/* Stack evaluation of Polish. */

int evaluate(TOP *polish)
{
   TOP    eval = NULL;       /* empty evaluation stack */
   DATA   temp;
   char   bin_op;
   int    arg1, arg2;

   while (!isempty(*polish)) {
      pop(polish, &temp);
      if (temp.kind == operator) {
	 bin_op = temp.u.op;
	 pop(&eval, &temp);
	 arg2 = temp.u.val;
	 pop(&eval, &temp);
	 arg1 = temp.u.val;
	 temp.kind = value;
	 switch (bin_op) {
	 case '+':
	    temp.u.val = arg1 + arg2;
	    break;
	 case '-':
	    temp.u.val = arg1 - arg2;
	    break;
	 case '*':
	    temp.u.val = arg1 * arg2;
	 }
	 push(&eval, temp);
      }
      else                      /* temp.kind == value */
	 push(&eval, temp);
   }
   pop(&eval, &temp);
   return (temp.u.val);
}