ITSC 1213 - Introduction to CS 2 How to Design Programs with Object Oriented Programming

A for-loop combines all 3 parts of writing a loop in one line to initialize, test, and change the loop control variable. The 3 parts are separated by semicolons (;). Each of the three parts of a for loop declaration is optional (initialization, condition, and change), but the semicolons are not optional.

for (initialize; test condition; change)
{
   loop body
}

The for-loop is almost a shortcut way to write a while loop with all three steps that you need in one line.

Watch the following video which compares a while loop and for loop line by line.

Here is a control flow diagram for a for loop. The code in the initialization area is executed only one time before the loop begins, the test condition is checked each time through the loop and the loop continues as long as the condition is true, and the loop control variable change is done at the end of each execution of the body of the loop, just like a while loop. When the loop condition is false, execution will continue at the next statement after the body of the loop.

// These loops both run 10 times
// If you start at 0, use <
for(int i = 0; i < 10; i++)
{
   System.out.println(i);
}
// If you start at 1, use <=
for(int i = 1; i <= 10; i++)
{
   System.out.println(i);
}

Practice

for (int i = 3; i < 8; i++)
{
   System.out.print(i + " ");
}

for (int i = 1; i <= 10; i++)
{
   System.out.print(i + " ");
}

for (int i = 3; i <= 9; i++)
{
   System.out.print("*");
}

public static void printEvens()
{
---
   for (int i = 0;
        i <= 10;
        i+=2)
   {
---
      System.out.println(i);
---
   } // end for
---
} // end method

You can also count backwards in a loop starting from the last number and decrementing down to 0 or 1. All 3 parts of the loop must change to count backwards including the test of when to stop. For example, for (int i=5; i > 0; i–) counts from 5 down to 1.

The method printPopSong prints the words to a song. It initializes the value of the variable i equal to 5 and then checks if i is greater than 0. Since 5 is greater than 0, the body of the loop executes. Before the condition is checked again, i is decreased by 1. When the value in i is equal to 0 the loop stops executing.