• Initialization
• Test
• Body
• Update

• while
  • particularly good for loops controlled by flags or complex logic
  • test occurs before loop body is executed
• for
  • particularly good for counting or going through a sequence
  • test occurs before loop body is executed
  • initialization, test and increment explicitly listed as part of the for
• do-while
  • particularly good for menu-driven loops
  • test occurs after the body of the loop --- so it's always done at least once

Form: for( initialization; test; update )
                        loop-statement

Example: Printing n asterisks on a line

   for (count = 0; count < n; count++)
      System.out.print("*");
   System.out.println();

Interpretation: The initialization is done first. Then the test is performed. If the test is true, execute the body of the loop. Then perform the update step and do the test again. As soon as the test is false, control passes to the statement after the for loop.

Form: do {
                        loop-body
              } while ( repetition-condition)

Example: Write code to prompt the user to enter an integer greater than 0.

   SimpleGUI wind = new SimpleGUI();
   int theNumber;
   do {
      theNumber = wind.GetInt("Please enter an integer  > 0:");
   } while (theNumber <= 0); 
   System.out.println("The number you have entered is " + theNumber);
}

private int findFirstNonBlank(String s) {
   static final char BLANK = ' ';
   int i = 0;
   while (i < s.length() && s.charAt(i) == BLANK){
      i++;
   } 
   if (i < s.length())
      return i;
   else
      return -1;
}

The condition i < s.length() && s.charAt(i) == BLANK is an example of a short-circuit evaluation as described in the next item.

Compare the following implementations from the previous example:

• i < s.length() && s.charAt(i) == BLANK
• s.charAt(i) == BLANK && i < s.length()

• Infinite loops - make sure that the test condition is changed in the body of the loop.

• Off-by-one - watch out for loops that are executed one too many times or one too few times. This is often fixed by changing the initialization slightly or changing whether equality is included in the test

• Incorrect ordering in a short circuit - as above the order in compound boolean expressions often matters.

• Scoping of variables - a variable declared within the initialization part of a for loop is in existence only within that loop and cannot be referred to outside the loop.

• Fence post problems - occur when the endpoints need to be treated differently from the middle items in a sequence. For example, the following code to print the numbers from 1 to 10 separated by commas:

   int num = 1; 
   while ( num <= 10 ) { 
       System.out.print( num + ","); 
       num ++; 
   } 
   System.out.println(); 
  

  has a trailing comma in the output:

   1,2,3,4,5,6,7,8,9,10,  
  

  Three approaches for dealing with fence posts:

  • Print the first fence post (number) before the loop:

        int num = 1;                         
        System.out.print(num);              
        num++;                             
        while ( num <= 10 ) {               
           System.out.print("," + num); 
           num++;                      
        }
        System.out.println();
    

  • Print the last post (number) after the loop:

    
        int num = 1;
        while ( num < 10 ) {
           System.out.print(num + ",");
           num ++;
        }
        System.out.println(num);
    

  • Guard the printing of the crosspiece inside the loop:

        int num = 1; 
        while( num <= 10) { 
           System.out.print(num); 
           if( num < 10 ) 
              System.out.print(","); 
           num++; 
        } 
        System.out.println();