Prolog List Plateau

Question

Just got introduced to prolog, trying to get through some simple exercises, but I\'ve been getting kind of stuck on this one. I\'m trying to write a program that outputs all

Answer 1

I tried using nth1/3 builtin, but had more trouble to get it working... anyway, here another implementation:

plateau(L, I, Len) :-
    plateau(L, 1, I, Len).
plateau(L, P, I, Len) :-
    nth1(P, L, E),
    skipseq(P, L, E, J),
    (   J > P,
        Len is J - P + 1,
        I is P
    ;   Q is J + 1,
        plateau(L, Q, I, Len)
    ).

% get the index J of last element E (after I)
skipseq(I, L, E, J) :-
    T is I + 1,
    nth1(T, L, E),
    !, skipseq(T, L, E, J).
skipseq(I, _, _, I).

test:

[debug]  ?- plateau([a,x,x,x,u,u,h,w],I,N).
I = 2,
N = 3 ;
I = 5,
N = 2 ;
false.