1.3.3 Average of solutions

This example is a bit harder. The predicate average/3 is defined to take the template average(+Var, :Goal, -Average) , where Goal binds Var and will unify Average with average of the (integer) results.

PlQuery takes the name of a predicate and the goal-argument vector as arguments. From this information it deduces the arity and locates the predicate. the member-function next_solution() yields TRUE if there was a solution and FALSE otherwise. If the goal yielded a Prolog exception it is mapped into a C++ exception.

PREDICATE(average, 3)
{ long sum = 0;
  long n = 0;

  PlQuery q("call", PlTermv(A2));
  while( q.next_solution() )
  { sum += (long)A1;
    n++;
  }
  return A3 = (double)sum/(double)n;
}