Check if item is in a list (Lisp)

Question

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 1

Use MEMBER to test whether an item is in a list:

(member 1 '(5 9 1 2))  ; (1 2) 

Unlike FIND, it is also able to test whether NIL is in the list.

Answer 2

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