns (assoc 'birch trees) => nil Here is another example, in which the keys and values are not symbols: (setq needles-per-cluster '((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine"))) (cdr (assoc 3 needles-per-cluster)) => ("Pitch Pine") (cdr (assoc 2 needles-per-cluster)) => ("Austrian Pine" "Red Pine") - Function: rassoc VALUE ALIST This function returns the first association with value VALUE in ALIST. It returns `nil' if no association in ALIST has a CDR `equal' to VALUE. `rassoc' is like `assoc' except that it compares the CDR of each ALIST association instead of the CAR. You can think of this as "reverse `assoc'", finding the key for a given value. - Function: assq KEY ALIST This function is like `assoc' in that it returns the first association for KEY in ALIST, but it makes the comparison using `eq' instead of `equal'. `assq' returns `nil' if no association in ALIST has a CAR `eq' to KEY. This function is used more often than `assoc', since `eq' is faster than `equal' and most alists use symbols as keys. *Note Equality Predicates::. (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) => ((pine . cones) (oak . acorns) (maple . seeds)) (assq 'pine trees) => (pine . cones) On the other hand, `assq' is not usually useful in alists where the keys may not be symbols: (setq leaves '(("simple leaves" . oak) ("compound leaves" . horsechestnut))) (assq "simple leaves" leaves) => nil (assoc "simple leaves" leaves) => ("simple leaves" . oak) - Function: rassq VALUE ALIST This function returns the first association with value VALUE in ALIST. It returns `nil' if no association in ALIST has a CDR `eq' to VALUE. `rassq' is like `assq' except that it compares the CDR of each ALIST association instead of the CAR. You can think of this as "reverse `assq'", finding the key for a given value. For example: (setq trees '((pine . cones) (oak . acorns) (maple . seeds))) (rassq 'acorns trees) => (oak . acorns) (rassq 'spores trees) => nil Note that `rassq' cannot search for a value stored in the CAR of the CDR of an element: (setq colors '((rose red) (lily white) (buttercup yellow))) (rassq 'white colors) => nil In this case, the CDR of the association `(lily white)' is not the symbol `white', but rather the list `(white)'. This becomes clearer if the association is written in dotted pair notation: (lily white) == (lily . (white)) - Function: remassoc KEY ALIST This function deletes by side effect any associations with key KEY in ALIST - i.e. it removes any elements from ALIST whose `car' is `equal' to KEY. The modified ALIST is returned. If the first member of ALIST has a `car' that is `equal' to KEY, there is no way to remove it by side effect; therefore, write `(setq foo (remassoc key foo))' to be sure of changing the value of `foo'. - Function: remassq KEY ALIST This function deletes by side effect any associations with key KEY in ALIST - i.e. it removes any elements from ALIST whose `car' is `eq' to KEY. The modified ALIST is returned. This function is exactly like `remassoc', but comparisons between KEY and keys in ALIST are done using `eq' instead of `equal'. - Function: remrassoc VALUE ALIST This function deletes by side effect any associations with value VALUE in ALIST - i.e. it removes any elements from ALIST whose `cdr' is `equal' to VALUE. The modified ALIST is returned. If the first member of ALIST has a `car' that is `equal' to VALUE, there is no way to remove it by side effect; therefore, write `(setq foo (remassoc value foo))' to be sure of changing the value of `foo'. `remrassoc' is like `remassoc' except that it compares the CDR of each ALIST association instead of the CAR. You can think of this as "reverse `remassoc'", removing an association based on its value instead of its key. - Function: remrassq VALUE ALIST This function deletes by side effect any associations with value VALUE in ALIST - i.e. it removes any elements from ALIST whose `cdr' is `eq' to VALUE. The modified ALIST is returned. This function is exactly like `remrassoc', but comparisons between VALUE and values in ALIST are done using `eq' instead of `equal'. - Function: copy-alist ALIST This function returns a two-level deep copy of ALIST: it creates a new copy of each association, so that you can alter the associations of the new alist without changing the old one. (setq needles-per-cluster '((2 . ("Austrian Pine" "Red Pine")) (3 . ("Pitch Pine")) (5 . ("White Pine")))) => ((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine")) (setq copy (copy-alist needles-per-cluster)) => ((2 "Austrian Pine" "Red Pine") (3 "Pitch Pine") (5 "White Pine")) (eq needles-per-cluster copy) => nil (equal needles-per-cluster copy) => t (eq (car needles-per-cluster) (car copy)) => nil (cdr (car (cdr needles-per-cluster))) => ("Pitch Pine") (eq (cdr (car (cdr needles-per-cluster))) (cdr (car (cdr copy)))) => t This example shows how `copy-alist' makes it possible to change the associations of one copy without affecting the other: (setcdr (assq 3 copy) '("Martian Vacuum Pine")) (cdr (assq 3 needles-per-cluster)) => ("Pitch Pine") ---------- Footnotes ---------- (1) 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 the alist associations are the items.  File: lispref.info, Node: Property Lists, Next: Weak Lists, Prev: Association Lists, Up: Lists Property Lists ============== A "property list" (or "plist") is another way of representing a mapping from keys to values. Instead of the list consisting of conses of a key and a value, the keys and values alternate as successive entries in the list. Thus, the association list ((a . 1) (b . 2) (c . 3)) has the equivalent property list form (a 1 b 2 c 3) Property lists are used to represent the properties associated with various sorts of objects, such as symbols, strings, frames, etc. The convention is that property lists can be modified in-place, while association lists generally are not. Plists come in two varieties: "normal" plists, whose keys are compared with `eq', and "lax" plists, whose keys are compared with `equal', - Function: valid-plist-p PLIST Given a plist, this function returns non-`nil' if its format is correct. If it returns `nil', `check-valid-plist' will signal an error when given the plist; that means it's a malformed or circular plist or has non-symbols as keywords. - Function: check-valid-plist PLIST Given a plist, this function signals an error if there is anything wrong with it. This means that it's a malformed or circular plist. * Menu: * Working With Normal Plists:: Functions for normal plists. * Working With Lax Plists:: Functions for lax plists. * Converting Plists To/From Alists:: Alist to plist and vice-versa.  File: lispref.info, Node: Working With Normal Plists, Next: Working With Lax Plists, Up: Property Lists Working With Normal Plists -------------------------- - Function: plist-get PLIST PROP &optional DEFAULT This function extracts a value from a property list. The function returns the value corresponding to the given PROP, or DEFAULT if PROP is not one of the properties on the list. - Function: plist-put PLIST PROP VAL This function changes the value in PLIST of PROP to VAL. If PROP is already a property on the list, its value is set to VAL, otherwise the new PROP VAL pair is added. The new plist is returned; use `(setq x (plist-put x prop val))' to be sure to use the new value. The PLIST is modified by side effects. - Function: plist-remprop PLIST PROP This function removes from PLIST the property PROP and its value. The new plist is returned; use `(setq x (plist-remprop x prop val))' to be sure to use the new value. The PLIST is modified by side effects. - Function: plist-member PLIST PROP This function returns `t' if PROP has a value specified in PLIST. In the following functions, if optional arg NIL-MEANS-NOT-PRESENT is non-`nil', then a property with a `nil' value is ignored or removed. This feature is a virus that has infected old Lisp implementations (and thus E-Lisp, due to RMS's enamorment with old Lisps), but should not be used except for backward compatibility. - Function: plists-eq A B &optional NIL-MEANS-NOT-PRESENT This function returns non-`nil' if property lists A and B are `eq' (i.e. their values are `eq'). - Function: plists-equal A B &optional NIL-MEANS-NOT-PRESENT This function returns non-`nil' if property lists A and B are `equal' (i.e. their values are `equal'; their keys are still compared using `eq'). - Function: canonicalize-plist PLIST &optional NIL-MEANS-NOT-PRESENT This function destructively removes any duplicate entries from a plist. In such cases, the first entry applies. The new plist is returned. If NIL-MEANS-NOT-PRESENT is given, the return value may not be `eq' to the passed-in value, so make sure to `setq' the value back into where it came from.  File: lispref.info, Node: Working With Lax Plists, Next: Converting Plists To/From Alists, Prev: Working With Normal Plists, Up: Property Lists Working With Lax Plists ----------------------- Recall that a "lax plist" is a property list whose keys are compared using `equal' instead of `eq'. - Function: lax-plist-get LAX-PLIST PROP &optional DEFAULT This function extracts a value from a lax property list. The function returns the value corresponding to the given PROP, or DEFAULT if PROP is not one of the properties on the list. - Function: lax-plist-put LAX-PLIST PROP VAL This function changes the value in LAX-PLIST of PROP to VAL. - Function: lax-plist-remprop LAX-PLIST PROP This function removes from LAX-PLIST the property PROP and its value. The new plist is returned; use `(setq x (lax-plist-remprop x prop val))' to be sure to use the new value. The LAX-PLIST is modified by side effects. - Function: lax-plist-member LAX-PLIST PROP This function returns `t' if PROP has a value specified in LAX-PLIST. In the following functions, if optional arg NIL-MEANS-NOT-PRESENT is non-`nil', then a property with a `nil' value is ignored or removed. This feature is a virus that has infected old Lisp implementations (and thus E-Lisp, due to RMS's enamorment with old Lisps), but should not be used except for backward compatibility. - Function: lax-plists-eq A B &optional NIL-MEANS-NOT-PRESENT This function returns non-`nil' if lax property lists A and B are `eq' (i.e. their values are `eq'; their keys are still compared using `equal'). - Function: lax-plists-equal A B &optional NIL-MEANS-NOT-PRESENT This function returns non-`nil' if lax property lists A and B are `equal' (i.e. their values are `equal'). - Function: canonicalize-lax-plist LAX-PLIST &optional NIL-MEANS-NOT-PRESENT This function destructively removes any duplicate entries from a lax plist. In such cases, the first entry applies. The new plist is returned. If NIL-MEANS-NOT-PRESENT is given, the return value may not be `eq' to the passed-in value, so make sure to `setq' the value back into where it came from.  File: lispref.info, Node: Converting Plists To/From Alists, Prev: Working With Lax Plists, Up: Property Lists Converting Plists To/From Alists -------------------------------- - Function: alist-to-plist ALIST This function converts association list ALIST into the equivalent property-list form. The plist is returned. This converts from ((a . 1) (b . 2) (c . 3)) into (a 1 b 2 c 3) The original alist is not modified. - Function: plist-to-alist PLIST This function converts property list PLIST into the equivalent association-list form. The alist is returned. This converts from (a 1 b 2 c 3) into ((a . 1) (b . 2) (c . 3)) The original plist is not modified. The following two functions are equivalent to the preceding two except that they destructively modify their arguments, using cons cells from the original list to form the new list rather than allocating new cons cells. - Function: destructive-alist-to-plist ALIST This function destructively converts association list ALIST into the equivalent property-list form. The plist is returned. - Function: destructive-plist-to-alist PLIST This function destructively converts property list PLIST into the equivalent association-list form. The alist is returned.  File: lispref.info, Node: Weak Lists, Prev: Property Lists, Up: Lists Weak Lists ========== A "weak list" is a special sort of list whose members are not counted as references for the purpose of garbage collection. This means that, for any object in the list, if there are no references to the object anywhere outside of the list (or other weak list or weak hash table), that object will disappear the next time a garbage collection happens. Weak lists can be useful for keeping track of things such as unobtrusive lists of another function's buffers or markers. When that function is done with the elements, they will automatically disappear from the list. Weak lists are used internally, for example, to manage the list holding the children of an extent - an extent that is unused but has a parent will still be reclaimed, and will automatically be removed from its parent's list of children. Weak lists are similar to weak hash tables (*note Weak Hash Tables::.). - Function: weak-list-p OBJECT This function returns non-`nil' if OBJECT is a weak list. Weak lists come in one of four types: `simple' Objects in the list disappear if not referenced outside of the list. `assoc' Objects in the list disappear if they are conses and either the car or the cdr of the cons is not referenced outside of the list. `key-assoc' Objects in the list disappear if they are conses and the car is not referenced outside of the list. `value-assoc' Objects in the list disappear if they are conses and the cdr is not referenced outside of the list. - Function: make-weak-list &optional TYPE This function creates a new weak list of type TYPE. TYPE is a symbol (one of `simple', `assoc', `key-assoc', or `value-assoc', as described above) and defaults to `simple'. - Function: weak-list-type WEAK This function returns the type of the given weak-list object. - Function: weak-list-list WEAK This function returns the list contained in a weak-list object. - Function: set-weak-list-list WEAK NEW-LIST This function changes the list contained in a weak-list object.  File: lispref.info, Node: Sequences Arrays Vectors, Next: Symbols, Prev: Lists, Up: Top Sequences, Arrays, and Vectors ****************************** Recall that the "sequence" type is the union of four other Lisp types: lists, vectors, bit vectors, and strings. In other words, any list is a sequence, any vector is a sequence, any bit 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 "array" is a single primitive object that has a slot for each elements. All the elements are accessible in constant time, but the length of an existing array cannot be changed. Strings, vectors, and bit vectors are the three 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 Nth element requires looking through 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: ___________________________________ | | | Sequence | | ______ ______________________ | | | | | | | | | List | | Array | | | | | | ________ _______ | | | |______| | | | | | | | | | | Vector | | String| | | | | |________| |_______| | | | | __________________ | | | | | | | | | | | Bit Vector | | | | | |__________________| | | | |______________________| | |___________________________________| The elements of vectors and lists may be any Lisp objects. The elements of strings are all characters. The elements of bit vectors are the numbers 0 and 1. * Menu: * Sequence Functions:: Functions that accept any kind of sequence. * Arrays:: Characteristics of arrays in XEmacs Lisp. * Array Functions:: Functions specifically for arrays. * Vectors:: Special characteristics of XEmacs Lisp vectors. * Vector Functions:: Functions specifically for vectors. * Bit Vectors:: Special characteristics of XEmacs Lisp bit vectors. * Bit Vector Functions:: Functions specifically for bit vectors.