assoc

Assoc is an abbreviation that can refer to several concepts, most commonly in computing as association or as a specific function in Lisp-like languages. In this context, assoc is a function that retrieves a cons pair from an association list (alist) by matching a given key. An alist is a list of pairs, where each pair consists of a key and a value. The function searches the list from the beginning and returns the first pair whose key matches the provided key according to a comparison predicate, often eql. If no match is found, the function returns nil.

Variations exist across Lisp dialects. Some versions allow specifying a different test function or a key function to adjust how keys are compared or extracted. Related helpers include assq, which uses eq for symbol keys, and rassoc, which searches by the cdr instead of the car. Beyond Lisp, many languages implement similar key–value retrieval patterns through dictionaries, maps, or hash tables, sometimes using functions named assoc in introductory libraries or tutorials.

In broader mathematics and logic, the term association denotes a relation that links inputs to outputs, and in computer science it is often used to describe a mapping between keys and values. As a general concept, associations underpin data structures that store and retrieve information, including simple alists and more complex hash-based structures.

See also: association list, hash table, dictionary, key-value store, map.