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setcar
Changing the CAR of a cons cell is done with setcar
. When
used on a list, setcar
replaces one element of a list with a
different element.
This function stores object as the new CAR of cons-cell, replacing its previous CAR. It returns the value object. For example:
(setq x '(1 2)) ⇒ (1 2)
(setcar x 4) ⇒ 4
x ⇒ (4 2)
When a cons cell is part of the shared structure of several lists, storing a new CAR into the cons changes one element of each of these lists. Here is an example:
;; Create two lists that are partly shared.
(setq x1 '(a b c))
⇒ (a b c)
(setq x2 (cons 'z (cdr x1)))
⇒ (z b c)
;; Replace the CAR of a shared link. (setcar (cdr x1) 'foo) ⇒ foo x1 ; Both lists are changed. ⇒ (a foo c) x2 ⇒ (z foo c)
;; Replace the CAR of a link that is not shared. (setcar x1 'baz) ⇒ baz x1 ; Only one list is changed. ⇒ (baz foo c) x2 ⇒ (z foo c)
Here is a graphical depiction of the shared structure of the two lists
in the variables x1
and x2
, showing why replacing b
changes them both:
___ ___ ___ ___ ___ ___ x1---> |___|___|----> |___|___|--> |___|___|--> nil | --> | | | | | | --> a | --> b --> c | ___ ___ | x2--> |___|___|-- | | --> z
Here is an alternative form of box diagram, showing the same relationship:
x1: -------------- -------------- -------------- | car | cdr | | car | cdr | | car | cdr | | a | o------->| b | o------->| c | nil | | | | -->| | | | | | -------------- | -------------- -------------- | x2: | -------------- | | car | cdr | | | z | o---- | | | --------------
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