rassoc item alist &key key test test-not → entry

rassoc-if predicate alist &key key → entry

rassoc-if-not predicate alist &key key → entry

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.

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.

 (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)

assoc, Section 3.6 (Traversal Rules and Side Effects)

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.