Question

I have a problem with a small program that i made. Below is the part with the problem.

failed(X, C, Y) :- 
       registered_for(
            student(id(X),_,_,_,_,_),  
            course(name(C),year(Y),_),absences(A)), 
       A>2.

failed(X, C, Y) :- 
       grades(
            course(name(C),year(Y),_),
            student(id(X),_,_,_,_,_),_,normal(G),_,_), 
       G<5.

 count_failed(X, C, Y, N) :- 
       failed(X, C, Y), 
       append(L, C, L), 
       length(_,L,N).

The first 2 lines are working as intended. The problem is somewhere in the last. I want to add C on the list L but all i get is “ERROR : Out of global stack”. I am using SWI-prolog.

If needed i can post the rest of the code. Thanks in advance for your time and effort 🙂

Answer 1

Wouldn’t you use findall to assemble the list?
The reason for the stackoverflow is probably append(L, C, L), since L is never unified to anything inside this clause.