1.7.3 Average of solutions - calling a Prolog goal

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 method PlQuery::next_solution() yields true if there was a solution and false otherwise. If the goal yields a Prolog exception, it is mapped into a C++ exception. A return to Prolog does an implicit “cut” (PL_cut_query()); this can also be done explicitly by the PlQuery::cut() method.

PREDICATE(average, 3) /* average(+Templ, :Goal, -Average) */
{ long sum = 0;
  long n = 0;

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

?- [user].
|: p(1).
|: p(10).
|: p(20).
|:
% user://1 compiled 0.00 sec, 3 clauses
true.

?- average(X, p(X), Average).
Average = 10.333333333333334.