Check if item is in a list (Lisp)

list lisp common-lisp
Jeff picture Jeff · May 26, 2011 · Viewed 47.7k times · Source

What's a simple way to check if an item is in a list?

Something like

(in item list)

might return true if item=1 and list=(5 9 1 2) and false if item=7

Answer

Rainer Joswig picture Rainer Joswig · May 26, 2011

Common Lisp

FIND is not a good idea:

> (find nil '(nil nil))
NIL

Above would mean that NIL is not in the list (NIL NIL) - which is wrong.

The purpose of FIND is not to check for membership, but to find an element, which satisfies a test (in the above example the test function is the usual default EQL). FIND returns such an element.

Use MEMBER:

> (member nil '(nil nil))
(NIL NIL)  ; everything non-NIL is true

or POSITION:

> (numberp (position nil '()))
NIL

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