/** Driver.java -- Let's illustrate stack operations.
 *  Let's create and evaluate an expression.
 */
import java.io.*;
import java.util.*;

public class Driver
{
  public static void main(String [] args) throws IOException
  {
    BufferedReader kbd = new BufferedReader(new InputStreamReader(System.in));
    System.out.print("Please enter a postfix expression:  ");
    String line = kbd.readLine();
    System.out.println("You entered " + line);
    StringTokenizer tok = new StringTokenizer(line, " ");
    LinkedListStack stack = new LinkedListStack();

    // 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.
    // The stack holds Objects, so we'll let them be String.  We could also
    // have used Double.  Because of the necessary conversions (operations can
    // only be done on double), the code is a little tedious.
    // 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((String) stack.pop());
        a = Double.parseDouble((String) stack.pop());
        stack.push("" + (a+b));
      }
      else if (token.equals("-"))
      {
        b = Double.parseDouble((String) stack.pop());
        a = Double.parseDouble((String) stack.pop());
	stack.push("" + (a-b));
      }
      else if (token.equals("*"))
      {
        b = Double.parseDouble((String) stack.pop());
        a = Double.parseDouble((String) stack.pop());
	stack.push("" + (a*b));
      }
      else if (token.equals("/"))
      {
        b = Double.parseDouble((String) stack.pop());
        a = Double.parseDouble((String) stack.pop());
	stack.push("" + (a/b));
      }
      else
	stack.push(token); 
    }

    System.out.println("The value of the expression is " + 
		       Double.parseDouble((String) stack.pop()));
  }
}