问题
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
回答1:
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
回答2:
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.
回答3:
You can use find:
(find 1 '(5 9 1 2)) ; 1
(find 7 '(5 9 1 2)) ; nil
Consider using :test argument:
(find "a" '("a" "b") :test #'equal)
来源：https://stackoverflow.com/questions/6144435/check-if-item-is-in-a-list-lisp