How do you reverse the words in a string using python (manually)? [duplicate]

Answer 1

Split the string, make a reverse iterator then join the parts back.

' '.join(reversed(my_string.split()))

If you are concerned with multiple spaces, change split() to split(' ')

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As requested, I'm posting an implementation of split (by GvR himself from the oldest downloadable version of CPython's source code: Link)

def split(s,whitespace=' \n\t'):
   res = []
   i, n = 0, len(s)
   while i < n:
       while i < n and s[i] in whitespace:
           i = i+1
       if i == n:
           break
       j = i
       while j < n and s[j] not in whitespace:
           j = j+1
       res.append(s[i:j])
       i = j
   return res

I think now there are more pythonic ways of doing that (maybe groupby) and the original source had a bug (if i = n:, corrrected to ==)

Answer 2

You have a string:

str = "A long string to test this algorithm"

Split the string (at word boundary -- no arguments to split):

splitted = str.split()

Reverse the array obtained -- either using ranges or a function

reversed = splitted[::-1]

Concatenate all words with spaces in between -- also known as joining.

result = " ".join(reversed)

Now, you don't need so many temps, combining them into one line gives:

result = " ".join(str.split()[::-1])

Answer 3

Original Answer

from array import array

def reverse_array(letters, first=0, last=None):
    "reverses the letters in an array in-place"
    if last is None:
        last = len(letters)
    last -= 1
    while first < last:
        letters[first], letters[last] = letters[last], letters[first]
        first += 1
        last -= 1

def reverse_words(string):
    "reverses the words in a string using an array"
    words = array('c', string)
    reverse_array(words, first=0, last=len(words))
    first = last = 0
    while first < len(words) and last < len(words):
        if words[last] != ' ':
            last += 1
            continue
        reverse_array(words, first, last)
        last += 1
        first = last
    if first < last:
        reverse_array(words, first, last=len(words))
    return words.tostring()

Answer using list to match updated question

def reverse_list(letters, first=0, last=None):
    "reverses the elements of a list in-place"
    if last is None:
        last = len(letters)
    last -= 1
    while first < last:
        letters[first], letters[last] = letters[last], letters[first]
        first += 1
        last -= 1

def reverse_words(string):
    """reverses the words in a string using a list, with each character
    as a list element"""
    characters = list(string)
    reverse_list(characters)
    first = last = 0
    while first < len(characters) and last < len(characters):
        if characters[last] != ' ':
            last += 1
            continue
        reverse_list(characters, first, last)
        last += 1
        first = last
    if first < last:
        reverse_list(characters, first, last=len(characters))
    return ''.join(characters)

Besides renaming, the only change of interest is the last line.

Answer 4

str = "hello world"
" ".join(str.split()[::-1])