import java.util.*;

public class PostfixEvaluator {

	public static String evaluate(String expr) {

		if (expr.length() == 0) {
			return "No input.";
		}

		Stack<Double> operands = new Stack<Double>();
		TokenScanner input = new TokenScanner(expr);
		Token tkn;

		try {
			if (!input.hasNextToken()) {
				return "No input.";
			}

			while (input.hasNextToken()) {
				tkn = input.nextToken();

				if (tkn.isNumber()) {
					operands.push(tkn.numberValue());
				} else if (tkn.isOperator()) {
					operation(operands, tkn.operatorCharValue());
				} else if (tkn.isLeftParen() || tkn.isRightParen()) {
					return ("( and ) have no meaning here");
				} else {
					return ("Bad input.");
				}
			}

			Double x = operands.pop();

			if (!operands.empty()) {
				return ("Computed " + x + " but values remain on stack.");
			}

			if (!input.reachedEnd()) {
				return ("Computed " + x + " but not all input used.");
			}

			return x.toString();

		}
		catch (ArithmeticException e1) {
			return e1.getMessage();

		}
		catch (EmptyStackException e2) {
			return "Stack underflow.  Not enough operands on stack.";
		}
	}

	private static void operation(Stack<Double> operands, Character op) {
		Double op1, op2;

		op2 = operands.pop();
		op1 = operands.pop();

		switch (op) {
		case '+':
			operands.push(op1 + op2);
			break;
		case '-':
			operands.push(op1 - op2);
			break;
		case '*':
			operands.push(op1 * op2);
			break;
		case '/':
			try {
				operands.push(op1 / op2);
			} catch (ArithmeticException e1) {
				System.out.println("We tried to divide by zero");
				System.exit(-1);
			}
			break;
		}
	}
}