Question

I am trying to count how many times a element appears in a list, so far I have came up with

rate(X,[H|T],N):-
  X == H,
  N is N+1,
  rate(X,T,N).
rate(X,[_|T],N) :-
  rate(X,T,N).  
rate(_,[],N) :-
  N is 0.

I’ve covered when the a match is found, when there isn’t a match and when it reaches the end of the list. But when i test i get

43 ?- rate(4,[4,2,3,4,4,2],X).
ERROR: is/2: Arguments are not sufficiently instantiated
Exception: (6) frequency(4, [4, 2, 3, 4, 4, 2], _G393) ?     

What arguments am i missing exactly?

Answer 1

You can use is/2 (like in N is X) if and only if X is a number. You can’t use is/2 if X is a free variable. In your first clause you have: N is N+1. This is bad, since N is a free variable (it has not a value at this point, and so is for N+1).

There’s another error. In:

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

since you use this ONLY when X is not the first element in the list, you should check for this to be true! Here’s the code:

count(_, [], 0) :- !. /* empty list, base case */

count(X, [X|T], N) :- /* if X is in the head of the list */
    count(X, T, N2), /* count on the tail (let this N2) */
    N is N2 + 1.     /* and N is N2 + 1  */

count(X, [Y|T], N) :- 
    X \= Y,          /* if X is not in the head */
    count(X, T, N).  /* just count the rest */