This is Info file elisp, produced by Makeinfo-1.55 from the input file elisp.texi. This version is the edition 2.3 of the GNU Emacs Lisp Reference Manual. It corresponds to Emacs Version 19.23. Published by the Free Software Foundation 675 Massachusetts Avenue Cambridge, MA 02139 USA Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. 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: elisp, Node: Character Case, Next: Case Table, Prev: Formatting Strings, Up: Strings and Characters Character Case ============== The character case functions change the case of single characters or of the contents of strings. The functions convert only alphabetic characters (the letters `A' through `Z' and `a' through `z'); other characters are not altered. The functions do not modify the strings that are passed to them as arguments. The examples below use the characters `X' and `x' which have ASCII codes 88 and 120 respectively. - Function: downcase STRING-OR-CHAR This function converts a character or a string to lower case. When the argument to `downcase' is a string, the function creates and returns a new string in which each letter in the argument that is upper case is converted to lower case. When the argument to `downcase' is a character, `downcase' returns the corresponding lower case character. This value is an integer. If the original character is lower case, or is not a letter, then the value equals the original character. (downcase "The cat in the hat") => "the cat in the hat" (downcase ?X) => 120 - Function: upcase STRING-OR-CHAR This function converts a character or a string to upper case. When the argument to `upcase' is a string, the function creates and returns a new string in which each letter in the argument that is lower case is converted to upper case. When the argument to `upcase' is a character, `upcase' returns the corresponding upper case character. This value is an integer. If the original character is upper case, or is not a letter, then the value equals the original character. (upcase "The cat in the hat") => "THE CAT IN THE HAT" (upcase ?x) => 88 - Function: capitalize STRING-OR-CHAR This function capitalizes strings or characters. If STRING-OR-CHAR is a string, the function creates and returns a new string, whose contents are a copy of STRING-OR-CHAR in which each word has been capitalized. This means that the first character of each word is converted to upper case, and the rest are converted to lower case. The definition of a word is any sequence of consecutive characters that are assigned to the word constituent syntax class in the current syntax table (*Note Syntax Class Table::). When the argument to `capitalize' is a character, `capitalize' has the same result as `upcase'. (capitalize "The cat in the hat") => "The Cat In The Hat" (capitalize "THE 77TH-HATTED CAT") => "The 77th-Hatted Cat" (capitalize ?x) => 88  File: elisp, Node: Case Table, Prev: Character Case, Up: Strings and Characters The Case Table ============== You can customize case conversion by installing a special "case table". A case table specifies the mapping between upper case and lower case letters. It affects both the string and character case conversion functions (see the previous section) and those that apply to text in the buffer (*note Case Changes::.). You need a case table if you are using a language which has letters other than the standard ASCII letters. A case table is a list of this form: (DOWNCASE UPCASE CANONICALIZE EQUIVALENCES) where each element is either `nil' or a string of length 256. The element DOWNCASE says how to map each character to its lower-case equivalent. The element UPCASE maps each character to its upper-case equivalent. If lower and upper case characters are in one-to-one correspondence, use `nil' for UPCASE; then Emacs deduces the upcase table from DOWNCASE. For some languages, upper and lower case letters are not in one-to-one correspondence. There may be two different lower case letters with the same upper case equivalent. In these cases, you need to specify the maps for both directions. The element CANONICALIZE maps each character to a canonical equivalent; any two characters that are related by case-conversion have the same canonical equivalent character. The element EQUIVALENCES is a map that cyclicly permutes each equivalence class (of characters with the same canonical equivalent). (For ordinary ASCII, this would map `a' into `A' and `A' into `a', and likewise for each set of equivalent characters.) When you construct a case table, you can provide `nil' for CANONICALIZE; then Emacs fills in this string from UPCASE and DOWNCASE. You can also provide `nil' for EQUIVALENCES; then Emacs fills in this string from CANONICALIZE. In a case table that is actually in use, those components are non-`nil'. Do not try to specify EQUIVALENCES without also specifying CANONICALIZE. Each buffer has a case table. Emacs also has a "standard case table" which is copied into each buffer when you create the buffer. Changing the standard case table doesn't affect any existing buffers. Here are the functions for working with case tables: - Function: case-table-p OBJECT This predicate returns non-`nil' if OBJECT is a valid case table. - Function: set-standard-case-table TABLE This function makes TABLE the standard case table, so that it will apply to any buffers created subsequently. - Function: standard-case-table This returns the standard case table. - Function: current-case-table This function returns the current buffer's case table. - Function: set-case-table TABLE This sets the current buffer's case table to TABLE. The following three functions are convenient subroutines for packages that define non-ASCII character sets. They modify a string DOWNCASE-TABLE provided as an argument; this should be a string to be used as the DOWNCASE part of a case table. They also modify the standard syntax table. *Note Syntax Tables::. - Function: set-case-syntax-pair UC LC DOWNCASE-TABLE This function specifies a pair of corresponding letters, one upper case and one lower case. - Function: set-case-syntax-delims L R DOWNCASE-TABLE This function makes characters L and R a matching pair of case-invariant delimiters. - Function: set-case-syntax CHAR SYNTAX DOWNCASE-TABLE This function makes CHAR case-invariant, with syntax SYNTAX. - Command: describe-buffer-case-table This command displays a description of the contents of the current buffer's case table. You can load the library `iso-syntax' to set up the standard syntax table and define a case table for the 256-bit ISO Latin 1 character set.  File: elisp, Node: Lists, Next: Sequences Arrays Vectors, Prev: Strings and Characters, Up: Top Lists ***** A "list" represents a sequence of zero or more elements (which may be any Lisp objects). The important difference between lists and vectors is that two or more lists can share part of their structure; in addition, you can insert or delete elements in a list without copying the whole list. * Menu: * Cons Cells:: How lists are made out of cons cells. * Lists as Boxes:: Graphical notation to explain lists. * List-related Predicates:: Is this object a list? Comparing two lists. * List Elements:: Extracting the pieces of a list. * Building Lists:: Creating list structure. * Modifying Lists:: Storing new pieces into an existing list. * Sets And Lists:: A list can represent a finite mathematical set. * Association Lists:: A list can represent a finite relation or mapping.  File: elisp, Node: Cons Cells, Next: Lists as Boxes, Up: Lists Lists and Cons Cells ==================== Lists in Lisp are not a primitive data type; they are built up from "cons cells". A cons cell is a data object that represents an ordered pair. It records two Lisp objects, one labeled as the CAR, and the other labeled as the CDR. These names are traditional; see *Note Cons Cell Type::. 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 CARs of the cons cells are the elements of the list, and the CDRs are used to chain the list: the CDR of each cons cell is the following cons cell. The CDR of the last cons cell is `nil'. This asymmetry between the CAR and the CDR is entirely a matter of convention; at the level of cons cells, the CAR and CDR slots have the same characteristics. Because most cons cells are used as part of lists, the phrase "list structure" has come to mean any structure made out of cons cells. The symbol `nil' is considered a list as well as a symbol; it is the list with no elements. For convenience, the symbol `nil' is considered to have `nil' as its CDR (and also as its CAR). The CDR of any nonempty list L is a list containing all the elements of L except the first.  File: elisp, Node: Lists as Boxes, Next: List-related Predicates, Prev: Cons Cells, Up: Lists Lists as Linked Pairs of Boxes ============================== A cons cell can be illustrated as a pair of boxes. The first box represents the CAR and the second box represents the CDR. Here is an illustration of the two-element list, `(tulip lily)', made from two cons cells: --------------- --------------- | car | cdr | | car | cdr | | tulip | o---------->| lily | nil | | | | | | | --------------- --------------- 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 CAR of the first cons cell, contains the symbol `tulip'. The arrow from the CDR of the first cons cell to the second cons cell indicates that the CDR of the first cons cell points to the second cons cell. The same list can be illustrated in a different sort of box notation like this: ___ ___ ___ ___ |___|___|--> |___|___|--> nil | | | | --> tulip --> lily Here is a more complex illustration, showing the three-element list, `((pine needles) oak maple)', the first element of which is a two-element list: ___ ___ ___ ___ ___ ___ |___|___|--> |___|___|--> |___|___|--> nil | | | | | | | --> oak --> maple | | ___ ___ ___ ___ --> |___|___|--> |___|___|--> nil | | | | --> pine --> needles The same list represented in the first box notation looks like this: -------------- -------------- -------------- | car | cdr | | car | cdr | | car | cdr | | o | o------->| oak | o------->| maple | nil | | | | | | | | | | | -- | --------- -------------- -------------- | | | -------------- ---------------- | | car | cdr | | car | cdr | ------>| pine | o------->| needles | nil | | | | | | | -------------- ---------------- *Note Cons Cell Type::, for the read and print syntax of cons cells and lists, and for more "box and arrow" illustrations of lists.  File: elisp, Node: List-related Predicates, Next: List Elements, Prev: Lists as Boxes, Up: Lists Predicates on Lists =================== The following predicates test whether a Lisp object is an atom, is a cons cell or is a list, or whether it is the distinguished object `nil'. (Many of these predicates can be defined in terms of the others, but they are used so often that it is worth having all of them.) - Function: consp OBJECT This function returns `t' if OBJECT is a cons cell, `nil' otherwise. `nil' is not a cons cell, although it *is* a list. - Function: atom OBJECT This function returns `t' if OBJECT is an atom, `nil' otherwise. All objects except cons cells are atoms. The symbol `nil' is an atom and is also a list; it is the only Lisp object that is both. (atom OBJECT) == (not (consp OBJECT)) - Function: listp OBJECT This function returns `t' if OBJECT is a cons cell or `nil'. Otherwise, it returns `nil'. (listp '(1)) => t (listp '()) => t - Function: nlistp OBJECT This function is the opposite of `listp': it returns `t' if OBJECT is not a list. Otherwise, it returns `nil'. (listp OBJECT) == (not (nlistp OBJECT)) - Function: null OBJECT This function returns `t' if OBJECT is `nil', and returns `nil' otherwise. This function is identical to `not', but as a matter of clarity we use `null' when OBJECT is considered a list and `not' when it is considered a truth value (see `not' in *Note Combining Conditions::). (null '(1)) => nil (null '()) => t  File: elisp, 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)  File: elisp, 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. Integers are also allowed as arguments to `append'. They are converted to strings of digits making up the decimal print representation of the integer, and these strings are then appended. Here's what happens: (setq trees '(pine oak)) => (pine oak) (char-to-string 54) => "6" (setq longer-list (append trees 6 '(spruce))) => (pine oak 54 spruce) (setq x-list (append trees 6 6)) => (pine oak 54 . 6) This special case exists for compatibility with Mocklisp, and we don't recommend you take advantage of it. If you want to convert an integer in this way, use `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: elisp, 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: elisp, 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: elisp, 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: elisp, 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; each cons cell in the result contains the same element that it contained before. The result differs from the argument LIST because the cells themselves have been reordered. 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: elisp, 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 GNU Emacs 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 c (4)) sample-list => (a c (4)) Note that `(delq 'b sample-list)' modifies `sample-list' to splice out the second 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 GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not use `equal' to compare elements.  File: elisp, Node: Association 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 Emacs 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: 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: 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 needles-per-cluster) '("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: elisp, Node: Sequences Arrays Vectors, Next: Symbols, Prev: Lists, Up: Top Sequences, Arrays, and Vectors ****************************** Recall that the "sequence" type is the union of three other Lisp types: lists, vectors, and strings. In other words, any list is a sequence, any vector is a sequence, and any string is a sequence. The common property that all sequences have is that each is an ordered collection of elements. An "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. Both strings and vectors are arrays. A list is a sequence of elements, but it is not a single primitive object; it is made of cons cells, one cell per element. 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 | | | | | | ________ _______ | | | |______| | | | | | | | | | | String | | Vector| | | | | |________| |_______| | | | |______________________| | |___________________________________| The elements of vectors and lists may be any Lisp objects. The elements of strings are all characters. * Menu: * Sequence Functions:: Functions that accept any kind of sequence. * Arrays:: Characteristics of arrays in Emacs Lisp. * Array Functions:: Functions specifically for arrays. * Vectors:: Special characteristics of Emacs Lisp vectors. * Vector Functions:: Functions specifically for vectors.