How not to teach recursion (2021)

I've been a SWE for 12 years and has been teaching people how to code during my free time ever since I started coding (so more than 12 years of teaching).

I have taught around 30 students over the past 12 years who went from knowing nothing to getting a SWE job and from my limited dataset I have observed that:

1. Students who were taught for/while loops first has a hard time grasping recursion. I suspect it is because following the recursive callstack gets tricky and feels unintuitive. This inspired me to try a curriculum where I teach students recursion first and don't expose them for/while loops until they are prepping for interviews.

2. Students who were taught recursion first has no problem understanding for/while loops when they were exposed to it.

For people who are curious, I used to teach students at my local library but recently created a free online curriculum at: https://c0d3.com

I completely agree with this. The trouble also is that "real" recursive problems tend to be _very_ hard: an example from physics would be magnetohydrodynamics where a plasma moves in accordance to an applied external B field, causing a current to flow, which generates a B field that changes how the plasma flows...you get the idea.

Everyone knows that you don't _really_ want to try doing 123! using your "my first recursion" code in C. Similarly everyone knows that factorial gets pretty big pretty quickly – I remember finding out the limits of calculators in school by finding at which point x! went from "big number" to "error". It's pedagogically useful for those things and you can see (a) if your answer is right, compared to a BigNum library, and (b) how long it takes. There's a whole can of worms you can go down, from Sterling's approximation to a discussion about time complexity, ints and IEEE 754, and stack overflows. You could then go waffling about, e.g. Haskell and lazy evaluation and functional programming and so on.

Factorial isn't a good problem but it's common enough that students will have heard of it, deep enough to be interesting, and used so frequently as a programming example that if you see a recursive definition of factorial in another programming language you get that they're trying to teach you about recursion. It's a good teaching tool for all those reasons.

I agree you should start with the data structure. Give someone a recursive data structure and have them try to write a function that does stuff with it.

Maybe the trick is to just teach it young. I first learned about the towers of Hanoi when I was about 14. Blew my mind. It was like circular reasoning that worked somehow.

Unlike a lot of people, I don't have this fear/anxiety about recursion. In fact the opposite. I friggin love recursion. I actively seeking out the recursive answer. It doesn't hurt my brain. In fact the opposite, it's gratifying on an almost sexual level. At some point in my apartment we had a recursion bin next to the recycling and normal trash. Inside of it was a power strip plugged into itself.

Maybe the trick with recursion is to teach it while the brain is still a bit plastic. Don't assume teens are too dumb. Talk to your kids about recursion, before it's too late.

> For instance, even a child informed about biology can answer basic questions like these: - Does a child have (biological) parents? - Yes. - How many? - Two, a (biological) male and (biological) female. [...] They can similarly see other kinds of self-referential data even at a young age, well before they program.

Isn’t this a horrific example for recursion? Wouldn’t most adults realise that the structure isn’t a tree (hi uncle grandma), and that it’s leaf boundaries are fuzzy. Or is that the point?

Apart from the fact that in a modern children’s classroom, bringing up parental discussions is going to be very complicated by other factors.

> These confuse recursion with cyclicity. If you don’t understand the difference, don’t use the dumb jokes.

Well, except cyclicity has a bunch of different meanings - I did a quick search and didn’t find a good reference for the sense they are using.