The functions cdr
and car
are used to refer to the tail (cdr
) and head (car
) of a list:
(cdr '(A B C))
yields (B C)
(car '(A B C))
yields A
The names have their origin in the first implementation of Lisp on an IBM 704 computer. On the 704, an atom[?] was represented by a single 36-bit machine word containing a so-called address part and a decrement part. Each of these parts had a length of 15 bits. The address part was used to point to the head of a list and the decrement part was used to address its tail. The functions used to extract either part of a machine word were called car
(Contents of Address Register) and cdr
(Contents of Decrement Register).
Portions from NILS' LISP PAGES - http://t3x.dyndns.org/LISP/QA/carcdr
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