This proposal supersedes the association-list section of SRFI 1, maintaining compatibility but bringing it to parity with Common Lisp.
FIXME: More will be added later.
An "association list" (or "alist") is a list of pairs. The car of each pair contains a key value, and the cdr contains the associated data value. They can be used to construct simple look-up tables in Scheme. Note that association lists are probably inappropriate for performance-critical use on large data; in these cases, hash tables or some other alternative should be employed.
(assoc key alist [ = ])
(assq key alist)
(assv key alist)
The alist argument must be an association list -- a list of pairs. These procedures find the first pair in alist whose car field is key, and returns that pair. If no pair in alist has key as its car, then #f is returned. assq uses eq? to compare key with the car fields of the pairs in alist, while assv uses eqv? and assoc uses = if given and equal? otherwise. (R5RS, SRFI 1, Common Lisp ASSOC.)
(define e '((a 1) (b 2) (c 3))) (assq 'a e) => (a 1) (assq 'b e) => (b 2) (assq 'd e) => #f (assq (list 'a) '(((a)) ((b)) ((c)))) => #f (assoc (list 'a) '(((a)) ((b)) ((c)))) => ((a)) (assq 5 '((2 3) (5 7) (11 13))) => *unspecified* (assv 5 '((2 3) (5 7) (11 13))) => (5 7)(reverse-assoc obj alist [ = ] )
(reverse-assq obj alist)
(reverse-assv obj alist)
The alist argument must be an association list -- a list of pairs. These procedures find the first pair in alist whose cdr (rather than car) field is key , and return that pair. If no pair in alist has obj as its cdr, then #f (not the empty list) is returned. The assq procedure uses eq? to compare obj with the cdr fields of the pairs in alist, while assv uses eqv? and assoc uses = if given and equal? otherwise. (Common Lisp RASSOC.)
(define e '((a 1) (b 2) (c 3))) (reverse-assq 1 e) => (a 1) (reverse-assq 2 e) => (b 2) (reverse-assq 4 e) => #f (reverse-assq 7 '((2 3) (5 7) (11 13))) => *unspecified* (reverse-assv 7 '((2 3) (5 7) (11 13))) => (5 7)The comparison procedure of assoc and reverse-assoc is used to compare the elements of alist to key in this way: The first argument is always key, and the second argument is one of the elements of list. Thus one can reliably find the first entry of alist whose key is greater than five with (assoc 5 alist <).
(make-alist keys values)
Returns a newly allocated alist whose pairs are constructed from the elements of the lists keys and values. (Common Lisp PAIRLIS.)
(alist-cons key datum alist)
Cons a new alist entry that maps key to datum onto the beginning of alist. (SRFI 1, Common Lisp ACONS.)
(alist-copy alist)
Make a newly allocated copy of alist. This means copying each pair that forms an association as well as the spine of the list. (SRFI 1, Common Lisp COPY-ALIST.)
(alist-delete key alist [ = ])
(alist-delete! key alist [ = ])
alist-delete deletes all associations from alist with the given key, using the key-comparison procedure =, which defaults to equal?. The dynamic order in which the various applications of = are made is not specified. Return values may share common tails with the alist argument. The alist is not disordered -- elements that appear in the result occur in the same order as they occur in alist. The comparison procedure is used in the same way as by assoc. (SRFI 1.)
alist-delete! is the linear-update variant of alist-delete. It is allowed, but not required, to alter cons cells from the alist parameter to construct the result. (SRFI 1.)