Question

I have written a set of two predicates to test whether a number is a square number.

patrat_perfect(A):- patr(A, 0).

patr(A, S):- A =:= S * S.

patr(A,S):- A > S * S, T is S+1, patrat(A, T).

patr(A,S):- A < S * S, fail.

The first predicate, patrat_perfect simply execute a call for the second predicate with the value 0 for the second parameter, and the number which should be tested whether or not it is a perfect square for the first parameter.

The patr predicate practically searches that number S from 0 to sqrt(A), which respects the equation S * S = A. In order to do so, the predicate increases the value of S with 1 if A > S * S. If at the next call, A = S * S, then the predicate returns true and if A < S * S, then the predicate returns false through the call of the built-in predicate fail.

The problem is that at every call of the patrat_perfect predicate, I receive the false value. Could you please tell me where do I go wrong?

Answer 1

When I fixed a typo in your code (you’re calling patrat, but don’t define it), it seems to work correctly (see http://ideone.com/j7fSA and http://ideone.com/SjIws).

Maybe you have copied the code incorrectly, or you have patrat defined somewhere else, which works differently.