Possible Duplicate:
Prolog delete: doesn't delete all elements that unify with Element
In Prolog if you write this:
delete([(1,1),(1,2),(1,1),(3,4)],(1,_),L).
the result will be:
L = [ (1, 2), (3, 4)].
What is normal because the _ variable binds with 1 in the first element and it searches for further elements of (1,1) and deletes them.
Is there a way to prevent this unification from happening and deleting all members of the form (1,_). In that case the result must be: L = [ (3, 4)].
delete_pattern([], _, []).
delete_pattern([H|T], P, O) :-
( H \= P
-> O = [H|O1],
delete_pattern(T, P, O1)
; delete_pattern(T, P, O) ).
You may like to use other predicates for filtering that would result in slightly different semantics as ==/2 or =@=/2.