Prolog add to list if element is not equal to string

Question

I am trying to create a list of pairs which have a non-null Guard element.

get_only_guarded(L) :-
    Guard \= null,
    findall([S,D],transition(S,D,_,Guard,_),L).

This is what ive tried but it doesnt seem to work. What would be the correct way?

score 3 · Answer 1 · answered Mar 28 '15 at 17:30

3

Use dif(Guard,null) instead. Handle (\=)/2 with care!

answered Mar 28 '15 at 17:30

repeat

2

Thank you very much, didnt know about this built in function – Evgeny Chulak Mar 28 '15 at 17:33
2

I have trouble letting go of my GNU Prolog (it doesn't have `dif/2`). ;) – lurker Mar 28 '15 at 22:44

score 2 · Answer 2 · answered Mar 28 '15 at 18:10

2

a more 'conventional ' way to do: push the test after the instantiation

get_only_guarded(L) :-
    findall([S,D], (transition(S,D,_,Guard,_),Guard \= null), L).

answered Mar 28 '15 at 18:10

CapelliC

2 Answers2