Question

I want to count how many g items appear in a list, below is the code I was trying now, but I got false when return.

g(E) :- memberchk(E, [apple, orange, pear, grape, lycee, pineapple,dragonfruit]).

countFruit([], No):- write(' >> No of Fruits : '), write(No), nl.
countFruit([H|T], No) :- not(g(H)), countFruit(T,No).
countFruit([H|T], No) :- No1 is No+1, countFruit(T,No1).

?countFruit(H,0). (H is a list).

Answer 1

By calling ?- countFruit(H,0). you are telling prolog to unify the No variable in your countfruit\2 predicate to 0. So the result can only ever be 0 or fail.

If I run your code as-is though, I get the following:

>>NoofFruits:0
Yes.

Here’s my take on this problem:

countFruit(Xs, No) :-
    countFruit(Xs, No, 0).

countFruit([], No, No).
countFruit([H|T], No, X0) :-
    member(H, [apple, orange, pear, grape, lycee, pineapple, dragonfruit]),
    !,
    X1 is X0 + 1,
    countFruit(T, No, X1).
countFruit([_|T], No, X) :-
    countFruit(T, No, X).

?- countFruit([orange, dog, dragonfruit, lycee], X),
    write(" >> No of Fruits : "),
    write(X),
    nl,
    fail.