Prolog: Count number of times e appears in a list in odd positions

Question

I can't figure out why this is constantly returning false. I can count the number of element "e" in a list in odd positions if I call another function, but I want to do it in a more clean and compact way. Here is the code:

count(_,[],0).
count(E,[E,_|T],C) :-
    count(E,T,D),
    C is D+1.
count(E,[H,_|T],C) :-
    E\=H,
    count(E,T,C).

Answer 1

Your current formulation is correct when the list has even number of elements:

?- count(x, [x, y, x, x], C).
C = 2 

But it will fail if the number of elements is odd.

To correct this you can just add couple of rules for one-element lists:

count(E,[E],1).
count(E,[H], 0):-
    E\=H.

Now it works for lists with odd number of elements:

?- count(x, [x, y, x, x, x], C).
C = 3 

?- count(y, [x, y, x, x, x], C).
C = 0 

?- count(x, [y, y, y, y, x], C).
C = 1