CAR and CDR

In <a href="/facts/Computer_programming/QmSsFnu0">computer programming</a>, CAR (car) /kɑːr/ ⓘ and CDR (cdr) (/ˈkʌdər/ ⓘ or /ˈkʊdər/ ⓘ) are primitive operations on <a href="/facts/Cons/KTYIDGwz">cons</a> cells (or "non-atomic <a href="/facts/S-expression/TmCqHcln">S-expressions</a>") introduced in the <a href="/facts/Lisp_(programming_language)/xJHA3CrX">Lisp programming language</a>. A cons cell is composed of two <a href="/facts/Pointer_(computer_programming)/fPoNzy9b">pointers</a>; the car operation extracts the first pointer, and the cdr operation extracts the second.
Thus, the expression (car (cons x y)) evaluates to x, and (cdr (cons x y)) evaluates to y.
When cons cells are used to implement <a href="/facts/Singly_linked_list/Ik4JTgJW">singly linked lists</a> (rather than <a href="/facts/Tree_(data_structure)/RcXz9noF">trees</a> and other more complicated <a href="/facts/Data_structure/yNsNvL1I">structures</a>), the car operation returns the first element of the list, while cdr returns the rest of the list. For this reason, the operations are sometimes given the names first and rest or head and tail.

CAR and CDR open-in-new

CAR and CDR