Counting with for loops

As you have seen in previous exercises, while loops and do-while loops are used when you want to repeat a set of instructions.

Those loops keep going as long as something is true. What if you want to do something a given number of times? What if you know the number of iterations? Then you want to use a for loop.

import java.util.Scanner;

public class BartsBlackboardAutomator
{
    public static void main( String[] args )
    {
        Scanner keyboard = new Scanner(System.in);

        String message;
        // whenever Bart Simpson gets in trouble he has to write something
        // on the blackboard. Now he can use this program to do it for him.
        // Leaving him more time for trouble!
        System.out.println( "Type in your message, " 
                         + "Bart, and I'll display it one hundred times." );
        System.out.print( "Message: " );
        message = keyboard.nextLine();

        //Array numbering starts with zero. But we're using a for loop 
        //so we can set the start point and end point to anything we want.
        for ( int n = 1 ; n <= 100 ; n++ )
        {
            //The counter variable, n, is within scope inside the loop
            //but not accessible outside the loop
            //What does the \t do? What happens if you remove it?
            System.out.println( n + ".\t" + message );
        }
        // Note: n is not visible outside the for loop in which it was declared
        // Uncomment the following line to see if this is true
        // System.out.println("value of n = " + n);



    }
}

Notes on for loops

The for loop contains three expressions in the parenthesis following the word for . They are called initialization, n = 1, condition n <= 100, and update, n++).

Each could also be missing but you would still need to include the ; anyway. If a condition is missing, it is assumed to be equal to true. If all three were missing you'd have an endless for loop. Which would be the same as a do-loop or a while-loop. To get out of such a loop you would need a terminating condition in the body that broke out with the keyword break.

Recommendations

• Use the three parts of the for loop according to their intended meaning described above, and with reference to a control variable for the loop
• Do not modify the control variable in the body of the loop