My backup program deals with just one directory level at a time, which is why it tests to see that a file, f, is not a directory before attempting to copy it. There are many times, however, when you may want to traverse the subdirectories. For an example of this, let’s write a program that calculates the sizes of all the subdirectories beneath a specified root directory. This might be useful if, for example, you wanted to locate the biggest files and directories in order to free up disk space by archiving or deleting them.

Navigating through subdirectories creates an interesting programming problem. When you begin searching for the presence of subdirectories, you have no idea whether you will find one, none, or many. Moreover, any subdirectory you find may contain yet another level of subdirectories, each of which may contain other subdirectories and so on through many possible levels.