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  ANSI Common Lisp   14 Conses   14.2 Dictionary of Conses

14.2.39 rassoc, rassoc-if, rassoc-if-not Function

Syntax:
rassoc item alist &key key test test-not    entry

rassoc-if predicate alist &key key    entry

rassoc-if-not predicate alist &key key    entry

Arguments and Values:
item - an object.

alist - an association list.

predicate - a designator for a function of one argument that returns a generalized boolean.

test - a designator for a function of two arguments that returns a generalized boolean.

test-not - a designator for a function of two arguments that returns a generalized boolean.

key - a designator for a function of one argument, or nil.

entry - a cons that is an element of the alist, or nil.

Description:
rassoc, rassoc-if, and rassoc-if-not return the first cons whose cdr satisfies the test. If no such cons is found, nil is returned.

If nil appears in alist in place of a pair, it is ignored.

Examples:
 (setq alist '((1 . "one") (2 . "two") (3 . 3))) 
 ((1 . "one") (2 . "two") (3 . 3))
 (rassoc 3 alist)  (3 . 3)
 (rassoc "two" alist)   NIL
 (rassoc "two" alist :test 'equal)  (2 . "two")
 (rassoc 1 alist :key #'(lambda (x) (if (numberp x) (/ x 3))))  (3 . 3)
 (rassoc 'a '((a . b) (b . c) (c . a) (z . a)))  (C . A)
 (rassoc-if #'stringp alist)  (1 . "one")
 (rassoc-if-not #'vectorp alist)  (3 . 3)

See Also:
assoc, Section 3.6 Traversal Rules and Side Effects

Notes:
The :test-not parameter is deprecated.

The function rassoc-if-not is deprecated.

It is possible to rplaca the result of rassoc, provided that it is not nil, in order to "update" alist.

The expressions

 (rassoc item list :test fn)
and

 (find item list :test fn :key #'cdr)
are equivalent in meaning, except when the item is nil and nil appears in place of a pair in the alist. See the function assoc.

Allegro CL Implementation Details:
None.

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