Nesting loops

This code is an example of nested loops:

The general pattern here is,

    while test1 :
        while test2 :
            statement

The key thing to be aware of is that the inner while loop (the one with test2 does all of its iterations for each of the iterations of the outer while loop (the one with test1).

Note that there is no limitation that stops us at two while loops. For example it is quite common in image processing operations to have four nested loops. An outer pair that is responsible for considering each pixel in the image, and an inner pair that processes that pixel in the context of its neighbouring pixels.