caar, cadr, cdar, cddr - compositions of car and cdr

LIBRARY

(import (rnrs))                     ;R6RS
(import (rnrs base))                ;R6RS
(import (scheme r5rs))              ;R7RS
(import (scheme base))              ;R7RS

SYNOPSIS

(caar pair)
(cadr pair)
(cdar pair)
(cddr pair)

DESCRIPTION

These procedures are compositions of car and cdr as follows: 

(define (caar x) (car (car x)))
(define (cadr x) (car (cdr x)))
(define (cdar x) (cdr (car x)))
(define (cddr x) (cdr (cdr x)))

These procedures can also be used with lists, which in Scheme are linked lists of pairs. The caar and cdar procedures return the car and cdr fields of the first element of the list. The cadr procedure returns the second element of the list. The cddr procedure returns the tail of the tail of the list.

There are two conventions for building lists where these procedures often show up. An association list or alist associates a key in the car field with a value in the cdr field, see assq(3scm). A property list or plist also associates a key with a value, but it uses a flat structure, see memq(3scm).

RETURN VALUES

Returns the contents of the car or cdr field of the pair which is in the car or cdr field of the argument.

EXAMPLES

(caar '((a . b) (d . e) (g . h))) => a
(cdar '((a . b) (d . e) (g . h))) => b
(cadr '(a b c))                   => b
(cddr '(a b c))                   => (c)
(map cadr '((a b) (d e) (g h)))   => (b e h)

APPLICATION USAGE

These procedures should primarily be used when walking lists or conventional list structures like alists. It can be tempting to write code that stores data as a new kind of tree of pairs and uses these procedures to walk that tree. While it is possible, it quickly becomes hard to follow. In modern code it is often better to define a new data type with define-record-type(3scm). An alternative is to use a pattern matching library like Alex Shinn's widely available (chibi match).

COMPATIBILITY

Most Lisp systems provide these procedures.

ERRORS

This procedure can raise exceptions with the following condition types:
&assertion (R6RS)
The wrong number of arguments was passed, an argument was not a pair or a referenced car or cdr field did not contain a pair.
R7RS
The assertions described above are errors. Implementations may signal an error, extend the procedure's domain of definition to include such arguments, or fail catastrophically.

SEE ALSO

car(3scm)

STANDARDS

R4RS, IEEE Scheme, R5RS, R6RS, R7RS

HISTORY

Present in MIT AI Memo No.~7 (1958 or 1959).

AUTHORS

This page is part of the scheme-manpages project. It includes materials from the RnRS documents. More information can be found at https://github.com/schemedoc/manpages/.

Markup created by unroff 1.0sc,    March 04, 2023.