Counter-Controlled Repetition
Click while / for to see the concepts.

Counter-controlled repetition requires

• a control variable (or loop counter)
• the initial value of the control variable
• the increment (or decrement) by which the control variable is modified each time through the loop (also known as each iteration of the loop)
• the loop-continuation condition that determines if looping should continue.

// Fig. 5.1: WhileCounter.java
// Counter-controlled repetition with the while repetition statement.

public class WhileCounter 
{
   public static void main( String[] args ) 
   {      
      int counter = 1; // declare and initialize control variable

      while ( counter <= 10 ) // loop-continuation condition
      {
         System.out.printf( "%d  ", counter );
         ++counter; // increment control variable by 1
      } // end while

      System.out.println(); // output a newline
   } // end main
} // end class WhileCounter

for repetition statement

• Specifies the counter-controlled-repetition details in a single line of code

// Fig. 5.2: ForCounter.java
// Counter-controlled repetition with the for repetition statement.

public class ForCounter 
{
   public static void main( String[] args ) 
   {
      // for statement header includes initialization,  
      // loop-continuation condition and increment
      for ( int counter = 1; counter <= 10; counter++ ) 
         System.out.printf( "%d  ", counter );

      System.out.println(); // output a newline
   } // end main
} // end class ForCounter

A common logic error with counter-controlled repetition is an off-by-one error. 
For example, if the above condition is changed to counter < 10, the loop would iterate only nine times.

The general format of the for statement is

• The two semicolons in the for header are required.

In most cases, the for statement can be represented with an equivalent while statement as follows:

initialization;
while ( loopContinuationCondition )
{
   statement
   increment;
}

Typically, for statements are used for counter-controlled repetition and while statements for sentinel-controlled repetition.

If the initialization expression in the for header declares the control variable, the control variable can be used only in that for statement.

A variable’s scope defines where it can be used in a program.
A local variable can be used only in the method that declares it and only from the point of declaration through the end of the method.

The increment expression in a for acts as if it were a standalone statement at the end of the for’s body, so

counter = counter + 1
counter += 1
++counter
counter++

are equivalent increment expressions in a for statement.