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

/**
 *  Programming Project 3-3
 *  
 *  @author:(your name here)
 *
 *  Evaluates a postfix expression that operates on integer operands
 *  using the arithmetic operators +, -, *, /, and %.  Inputs is
 *  assumed to be in valid postfix form.
*/
public class PostfixEvaluator {

    //Stack<Double> stack;

    /**
     * Helper method that parses the string and returns the int value.
     * if the string is invalid, return -1.
    */
    private static int getIntValue(String s)
    {
        int result = -1;
        try
        {
            result = Integer.parseInt(s);
        }
        catch (NumberFormatException e)
        {
            // does nothing
        }
        return result;
    }// end getIntValue
    
    /**
    *  Evaluates the postfix expression and returns the result
    */
    public static double evaluate(String expression)
    {
    char character;
    String word = "";
        Stack<Integer> stack = new Stack<Integer>();
    double first = 0.0;
    double second = 0.0;

    StringTokenizer tokenizer = new StringTokenizer(expression);

    while(tokenizer.hasMoreTokens()) {
        word = tokenizer.nextToken();
        if(word.equals("+") || word.equals( "-") || word.equals("*") || 
            word.equals("/")) 
{
            character = word.charAt(0);
            first = Double.parseDouble(stack.pop().toString());
            second = Double.parseDouble(stack.pop().toString());

        switch(character) {
            case '+': stack.push(new Double(first + second)); break;
            case '-': stack.push(new Double(second - first)); break;
            case '*': stack.push(new Double(first * second)); break;
            case '/': stack.push(new Double(second / first)); break;
        }
        }

        else {
            stack.push(new String(word));
        }   
    }           
        // finish this method
        
        return Double.parseDouble(stack.pop().toString());
    }// end evaluate
    
    /**
    *  main method, used for testing the static 
    *  method PostfixEvaluator.evaluate().  Can be
    *  removed when testing complete.
    */
    public static void main (String args[]) 
    {
        System.out.println("Postfix Evaluator");
        System.out.println("------- ---------");
        
        String expression = "5 2 + 8 5 - *";
        System.out.println(expression);
        System.out.println("Result : " + 
        PostfixEvaluator.evaluate(expression));
        
        expression = "2 4 - 5 2 * +";
        System.out.println("\n" + expression);
        System.out.println("Result : " + 
        PostfixEvaluator.evaluate(expression));
    
        expression = "9 3 / 6 / 4 * 10 -";
        System.out.println("\n" + expression);
        System.out.println("Result : " + 
        PostfixEvaluator.evaluate(expression));
    
        expression = "9 3 / 6 / 4 * 10 - 4 /";
        System.out.println("\n" + expression);
        System.out.println("Result : " + 
        PostfixEvaluator.evaluate(expression));
    }//end main
    
}//end PostfixEvaluator