convert string to list within recursive function

Question

I wrote this recursive function to check for a specific string in a list and if the string is found, it should be returned reversed within the list but I can't seem to keep the list as a list. my output is a joined string. can someone please suggest how to maintain the list type?

test_list = ['cat', 'hat', 'bat', 'sat']
test_string = 'bat'

def reverser(some_list, some_string):

    if some_list == []:
        return ''
    elif len(some_list) == 1 and item in some_list == some_string:
        return some_string[::-1] 
    else:
        if some_list[0] == some_string:
            return some_string[::-1] + reverser(some_list[1:], some_string)
        if some_list[0] != some_string:
            return some_list[0] + reverser(some_list[1:], some_string)
reverser(test_list, test_string)

output is:

'cathattabsat'

but i would like it to be:

['cat', 'hat', 'tab', 'sat']

NameError: global name 'item' is not defined

Stephen Rauch
– Stephen Rauch ♦

2017-04-21 02:42:40 +00:00
Commented Apr 21, 2017 at 2:42 — Stephen Rauch
– Stephen Rauch ♦, Commented Apr 21, 2017 at 2:42

Miriam Farber · Accepted Answer · 2017-04-21 02:51:30Z

If you want the function to return a list of strings, you need to make sure that in the base case as well as within the loops, you add the appropriate list and not a string.

In addition, in this row:

elif len(some_list) == 1 and item in some_list == some_string:

item is not defined. You can replace this row with:

elif len(some_list) == 1 and some_string in some_list

Finally, this will do the job:

test_list = ['cat', 'hat', 'bat', 'sat']
test_string = 'bat'


def reverser(some_list, some_string):

    if some_list == []:
        return []
    elif len(some_list) == 1 and some_string in some_list:
        return [some_string[::-1]] 
    else:
        if some_list[0] == some_string:
            return [some_string[::-1]] + reverser(some_list[1:], some_string)
        if some_list[0] != some_string:
            return [some_list[0]] + reverser(some_list[1:], some_string)
reverser(test_list, test_string)

This returns

['cat', 'hat', 'tab', 'sat']

ハセン · Accepted Answer · 2017-04-21 02:44:50Z

1

Your return statement returns a string always.

return some_string[::-1] + reverser(some_list[1:], some_string)

Note: some_string[::-1] is a string

Wrap it in [] to make it a list of one item.

return [some_string[::-1]] + reverser(some_list[1:], some_string)

However, there's a much simpler way than this weird loop you're doing. You can use map or list comprehensions.

def reverse_string(any, match):
    if any == match: return any[::-1]
    else: return any

def reverser(some_list, some_string):
    return [reverse_string(s, some_string) for s in some_list]

answered Apr 21, 2017 at 2:44

ハセン

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Comments

AChampion · Accepted Answer · 2017-04-21 02:54:19Z

1

Your recursive function is more complicated than it needs to be:

def reverser(some_list, some_string):
    if some_list == []:
        return []

    word = some_string[::-1] if some_list[0] == some_string else some_list[0]
    return [word] + reverser(some_list[1:], some_string)

But as mentioned by another answerer, this is much better done with a list comprehension:

[word[::-1] if word == some_string else word for word in some_list]

answered Apr 21, 2017 at 2:54

AChampion

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1 Comment

virgiliocyte Over a year ago

These are all great! I'm very new to python and my code is still clunky. I love list comprehensions, i'm just not very good at them...yet!

aghast · Accepted Answer · 2017-04-21 02:43:00Z

0

Think about the return type you want, and the expression you are returning:

return some_list[0] + reverser(some_list[1:], some_string)

You have a list of strings. So some_list[0] is going to be a string. You're returning a string, plus whatever.

If you want to return a list, make the string into a list somehow:

return some_list[0:1] + ...
return [some_list[0]] + ...

answered Apr 21, 2017 at 2:43

aghast

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1 Comment

virgiliocyte Over a year ago

I tried to use return [some_list[0]] + ... but adding the extra brackets was giving me an error because I forgot to change my base case to a list as well

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