import ch03.stacks.*;
import java.util.Scanner;
public class PostfixEval {
  public static double postfixEval(String expr) {
    UnboundedStackInterface<Double> stack = new LinkedStack<Double>();
    Scanner getInput = new Scanner(expr);
    double operand1, operand2;

    while (getInput.hasNext()) { // could be a value or an op
      if (getInput.hasNextDouble()) // its a value; push it
	stack.push(getInput.nextDouble());
      else {			// its an op; pop, pop, eval, push
	operand2 = stack.top();  stack.pop();
	operand1 = stack.top();  stack.pop();
	switch (getInput.next()) { // what is the operator ?
	case "+": stack.push(operand1 + operand2); break; // java 1.7
	case "-": stack.push(operand1 - operand2); break;
	case "*": stack.push(operand1 * operand2); break;
	case "/": stack.push(operand1 / operand2); break;
	}
      }
    }
    return stack.top();		// auto convert Double to double
  }
}