common-lisp Mapping functions over lists Examples of MAPCAR
Example
MAPCAR is the most used function of the family:
CL-USER> (mapcar #'1+ '(1 2 3))
(2 3 4)
CL-USER> (mapcar #'cons '(1 2 3) '(a b c))
((1 . A) (2 . B) (3 . C))
CL-USER> (mapcar (lambda (x y z) (+ (* x y) z))
'(1 2 3)
'(10 20 30)
'(100 200 300))
(110 240 390)
CL-USER> (let ((list '(a b c d e f g h i))) ; randomize this list
(mapcar #'cdr
(sort (mapcar (lambda (x)
(cons (random 100) x))
list)
#'<=
:key #'car)))
(I D A G B H E C F)
An idiomatic use of mapcar is to transpose a matrix represented as a list of lists:
CL-USER> (defun transpose (list-of-lists)
(apply #'mapcar #'list list-of-lists))
ROTATE
CL-USER> (transpose '((a b c) (d e f) (g h i)))
((A D G) (B E H) (C F I))
; +---+---+---+ +---+---+---+
; | A | B | C | | A | D | G |
; +---+---+---+ +---+---+---+
; | D | E | F | becomes | B | E | H |
; +---+---+---+ +---+---+---+
; | G | H | I | | C | F | I |
; +---+---+---+ +---+---+---+
For an explanation, see this answer.
