Question

I am working on a function that accepts two sets and returns an intersection of them. However, I dont want to use member function in my code. Here’s the member function :

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

Here’s what I have so far:

     set_int(_,[],_).   
     set_int([H|T],[H|T1],[H|T2]) :-
           set_int(T,T1,T2).
     set_int(T,[X|T1],T2) :-
           set_int(T,T1,T2). 

Please help me with my logic.

Answer 1

I think you can dispose of member (or equivalent) only if your sets are ordered. With such assumption, we can compare just heads:

set_int([X|Xs], [Y|Ys], Is) :-
    X @< Y,
    !, set_int(Xs, [Y|Ys], Is).
set_int([X|Xs], [Y|Ys], Is) :-
    X @> Y,
    !, set_int([X|Xs], Ys, Is).
set_int([Z|Xs], [Z|Ys], [Z|Is]) :-
    !, set_int(Xs, Ys, Is).
set_int(_, _, []).