Java - Postfix Notation Expression Evaluator

[schapman22]

Homework Statement

I need to write a program that evaluates a postfix notation expression using a stack.

Homework Equations

5 3 2 * + 4 - 5 + = 12

The Attempt at a Solution

/*
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 * and open the template in the editor.
 */

package edu.colorado.collections;
import java.util.Scanner;

/**
 *
 * @author schapman2344
 */
public class PostfixEvaluation
{

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args)
    {
        ArrayStack<Double> numbers = new ArrayStack();
        String input;
        double num1, num2, num3;

        Scanner keyboard = new Scanner(System.in);
        System.out.println("Enter an expression in postfix notation" +
                " then press <Enter>: ");
        input = keyboard.next();
        int i = 0;

        do
        {
            if(Character.isWhitespace(input.charAt(i)))
                i++;//go to next character
            else if(Character.isDigit(input.charAt(i)))
            {
                numbers.push((double)input.charAt(i));
                i++;
            }
            else
            {
                switch(input.charAt(i))
                {
                    case '*':
                        num2 = numbers.pop();
                        num1 = numbers.pop();
                        num3 = num1*num2;
                        numbers.push(num3);
                        break;

                    case '/':
                        num2 = numbers.pop();
                        num1 = numbers.pop();
                        num3 = num1/num2;
                        numbers.push(num3);
                        break;

                    case '+':
                        num2 = numbers.pop();
                        num1 = numbers.pop();
                        num3 = num1+num2;
                        numbers.push(num3);
                        break;

                    case '-':
                        num2 = numbers.pop();
                        num1 = numbers.pop();
                        num3 = num1-num2;
                        numbers.push(num3);
                        break;
                }
                i++;
            }
        }while(i<input.length());

        System.out.println("The result is: " + numbers.pop());
    }
}

When I input a postfix expression it is outputting an incorrect number. For example when I input "5 3 2 * + 4 - 5 +" which evaluates to 12, it outputs 53.

 

[schapman22]

Changed the input to keyboard.nextLine(). But I am still getting wrong answers.

 

[Joffan]

I suggest you add some simple debugging output, like

        System.out.println(input.charAt(i) +  " op gave " + num3);

immediately after the operations switch statement.

 

[schapman22]

The values I'm getting back I can't make sense of. If I only enter 0 I get 48. If I enter any other single number I get 48+n. When I use an operator the numbers get even stranger.

 

[schapman22]

Sorry just realized that these are the ascii values of the input. But I am unsure how to get it to recognize the numerical value and not the ascii value.

 

[Joffan]

That's your problem right there. Easy enough to subtract off the ascii value of '0' from any digit character, but there's probably a valueOf or parseInt function available too.

 

[schapman22]

Cool I got it working now. Thanks for your help!

 

[Joffan]

The remaining question is: did the problem specify single digits for numerical input? otherwise you should be considering how to handle multi-character numbers.