Common errors in writing while loops

• To forget to initialize a variable that is used in the condition of the loop. Remember that the first time the condition is checked is before starting the execution of the body of the loop.

  Example:

  int i;
  String s;
  while (s != null) {
    i = Integer.parseInt(s);
    System.out.println(i*i);
    s = JOptionPane.showInputDialog("Input an integer");
  }
  

• To forget to update the variables appearing in the condition of the loop. The loop will never terminate.

  Example: The statement to read the next element to be processed is missing.

  int i;
  String s;
  s = JOptionPane.showInputDialog("Input an integer");
  while (s != null) {
    i = Integer.parseInt(s);
    System.out.println(i*i);
  }
  

• Be off by 1 in the number of iterations. Typically this is due to a mismatch between the condition of the loop and the initialization of the variables used in the condition.

  Example: Print out 10 stars.

  int i = 0;
  while (i <= 10) {           // should be (i < 10)
    System.out.print("*");
    i++;
  }
  

  To avoid such types of errors, it is often convenient to test the loop with simple (i.e., small) values for the variables. In the example above, if we check the loop by printing 1 star, instead of 10 (by substituting 10 with 1 in the condition of the loop), we immediately notice that the loop would print 2 stars, instead of 1.