import java.util.Scanner;
import java.util.StringTokenizer;
import java.util.Stack;

/** Driver.java
 *  Let's create and evaluate an expression to illustrate stack operations.
 *  Rather than implementing our own stack class, let's use the API's version.
 *  We'll have a stack of strings, in other words:  Stack<String>.
 *  Alternatively, we could have stored Double objects in the stack.
 */
public class Driver
{
  public static void main(String [] args)
  {
    Scanner kbd = new Scanner(System.in);
    System.out.printf("Please enter a postfix expression:  ");
    String line = kbd.nextLine();
    System.out.printf("You entered %s\n", line);
    StringTokenizer tok = new StringTokenizer(line, " ");
    Stack<String> stack = new Stack<String>();

    // Each time we encounter an operator, we pop 2 numbers, evaluate, and
    // push the result on the stack.  If we encounter a number, we just push
    // it.  When the input is done, our answer should be the only number left
    // on the stack.
    // Note that we pop items off the stack in reverse order to how they
    // were pushed.
    while (tok.hasMoreTokens())
    {
      String token = tok.nextToken();
      double a, b;
      if (token.equals("+"))
      {
        b = Double.parseDouble(stack.pop());
        a = Double.parseDouble(stack.pop());
        stack.push("" + (a+b));
      }
      else if (token.equals("-"))
      {
        b = Double.parseDouble(stack.pop());
        a = Double.parseDouble(stack.pop());
        stack.push("" + (a-b));
      }
      else if (token.equals("*"))
      {
        b = Double.parseDouble(stack.pop());
        a = Double.parseDouble(stack.pop());
        stack.push("" + (a*b));
      }
      else if (token.equals("/"))
      {
        b = Double.parseDouble(stack.pop());
        a = Double.parseDouble(stack.pop());
        stack.push("" + (a/b));
      }
      else
        stack.push(token); 
    }

    System.out.printf("The value of the expression is %f\n", 
                      Double.parseDouble(stack.pop()));
  }
}