//PostfixMalik.java
//simplified version of PostfixCalculator of page 408.
//input a string of a valid postfix expression of integers, evaluate it.
//assumes input is a valid postfix expression (i.e. no error checking is done).


import java.util.*;     
import javax.swing.*;

public class PostfixMalik {
	
    public static void main(String[] args) {
	String expression;
	String token;
	String operator;
	StringTokenizer tokens;
	int operand1, operand2, result=0;
		
	//create empty stack of pending integer operands
	Stack operandStack = new Stack();  
	
	expression = JOptionPane.showInputDialog( "Enter a postfix expression" );
	tokens = new StringTokenizer(expression);
	
	while (tokens.hasMoreTokens()) {
	    token = tokens.nextToken();
	    
	    if (!isOperator(token))   //token is an operand (i.e. integer)
	        //extract int from String token, create Integer object, push onto stack
	        operandStack.push(new Integer(Integer.parseInt(token)));  
	        
	    //  printStack(operandStack);   //display stack at each step. Debugging

	    else {                    //token is an operator
	        //pop two ints off stack.
	        //convert popped Object to Integer, extract int from the Integer
	        operand2 = ((Integer)operandStack.pop()).intValue();
	        operand1 = ((Integer)operandStack.pop()).intValue();  //"first" one
	        
		if (token.equals("+"))
		    result = operand1 + operand2;
		else if (token.equals("-"))
		    result = operand1 - operand2;
		else if (token.equals("*"))
		    result = operand1 * operand2;
		else if (token.equals("/"))
		    result = operand1 / operand2;
		else if (token.equals("%"))
		    result = operand1 % operand2;
		//push the resulting integer onto the stack
	        operandStack.push(new Integer(result));  
	    }
	}
	
	//only object in stack is the answer
	result = ((Integer)operandStack.pop()).intValue();
	JOptionPane.showMessageDialog(null,expression+" = "+result);

	System.exit(0);
    }
    
    
    static boolean isOperator(String t) {
    	if (t.equals("+") || t.equals("-") || t.equals("*") || t.equals("/") ||
    	    t.equals("%"))
    	    return true;
    	else
    	    return false;
    }
    
    /* //if needed for debugging
    static void printStack( Stack s) {
	System.out.print("Stack is: ");
        //don't normally iterate over a stack.  here for illustration purposes.
        Iterator items = s.iterator();
        while (items.hasNext())
            System.out.print(items.next()+"  ");
        System.out.println();
    }
    */
}
/*
page 399 example:
 6 3 + 2 *
   postfix for: (6 + 3) * 2  = 18
   
   
other examples:
4 5 7 2 + - *
  postfix for: 4 * (5 - (7 + 2))  = -16

3 4 + 2 * 7 /
  postfix for: (3 + 4) * 2 / 7  = 2
  
5 7 + 6 2 - *
  postfix for: (5 + 7) * (6 - 2)  = 48
  
4 2 + 3 5 1 - * +
  postfix for: 4 + 2 + 3 * (5 - 1)  = 18
 
bonus:
 2 3 + 4 * 6 4 1 - % 5 2 / - +
which is the postfix for: (2+3)*4 + 6%(4-1) - 5/2  = 18
*/