Recursion Function Isn't Working

Question

Okay, so I'm trying to make a recursive function that returns True if the function is a palindrome, and False otherwise. However, it doesn't go to the very end, and randomly stops.

Code:

def is_palindrome(word):

    if len(word) == 1 or len(word) == 0:
        return True
    else:
        lst = len(word) - 1
        if word[0] == word[lst]:
            print(len(word), " --> ", word)
            print(word[0], " # ", word[lst])
            is_palindrome(word[0+1:lst])
        else: 
            return False

For the life of me, I can't figure out why. Here's a sample output:

7  -->  racecar
r  #  r
5  -->  aceca
a  #  a
3  -->  cec
c  #  c

^ It stops right here. Why doesn't it continue and return True when length = 1?

`return list(word) == reversed(list(word))` – Klaus D. Feb 05 '18 at 04:51 — Klaus D., Feb 05 '18 at 04:51

score 6 · Accepted Answer · answered Feb 05 '18 at 04:42

You need to return your call to the recursive function:

def is_palindrome(word):

    if len(word) == 1 or len(word) == 0:
        return True
    else:
        lst = len(word) - 1
        if word[0] == word[lst]:
            print(len(word), " --> ", word)
            print(word[0], " # ", word[lst])
            return is_palindrome(word[0+1:lst])     # change here
        else: 
            return False

The reason you code appears to stop at the final step of recursion is because you never actually return a value in that case. In some programming languages, such as C or maybe Java, such code would not even compile. Python appears to tolerate it, though it results in your current behavior.

That makes so such sense. Thank you very much. I'll be more careful with my recursive statements now. Thank you!!! — , Feb 05 '18 at 05:01

score 1 · Answer 2 · answered Feb 05 '18 at 04:51

1

your function is too long. I prefer to use string[::-1]

#

def is_palindrome(word):

    return word == word[::-1]

answered Feb 05 '18 at 04:51

define is_tofu 1

56
1
3

Haha, I'm aware of that. But I don't quite understand it, so I'm gonna avoid it. But thank you! – Feb 05 '18 at 05:00

Recursion Function Isn't Working

2 Answers2