WITH Integer_Stacks; USE Integer_Stacks;
WITH VStrings; USE VStrings;
FUNCTION Evaluate_RPN (X : IN VString) RETURN Integer IS
-- Pre:  X is defined and represents an RPN arithmetic expression
--   of single digits and operators
-- Post: returns the value of the expression.

  ZeroPos   : Integer := Character'Pos ('0');
  C         : Character;
  T         : VString(MaxLength(X)) := X;
  S         : Stack(Capacity => 100);
  Y, Z      : Integer;

BEGIN -- Evaluate_RPN

  IF IsEmpty (T) THEN
    RETURN 0;
  END IF;
  LOOP
    C := Head (T);
    IF C IN '0' .. '9' THEN
      Push (S, Character'POS (C) - ZeroPos); -- convert to integer first
    ELSE
      Y := Top (S);
      Pop (S);
      Z := Top (S);
      Pop (S);
      CASE C IS
        WHEN '+' =>
          Push (S, Z + Y);
        WHEN '-' =>
          Push (S, Z - Y);
        WHEN '*' =>
          Push (S, Z * Y);
        WHEN '/' =>
          Push (S, Z / Y);
        WHEN OTHERS =>
          NULL;                  -- skip bad characters, if any
      END CASE;
    END IF;
    T := Tail (T);
    EXIT WHEN IsEmpty (T);
  END LOOP;

  RETURN Top (S);

END Evaluate_RPN;