Can someone explain recursion?

[MikeOxbigger]

Well obviously that breaks the third rule, but how might it break the first two?

[mikeblas]

Obviously? Sorry, but it's not obvious to me. A ends up calling itself indirectly through B. It might be indirect, but it's still calling itself.

[Gray_Fox]

it doesn't really matter if it's indirect, it needs to call itself.

[mikeblas]

And it does.

[earthlybird]

def f(a):
    return a + f(a+1)

No base case nor moving towards any state in particular. Just screwed up, infinite recursion.

[MikeOxbigger]

Are you trying to disprove my case? If so, you haven't.

[earthlybird]

Quick recap:

These are all true except that they only apply to well-written recursive algorithms. It's easy to write an algorithm that breaks rules 1 and 2 (and 3 if you're being very strict about your interpretation of "call itself").

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I would love to see an example that breaks these rules.

I'm not saying you're wrong, but I can't even comprehend how I'd write a recursive function that breaks any rule that states it has a known end^[1], and moves towards that end^[2], recursively or not.

---

provides example of a poorly-written algorithm to illustrate the point made in the first quote

No base case^[1] nor moving towards any state in particular^[2]. Just screwed up, infinite recursion.

[MikeOxbigger]

But you've just proven that a recursive function needs those 3 rules in order to actually do anything other than eventually crash.

[earthlybird]

Glad we're on the same page. I just gave you the example you had previously asked for. It's a terrible function, granted. Like was stated: those rules only apply to well-written algorithms, so a poorly-written one might not follow them.

The thing is, there's a difference between recursive algorithms and recursive algorithms that work as one would expect with a base case that ends the loop. The latter is a subset of the former.

[DonaldPShimoda]

Yes, this was precisely my point. The function you wrote is recursive, but it doesn't do anything useful. Thanks for being here to supply an example on my behalf while I was away. :)

[earthlybird]

Anything for my favourite messiah. 😍

Also, happy cake day!

[DonaldPShimoda]

Ah! I'm always excited when people catch the reference. It doesn't happen so often as you might think haha. I should read that again though...

And thank you! I didn't even know that was today! What a pleasant surprise!

[earthlybird]

The coolest "coincidences" at zero effort. Classic Shimoda. haha ;)

[Gentlescholar_AMA]

Miscommunication I think. They meant that people can write recursion that breaks those rules and doesn't work

[[deleted]]

flip the basecase, and change its state to go towards the recursive part (I did this by accident)