Prolog program to find equality of two lists in any order

list prolog any failure-slice
Happy Mittal picture Happy Mittal · Apr 26, 2010 · Viewed 14.6k times · Source

I wanted to write a Prolog program to find equality of two lists, where the order of elements
doesn't matter. So I wrote the following:

del(_, [], []) .
del(X, [X|T], T).  
del(X, [H|T], [H|T1]) :-
   X \= H,
   del(X, T, T1).

member(X, [X|_]).  
member(X, [_|T]) :- 
   member(X, T).

equal([], []).  
equal([X], [X]).  
equal([H1|T], L2) :-
   member(H1, L2),
   del(H1, L2, L3),
   equal(T, L3).

But when I give input like equal([1,2,3],X)., it doesn't show all possible values of X. Instead, the program hangs in the middle. What could be the reason?

Answer

Mr. Berna picture Mr. Berna · Apr 27, 2010 
isSubset([],_).
isSubset([H|T],Y):-
    member(H,Y),
    select(H,Y,Z),
    isSubset(T,Z).
equal(X,Y):-
    isSubset(X,Y),
    isSubset(Y,X).

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