rassoc

rassoc is a function found in Lisp dialects such as Common Lisp. It searches an association list (alist) for the first pair whose cdr matches a given key, making it the reverse of the more common assoc function, which compares the car of pairs to a key. In use, rassoc takes a key and an alist and returns the first cons cell where the cdr is considered equal to the key according to a specified test.

The typical Lisp signature is (rassoc key alist &key test test-not-found). The test argument designates the function used to compare the cdr of a candidate pair with the key; by default this is a straightforward equality test (commonly eql). The test-not-found keyword lets you specify a value to return when no matching pair is found, with nil being the default if not provided.

As a linear search through the alist, rassoc has O(n) time complexity in the length of the list. It is generally used when the association is organized around the cdr values rather than the car values. Related functions include assoc, which matches on the car, and rassq, which is a variant that typically uses eq for comparing the car component.

Example: (rassoc 'bar '((a . foo) (b . bar))) yields (b . bar). If there is no matching pair, the result is nil unless a test-not-found value is supplied. Rassocs are convenient for reverse lookups in lists of key-value pairs where the value (cdr) is the primary criterion. See also: assoc, rassq.