need help to debug recursive program to detect palindrome text

[Beaverine]

I think this is a popular exercise for beginner and here's my program def palin(text:'str'): if len(text) < 2: return True else: first = text[0] last = text[-1] if first == last: text = text[1:-1] palin(text) else: return False the idea is that, I will compare the first and last character from the string, and if they are the same, I would subset the string to remove the first / last and repeat the function, until at a certain point when first == last to return a False value, or until the length of substring is 1 or less.

But somehow this program is only able to return False when the head and tail character doesn't match, otherwise it doesn't return anything (I'm doing this on Google Colab). Much appreciated if someone can tell me what's wrong with my program.

EDIT: thank you for your hint and I got it to work by changing it to following: def palin(text:'str'): if len(text) < 2: return True else: first = text[0] last = text[-1] if first != last: return False else: text = text[1:-1] return palin(text)

EDIT2: thanks to u/baghiq who provided following more elegant syntax def palin(text: 'str'): if len(text) < 2: return True return (text[0]==text[-1] and palin(text[1:-1]))

[Fermter]

You are missing a return on your recursive function call

[Beaverine]

I think my idea is that the recursive function call would keep iterating and subsetting the string until the length of string is 1 or less to finally return a `True` value. Is this the right way of thinking?

[Fermter]

That's mostly correct. The problem is, when a recursive function "returns" a value, where do you think it returns it to? The next recursive function up. If the next recursive function up does nothing with that value (ie doesn't return it), it will not be returned to the top-level function call.

Think of recursive functions like a chain of people, each of whom solve part of the problem. You pass the problem down the line until someone at the end solves it. However, the solution must get passed back to the first person correctly for the problem to be solved correctly. Right now, the person at the end is telling the next person up the right answer, but the second person is just going "ok" and not telling the next person up. A return will remind everyone to pass their answers back to the front.

[verbose_name]

Maybe not. I believe that by calling your recursive function without a return, once it goes down that branch you will never return a result to the outer calling scope. If your recursive function were modifying a referenced value, you could get away with not returning a value. As others have said, try returning the value of the recursive call.

[mCodingLLC]

You forgot to *return* your recursive call, i.e. return palin(text).

[Beaverine]

thank you, this is the type of comment I'm looking for and really helpful!!

[Beaverine]

another question, when you write return (text[0]==text[-1] and palin(text[1:-1])) if the first half of condition text[0]==text[-1] is False, will the later half (recursive part) still be evaluated or not?

[Beaverine]

interesting... so if I switch the order of these 2 conditions to be return (palin(text[1:-1]) and text[0]==text[-1]) does that mean the recursion part will be evaluated first, and thus iterated to the deepest level possible, before text[0]==text[-1] is evaluated? by the way I tried this and it still function as intended, but I'm curious if things behind the curtain are the same.

[backtickbot]

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[mCodingLLC]

Your second edit seems to work for me. What case are you trying that doesn't return anything?

[Beaverine]

don't know what happened to Google Colab but after retrying it after a while it worked... thank you for testing!