CODEWITHSUNDEEP
pythonstringrecursionreverse
Ojas Kale
Ojas Kale

Reputation: 2159

Strange behaviour with String reversal function

I am using recursion to reverse a string. I am able to reverse a string successfully using following code.

def rev(string):
    if len(string)==0:
        return string
    if len(string)==1:
        return string
    else:
        s=string

        string = s[-1]+rev(s[:-1])

    return string

But when i am replacing return with print it is printing only the first two characters of answer and then throwing an error saying

"TypeError: cannot concatenate 'str' and 'NoneType' objects"

Example. when give 'StackOverflow' to function and returned a value it returns 'wolfrevOkcatS' corrrectly but when I am trying to print the answer in the function itself, it is only printing "tS" giving the error above.

Upvotes: 1

Views: 108

Answers (4)

Garrett R
Garrett R

Reputation: 2662

I suspect that you did something like this: 

def rev(string):
    if len(string)==0:
        print string
    if len(string)==1:
        print string
    else:
        s=string

        string = s[-1]+rev(s[:-1])

    print string

Let's get right to the problem. Supposed the string you are trying to reverse is abc. In the first call, you will hit the else clause and try to evaluate:

'c' + rev('ab')

The problem is now that your function has no return statements, rev('ab') returns None. And python raises an error when you try to concatenate a string with None.

With the return statements. rev('ab') returns 'ba' and the plus sign can be used to concatenate 'c' with 'ba'.

As an aside, if you are ever trying to solve this problem in production code, you should be using the built in tools.

reversed_str = myStr[::-1]
reversed_str_iterator = reversed(myStr)

Upvotes: 2

Praveen
Praveen

Reputation: 9335

To reverse a string/list in Python use list[::-1]

Demo

>>> my_str = 'StackOverflow'
>>> my_str[::-1]
'wolfrevOkcatS'

Upvotes: 1

ShadowRanger
ShadowRanger

Reputation: 155363

If you don't return from a function, then the function implicitly returns None. By replacing return with print, you break the recursive functionality; it recurses, but the values computed in the recursive calls are not returned and used. If you want to print intermediate results for your own edification, do so, but do so just before each return statement, not in place of the return statements.

Upvotes: 1

Blckknght
Blckknght

Reputation: 104712

If you're replacing return with print, your function will not return anything. In Python, that is the same as returning None. This breaks your recursive step where you concatenate the last letter of the string with the return value of the recursive call. Instead, you should just let the recursive call do its own printing (just call it), after print the last letter in a separate statement:

else:
    print(s[-1], end="") # or print s[-1], (with the comma!) if you're on Python 2
    rev(s[:-1])

Note that you probably want your base cases to print a newline in this situation, since otherwise you won't get one at all.

Upvotes: 1

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