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You want to reverse the characters or words of a string.

Python, 42 lines
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# strings are immutable, so you need to make a copy -- and a list
# is the right intermediate datastructure, as it has a .reverse()
# method that does just what we want -- it works in-place, so...:
revchars = list(astring)        # string -> list of chars
revchars.reverse()              # inplace reverse the list
revchars = ''.join(revchars)    # list of strings -> string

# to flip words, we just work with a list-of-words instead:
revwords = astring.split()      # string -> list of words
revwords.reverse()              # inplace reverse the list
revwords = ' '.join(revwords)   # list of strings -> string

# note we use a ' ' (space) joiner for the list of words, but
# a '' (empty string) joiner for the list of characters.

# if you INSIST on oneliners, you need an auxiliary function
# (you can stick it in your builtins from sitecustomize.py...)
def reverse(alist):
    temp = alist[:]
    temp.reverse()
    return temp
# or maybe (NOT a good idea... it's messier & slower!!!):
def reverse_alternative(alist):
    return [alist[i] for i in range(-1, -len(alist)-1, -1)]
# which is "inlineable"... but *NOT* worth it...!!!

# anyway, now you CAN do brave oneliners such as:
revchars = ''.join(reverse(list(astring)))
revwords = ' '.join(reverse(astring.split()))

# the three-liners are faster and more readable, as well as
# more-idiomatic Python, but in the end Python does *NOT*
# twist your arm to make you choose the obviously-right approach:
# Python gives you the right tools, it's up to you to use them:-).

# to reverse-by-words while preserving untouched the intermediate
# whitespace, regular expression splitting can be used:
import re
revwords = re.split(r'(\s+)', astring)    # separators too since '(...)'
revwords.reverse()              # inplace reverse the list
revwords = ''.join(revwords)    # list of strings -> string
# *NOTE* the nullstring-joiner once again!

Inspired by recipe 1.6 of O'Reilly's Perl Cookbook. Again we see basically the same power, though packaged-up rather differently (no scalar-context issues in Python, but the inplace-operation idea that some people dislike...). And once again Perl tends to squeeze solutions into single dense lines (although it IS of course possible to split them up with intermediate auxiliary variables) while Python tends to divide them into a few statements (although it IS of course possible to squeeze them up with auxiliary functions).

2 comments

Anirudh Moudgal 19 years, 1 month ago  # | flag

Recursively reversing a string. The reverse of a string is the first letter prefixed by the reverse of the rest of the string:

def reverse(str):
    if len(str)==1:
        revstr=str
    else :
        revstr=reverse(str[1:])+str[0]

    return revstr
Raghunath Reddy Peesari 16 years, 10 months ago  # | flag

There is more simple way. a = 'abcdefghi'

print a[::-1]

'ihgfedcba'