# The Three Laws of Recursion

Like the robots of Asimov, all recursive algorithms must obey three important laws:

- A recursive algorithm must have a
**base case**. - A recursive algorithm must change its state and move toward the base case.
- A recursive algorithm must call itself, recursively.

Let’s look at each one of these laws in more detail and see how it was
used in the `sum_of`

algorithm. First, a base case is the condition
that allows the algorithm to stop recursing. A base case is typically a
problem that is small enough to solve directly. In the `sum_of`

algorithm the base case is a list of length 1.

To obey the second law, we must arrange for a change of state that moves
the algorithm toward the base case. A change of state means that some
data that the algorithm is using is modified. Usually the data that
represents our problem gets smaller in some way. In the `sum_of`

algorithm our primary data structure is a list, so we must focus our
state-changing efforts on the list. Since the base case is a list of
length 1, a natural progression toward the base case is to shorten the
list. This is exactly what happens in the `sum_of`

algorithm when we call `sum_of`

with a shorter
list.

The final law is that the algorithm must call itself. This is the very definition of recursion. Recursion is a confusing concept to many beginning programmers. As a novice programmer, you have learned that functions are good because you can take a large problem and break it up into smaller problems. The smaller problems can be solved by writing a function to solve each problem. When we talk about recursion it may seem that we are talking ourselves in circles. We have a problem to solve with a function, but that function solves the problem by calling itself! But the logic is not circular at all; the logic of recursion is an elegant expression of solving a problem by breaking it down into a smaller and easier problems.

In the remainder of this chapter we will look at more examples of recursion. In each case we will focus on designing a solution to a problem by using the three laws of recursion.