For the last loop we held your hand, so let’s try one on your own!

As an example of iteration, we have the first program ever to run on a stored-program computer (the EDSAC). It was written and run by David Wheeler in the computer laboratory at Cambridge University, England, on May 6th, 1948, to calculate and print a simple list of squares like the following:

0 0 1 1 2 4 3 9 4 16 5 25 6 36 7 49 8 64 9 81

On the left, there are numbers from 0 to 9. On the right are their squares. For example, for the number 9: 9 * 9 = 81.

Note: An algorithm like this is one of the easiest tells for needing a loop. But as you develop your programming skills, you’ll pick up more and more subtle hints and uses for where loops can be incorporated into your code!



Write a while loop that prints out a number, followed by a tab \t, then the square of that number, followed by a new line \n like so:

printf("%d\t%d\n", num, num * num);

Have the while loop calculate this from 0 to 9.

Note: If you notice the Run button spinning continuously or a “Lost connection to Codecademy” message in this exercise, you may have an infinite loop! If the stop condition for our loop is never met, we will create a loop that never exits which prevents the program from ever completing. In this exercise, you can simply refresh the page to stop the infinite loop — then fix the code for your loop. However, we’ll talk more about infinite loops later, and even how to use them effectively!


Now change your while loop to go backward from 9 and stop at 0. Your new output should look like this:

9 81 8 64 7 49 6 36 5 25 4 16 3 9 2 4 1 1 0 0

Take this course for free

Mini Info Outline Icon
By signing up for Codecademy, you agree to Codecademy's Terms of Service & Privacy Policy.

Or sign up using:

Already have an account?