Append elements of List1 that are different from List2 Different elements from in prolog

Question

I am currently trying to find all elements of Lists L1 and L2 that are different from each other and append them into List L.

So far, I tried the following:

 difference(L1, L2, L) :-
     findall(H, (member(H,L1),\+member(H, L2)), L).

However, I only get an empty list when I query the following:

 |?- difference([1,2,3],[1,2,3,4,5,6], L).

I also tried querying the following:

 |?- difference([1,2,3,4,5,6],[1,2,3], L).

Interestingly, for this one I get L = [4,5,6]. How can I rewrite my fact to for it to work also with the first query?

Possible duplicate of [Prolog lists difference](http://stackoverflow.com/questions/9687121/prolog-lists-difference) — Sergey Kalinichenko, Jan 04 '16 at 13:44
This is indeed a duplicate, however I am curious why my solution was not working :) — qwerty, Jan 04 '16 at 13:47

score 0 · Accepted Answer · answered Jan 04 '16 at 13:43

0

Your definition reads "all elements of L1 that are not in L2".
What you want is "all elements of L1 or L2 that are not in L1 and L2"

difference(L1, L2, L) :-
     findall(H, ((member(H,L1);member(H,L2)),\+(member(H,L1),member(H,L2))), L).

answered Jan 04 '16 at 13:43

Cephalopod

Ah, I see, such a simple mistake! Thank you so much! – qwerty Jan 04 '16 at 13:44

1 Answers1