Percentage always equal to 0

Question

I have this method. It basically uses popup windows to display simple addition questions:

public static void addition ()
  {
    JFrame frame = new JFrame("");
    JOptionPane.showMessageDialog(frame, "++ You chose Addition! ++");
    double percentage;
    String rank = "";
    int i = 0;
    int x = 0;
    int y = 0;

    while (true){
      int add1 = (int)(Math.random()*9) + 1;
      int add2 = (int)(Math.random()*9) + 1;

      i = i + 1;

      int addtotal = add1 + add2;

      try{
        String test = JOptionPane.showInputDialog(frame, i + "). What's " + add1 + " + " + add2 + "?");


        if (test == null){
          choose();
        }

        int convertToNum = Integer.parseInt (test);

        if (convertToNum == addtotal){ // if the user got it right
          x++;
          final ImageIcon icon = new ImageIcon(new URL("http://cdn2.iconfinder.com/data/icons/basicset/tick_64.png")); //Custom Icon for the JFrame below, The image destination uses a URL link to show the icon
          JOptionPane.showMessageDialog(null, "Nice Job!\nYou Currently Got " + x + " Out of " + i + " Correct!",":D",JOptionPane.INFORMATION_MESSAGE,icon);
        }
        else { // if the user got it wrong
          JOptionPane.showMessageDialog(frame, "Sorry, but that was wrong.\n" + add1 + " + " + add2 + " = " + addtotal + " . \n You Currently Got " + x + " Out of " + i + " Correct!","Whoops",JOptionPane.INFORMATION_MESSAGE);
          y++;
        }
      }
      catch (Exception e){
        JOptionPane.showMessageDialog(frame, "I Didn't Understand That.","...",JOptionPane.ERROR_MESSAGE);
        y++;
      }

      System.out.println ("x: " +x); 
      System.out.println ("i: " +i);

      percentage = (x - y) / i * 100;
      System.out.println ("% :" + percentage);
    }
  }

Say I keep getting perfect, Then percentage = 100.0 when displayed on the interactions pane. HOWEVER when I get one question wrong, instead of getting a percentage number, i automatically get a zero (ex say I got 2 out of 3, percentage = 0 instead of percentage = 66.6. I tried getting rid of the 0 when declaring it, but it only gives me "variable may not have been initialized".

NINCOMPOOP · Answer 1 · 2013-06-16T16:21:49.857

1

cast one of the integer operands inside the bracket to double to make use of floating point arithmetic .

double percentage = ((double)x - y) / i * 100;

All operands (x,y,i and literal 100) are integers, therefore integer arithmetic division is used which will remove everything after the decimal point.

edited Jun 16 '13 at 16:21

answered Jun 16 '13 at 16:16

NINCOMPOOP

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Alexis C. · Answer 2 · 2013-06-16T16:21:38.087

0

Welcome to integer arithmetic.

Try percentage = ((double)x - y) / i * 100;

From the Chapter 15 :

The type of a multiplicative expression is the promoted type of its operands.
If the promoted type is int or long, then integer arithmetic is performed.
If the promoted type is float or double, then floating-point arithmetic is performed.

edited Jun 16 '13 at 16:21

answered Jun 16 '13 at 16:16

Alexis C.

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Where is "integer arithmetic" defined exactly in the JLS? – ingyhere Dec 16 '13 at 00:25
@ingyhere http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.17 (see 15.17.2) – Alexis C. Dec 16 '13 at 08:27

score 0 · Answer 3 · answered Jun 16 '13 at 16:19

0

The problem with dividing ints is Java truncates the decimal portion. You can either make x and y floating point variables or just cast them to float for doing the division and cast them back to get an int as the final result.

answered Jun 16 '13 at 16:19

0x6C38

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Percentage always equal to 0

3 Answers3