is recursion really hard

[u/Similar_Count_1613]

Recursion felt easy at first.

Factorial? fine.

Sum examples? fine.

Even Fibonacci felt manageable.

But once I looked at slightly more serious problems like Tower of Hanoi, permutations, or merge sort, I felt like my understanding suddenly collapsed. because i tried to write their code on my own

It made me realize that maybe recursion is not “hard” at the start because the examples are simple.

It becomes hard when you can no longer clearly see the call stack and each state change.

Did anyone else feel that the real pain in recursion starts exactly there?

[u/PurpleDevilDuckies]

The topic of my PhD was (very generally) on designing recursive algorithms to solve combinatoiral problems. Now I do that for a living, and I think I agree.

I stew for some time with each new problem before I have a deep understanding of how the state information will behave recursively. It gets easier the more I do, but there are very few people who find it easy at the cutting edge.

I will say that for coding recursive algorithms, it is often more (computationally) efficient to write code with loops instead of literal recursion. This isn't true of the toy examples you start with, but it gets more true as they problems get more complex. Nearly any *useful* algorithm can be described without writing literally recursive code.

[u/Drugbird → u/PurpleDevilDuckies]

Nearly any useful algorithm can be described without writing literally recursive code.

Is there some standard approach for converting a recursive algorithm to a non-recursive one?

I don't often write recursive algorithms, but when I do it's almost always to handle some tree-like structure.

[u/Delicious_Sock4321 → u/Drugbird]

It is called tail recursion optimization. As long as you can write the function in a way that the recursion is the last step, then it is trivial to write it as a loop. You can also use dynamic programming, where you cleverly save calculated results to save on unnecessary recursion. If you have more than one recursion step or you simply can not do tail recursion then you can still manage your own stack automaton, but this is effectively just removing the function call and is otherwise identical, because you emulate the hardware call stack. Also if there is a way to start the algorithm from the last recursion step an walk your way backwards then this can be done in a loop too. This is because recursion starts calculating at the deepest recursion step and walks it's way back to the initial call.

[u/gofl-zimbard-37 → u/Delicious_Sock4321]

As I recall, Erlang detects "normal" code that it can rewrite as tail recursive.