how to test whether one list is a member of another

Question

Lets say I have two lists, ((1 2 3)) and (((1 2 3)) ((4 5))). I want to be able to tell if the first list is a member of the second list. I have t

Answer 1

If you want to have lists as elements of your sets for subsetp, you have to change the value of the :test keyword.

CL-USER 1 > (subsetp '(1 2 3) '(1 2 3 4 5))
T
CL-USER 2 > (subsetp '((1) (2) (3)) '((1) (2) (3) (4) (5)))
NIL

The first one gives T, the second one gives NIL. Why? Because equality is checked with #'eql which works for identical objects or numbers of the same value and of same type. Since two lists must not be identical objects, (eql '(1) '(1)) gives NIL. (That may depend on your CL implementation.) If you want to compare a tree of conses, tree-equal can help you.

CL-USER 3 > (subsetp '((1) (2) (3)) '((1) (2) (3) (4) (5)) :test #'tree-equal)
T

I don't understand the structure of the sets you gave as example completely, but I hope this helps.