Extra space added on reverse string function

user_2138024 0
Viewed 418

I'm trying to figure out how to reverse a string using Python without using the [::-1] solution.

My code seems to work fine for several test cases, but it adds an extra space for one instance and I can't figure out why.

def reverse(s):
    r = list(s)
    start, end = 0, len(s) - 1
    x = end//2
    for i in range(x):
        r[start], r[end] = r[end], r[start]
        start += 1
        end -= 1

    print(''.join(r))


reverse('A man, a plan, a canal: Panama')

# returns 'amanaP :lanac  a,nalp a ,nam A'
# note the double space ^^ - don't know why


reverse('a monkey named fred, had a banana')

# 'returns ananab a dah ,derf deman yeknom a'


reverse('Able was I ere I saw Elba')

# returns 'ablE was I ere I saw elbA'
python
4 Answers

Change

    x = end//2

to

    x = len(s)//2
user_116061930

The bug appears to be related to the handling of even-length strings. A much easier way to build a string reverse function would be:

def reverse(s):
    result = ""
    for character in reversed(s):  #Reversed returns an object that, when used in a for loop, outputs each object of a string, list, or other iterable, in reverse order.
        result += character #Add that character back to the result.
    return result

This function works regardless of string length. I hope this helps.

user_116269280

Using the technique you're using it's probably clearer to test start agains end directly rather than trying to manage lengths and indexes. You can do that with while start < end:. For example:

def reverse(s):
    r = list(s)
    start, end = 0, len(s) - 1
    while start < end:
        r[start], r[end] = r[end], r[start]
        start += 1
        end -= 1

    print(''.join(r))

reverse('A man, a plan, a canal: Panama')

prints

amanaP :lanac a ,nalp a ,nam A
user_38746230

Your bug is in the boundary condition for an even-length string:

start, end = 0, len(s) - 1
x = end//2
for i in range(x):

For instance, with 8 characters, your for iterator is (range(3)), which gets you only the first three positions.

You fail to swap the middle pair.

The "clean" way to fix this is to change your x calculation:

x = (end+1) // 2

or, as others have said

x = len(s) // 2
user_47851850
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