Recursion

Learn more about when and how to use recursion.

Introduction

Recursion is the idea that a function calls itself. Pretty simple, right? It's used to take a big problem and start breaking it down into smaller and smaller pieces ("Divide and Conquer") and continuing to feed their solutions back into the original function until some sort of answer is achieved and the whole chain unwinds.

From the Wikipedia entry on Divide and Conquer Algorithms:

In computer science, divide and conquer (D&C) is an important algorithm design paradigm based on multi-branched recursion. A divide and conquer algorithm works by recursively breaking down a problem into two or more sub-problems of the same (or related) type, until these become simple enough to be solved directly. The solutions to the sub-problems are then combined to give a solution to the original problem.

There's also a right and wrong way to use recursion. The fact is, any problem you can solve recursively you can also solve using the iterators that you know and love. If you find yourself saying "why didn't I just use a while loop here?" then you probably should have. You won't often end up using a recursive solution to a problem, but you should get a feel for when it might be a good idea. Some problems also break down into far too many pieces and totally overwhelm your computer's memory. There's a balance.

In this brief lesson, you'll get a chance to learn more about when and how to use recursion and then in the next project you will get the chance to apply some of that (since it probably won't really stick until you've had a chance to try it).

Learning outcomes

Look through these now and then use them to test yourself after doing the assignment

• Why is recursion a useful technique for solving a big problem?

• What are the limitations of using recursive solutions?

• What types of problems are more suited for simple loops than recursion?

• What is meant by "recursive depth?"

• What is a "stack overflow" (the concept, not the website)?

• Why is that relevant to a recursive problem?

Assignment

1. Read the Chapter on Recursion in the Bastards Book of Ruby by Dan Nguyen.
2. Watch this Video on Recursion from CS50.
3. Read the "Implementation Issues" section of the wiki article to get an overview of some of the limitations of recursion.
4. Complete the Code Quiz on Recursion.

Additional resources

This section contains helpful links to other content. It isn't required, so consider it supplemental for if you need to dive deeper into something.

• "What is Ruby Recursion and How Does It Work?" on Stack Overflow
• Efficient Recursion from U of Alberta
• Natashatherobot's blog post on Recursion in Ruby
• Functional Programming Techniques with Ruby post, which covers recursion at the top
• A breakdown of the recursive Fibonacci method from Khan Academy