Recursion
---------

You've seen functions that call other functions.

If a function calls *itself*, we call that **recursion**

Like with other functions, a call within a call establishes a *call stack*

With recursion, if you are not careful, this stack can get *very* deep.

Python has a maximum limit to how much it can recurse. This is intended to
save your machine from running out of RAM.

.. nextslide:: Recursion can be Useful

Recursion is especially useful for a particular set of problems.

For example, take the case of the *factorial* function.

In mathematics, the *factorial* of an integer is the result of multiplying that
integer by every integer smaller than it down to 1.

::

    5! == 5 * 4 * 3 * 2 * 1

We can use a recursive function nicely to model this mathematical function