Using recursion to concatenate ONLY the letters of an input string into a single output string

"Given a string of both letters and special characters/numbers, use recursion to concatenate the letters into a single string and return it."

My code is below, I’m still learning recursion and am stuck in trying to trace it. I tried a bunch of different lines in this code but idk how to fix what I do have so far:

def decoder(encryptedStr):
    if len(encryptedStr) != 0:
        if encryptedStr[0].isalpha() == True:
            decoded = encryptedStr[0]
            decoded.join(decoder(encryptedStr[1:]))
            print(decoded)
        else:
            decoder(encryptedStr[1:])

I haven’t had it return anything yet because I’m struggling with the part where I have to join the new letters to the output string. Instead of .join I also tried:

decoded += decoder(encryptedStr[1:])

but it doesn’t work bc Nonetype??

>Solution :

Your main issue is that you didnt return, but there are some issues with your approach that make this more complex than need-be.

Think tail-first when doing recursion- What is your end condition, and how do you decide to continue. Typically with this kind of method you do something like, 1) process a single value in the list, 2) let the recursive method handle the rest of it, 3) combine the results.

An easy indicator of the tail-first return here would be to return nothing if the string is empty:

def decoder(encryptedStr):
    if len(encryptedStr) == 0:
        return ""
    ...

Now in each run we want to operate on one letter and pass the rest to a recursive call. Ignoring the special character requirement, you’d get something like this:

def decoder(encryptedStr):
    if len(encryptedStr) == 0:
        return ""

    first = encryptedStr[0]
    rest = decoder(encryptedStr[1:])

    return first + rest

Now we can handle the special case where we want to omit letters.

def decoder(encryptedStr):
    if len(encryptedStr) == 0:
        return ""

    first = encryptedStr[0]
    rest = decoder(encryptedStr[1:])

    if not first.isalpha():
        first = ""

    return first + rest

And that’s all there is to it!

Bonus for some refactoring:

def clean(letter):
    return letter if letter.isalpha() else ""

def decoder(encrypted):
    if len(encrypted) == 0:
        return ""

    return clean(encrypted[0]) + decoder(encrypted[1:])