Question

I need to check if each element in second list has 3 times more instances then the same element in the first list. My function returns false all the time and I don’t know what I’m dong wrong.

Here is the code:

fourth(_,[ ]).
fourth(A,[HF|TF]) :-
    intersection(A, HF, NewA),
    intersection(TF, HF, NewB),
    append(HF, NewB, NewT),
    append(NewA, NewA, NewAA),
    append(NewA, NewAA, NewAAA),
    length(NewAAA) == length(NewT),
    select(HF, TF, NewTF),
    fourth(A, NewTF).

Example:

?- fourth([1,2,3], [1,1,1]).
true.

?- fourth([1,2,3], [1,1,1,1]).
false.

?- fourth([1,2,3], [1,1]).
false.

?- fourth([1,2,2,3], [1,1,2,2,1,2,2,2,2]).
true.

Answer 1

I would make myself a select/3 predicate: select(X,From,Left), and then for each elt of a first list I’d call it three times with same first argument on a second list, progressively passing it forward, getting me a final Left3 without the three occurences of X; iand I’d do that for each elt of a first list. Then if I’d succeed and end up with an empty list, that means it had exactly three times each elt from the first list.