Python Program To Reverse Words In A Given String

Example: Let the input string be “i like this program very much”. The function should change the string to “much very program this like i”

Examples: 

Input: s = “geeks quiz practice code” 
Output: s = “code practice quiz geeks”

Input: s = “getting good at coding needs a lot of practice” 
Output: s = “practice of lot a needs coding at good getting”

Algorithm:  

• Initially, reverse the individual words of the given string one by one, for the above example, after reversing individual words the string should be “i ekil siht margorp yrev hcum”.
• Reverse the whole string from start to end to get the desired output “much very program this like i” in the above example.

Below is the implementation of the above approach: 

Python3

# Python3 program to reverse a string

# Function to reverse each word in
# the string
def reverse_word(s, start, end):
    while start < end:
        s[start], s[end] = s[end], s[start]
        start = start + 1
        end -= 1

s = "i like this program very much"

# Convert string to list to use it as
# a char array
s = list(s)
start = 0
while True:

    # We use a try catch block because for
    # the last word the list.index() function
    # returns a ValueError as it cannot find
    # a space in the list
    try:
        # Find the next space
        end = s.index(' ', start)

        # Call reverse_word function
        # to reverse each word
        reverse_word(s, start, end - 1)

        #Update start variable
        start = end + 1

    except ValueError:

        # Reverse the last word
        reverse_word(s, start, len(s) - 1)
        break

# Reverse the entire list
s.reverse()

# Convert the list back to
# string using string.join() function
s = "".join(s)

print(s)

# This code is contributed by Prem Nagdeo

Output:

much very program this like i

Another Approach:

we can do the above task by splitting and saving the string in a reverse manner. 

Below is the implementation of the above approach:

Python3

# Python3 program to reverse a string
# s = input()
s = "i like this program very much"
words = s.split(' ')
string =[]
for word in words:
    string.insert(0, word)

print("Reversed String:")
print(" ".join(string))

# Solution proposed bu Uttam

Output:

Reversed String:
much very program this like i

Time Complexity: O(n) 

Without using any extra space:
The above task can also be accomplished by splitting and directly swapping the string starting from the middle. As direct swapping is involved, less space is consumed too.

Below is the implementation of the above approach:

Python3

# Python3 code to reverse a string

# Reverse the string
def RevString(s,l):

  # Check if number of words is even
  if l%2 == 0:

    # Find the middle word
    j = int(l/2)

    # Starting from the middle
    # start swapping words
    # at jth position and l-1-j position
    while(j <= l - 1):
      s[j], s[l - j - 1] = s[l - j - 1], s[j]
      j += 1

  # Check if number of words is odd
  else:

    # Find the middle word
    j = int(l/2 + 1)

    # Starting from the middle
    # start swapping the words
    # at jth position and l-1-j position
    while(j <= l - 1):
      s[j], s[l - 1 - j] = s[l - j - 1], s[j]
      j += 1

    # return the reversed sentence
    return s;

# Driver Code
s = 'getting good at coding needs a lot of practice'
string = s.split(' ')
string = RevString(string,len(string))
print(" ".join(string))

Output:

practice of lot a needs coding at good getting

Please refer complete article on Reverse words in a given string for more details!