How can I switch between 2 characters?

Question

I am using python 2.7 and i'm trying to find a solution to this problem, when i get a string from a certain function i need to switch between the first and the last character. for example if the string is "hello" it should return as "oellh" slicing won't work and i cant just replace the characters like i will normally do because i dont know what the string is going to be or how long it would be. I thought about replacing between the character in position 0 with the character in position -1 but i cant find something that will enable me to do it. Does anyone have any suggestions?

Answer 1

s[-1] + s[1:-1] + s[0] if len(s) > 1 else s

This will swap everything except for the edge cases of '' and 'a' (single char).

The snippet of code below slices the string into three chunks. The end chunks swap the first and last letters while the middle chunk grabs the middle of the string.

 s[-1] + s[1:-1] + s[0]

 # Below, I show each chunk in the interpreter.
 >>> s = 'hello'
 >>> s[-1]
 'o'
 >>> s[1:-1]
 'ell'
 >>> s[0]
 'h'

You can't slice up a string that's < 2 in length because of index errors and the fact that there is no reason to swap a 1 character string. That is what the if .. else protects against. If you don't understand how if .. else works, read the snippet below.

# This returns expr1 if expr2 is true, otherwise it returns expr3
a = expr1 if expr2 else expr3

# The above expression is the short hand of...
if expr2:
    a = expr1
else:
    a = expr3

I hope this made sense.

Answer 2

Strings are immutable so you can't modify them directly, but you can create a new string from an existing one. Try this:

if len(s) < 2:
    return s
else:
    return s[-1] + s[1:-1] + s[0] 

Answer 3

Old-fashioned Pythonistas don't need no steenking ternary operators:

Python 1.5.2 (#0, Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)] on win32
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> for s in ('abc', 'ab', 'a', ''):
...     print repr(s[-1:] + s[1:-1] + s[:-1][:1])
...
'cba'
'ba'
'a'
''
>>>

Answer 4

SINGLE LINE SOLUTION

Regexes are not as efficient as string replacements, but it should not make a big difference for short strings.

>>> s = 'hello'
>>> print re.sub('^(.)(.*?)(.)$',r'\3\2\1',s)
'oellh'