// RpnCalculator evaluates reverse Polish expressions.
// Uses a stack to store and supply operands to operations
//    as they are encountered in the input.

import java.io.*;
import structure.*;
import com.mw.io.*;

public class RpnCalculator  {

	static boolean isOperator (String M) {
	   if ( M.equals("+") || M.equals("-") || M.equals("*") || M.equals("/") ) 
	      return true;
	   else return false;
	}

	static void showError (String message) {
		System.out.println("Error: " + message);
	}

    public static void main (String args[]) {

      SystemInput sysIn = new SystemInput();
      ReadStream r = new ReadStream();
      Stack S = new StackList();
	  String nextWord;
	  int	i;
	  Integer p1, p2;

	  System.out.println("Enter a Polish expression, followed by cmnd-D");
      nextWord = r.readString();
      while (! r.eof()) {
 //  Show stack contents after each step. 
        System.out.println(nextWord + " " + S.toString());
 //  Push operands onto the stack. 
		if (!isOperator(nextWord)) 
		   S.push(new Integer(nextWord)); 
 //  Handle operators on a FIFO basis. 
		else if (S.size() >= 2) {  
		        p2 = (Integer)S.pop();
				p1 = (Integer)S.pop();
				if (nextWord.equals("+")) 
				        S.push(new Integer (p1.intValue() + p2.intValue()));
				else if (nextWord.equals("-")) 
				        S.push(new Integer (p1.intValue() - p2.intValue())); 
				else if (nextWord.equals("*")) 
				        S.push(new Integer (p1.intValue() * p2.intValue())); 
				else if (nextWord.equals("/")) 
				        S.push(new Integer (p1.intValue() / p2.intValue())); 
				else showError("Invalid operator");
		     } 
		else showError("Stack has too few operands"); 
        nextWord = r.readString();
     }
     if (S.size() == 1) 
        System.out.println("Final result = " + (Integer)S.pop());
     else showError("Expression has too few operators");
   }
}