Python | Extract Nth words in Strings List

Sometimes, while working with Python Lists, we can have problems in which we need to perform the task of extracting Nth word of each string in List. This can have applications in the web-development domain. Let’s discuss certain ways in which this task can be performed. 

Method #1: Using list comprehension + split() The combination of the above methods can be used to solve this problem. In this, we perform the task of getting Nth word using split and recreate list using list comprehension. 

# Python3 code to demonstrate working of 
# Extract Nth words in Strings List
# Using list comprehension + split()
    
# initializing list
test_list = ['Gfg best for', 'All geeks', 'It is for', 'all CS professionals']

# printing original list
print("The original list is :" + str(test_list))

# initializing N 
N = 2

# Extract Nth words in Strings List
# Using list comprehension + split()
res = [sub.split()[N - 1] for sub in test_list if len(sub.split()) > 1]

# printing result 
print("The Nth words in list are : " + str(res))

The original list is : ['Gfg best for', 'All geeks', 'It is for', 'all CS professionals']
The Nth words in list are : ['best', 'geeks', 'is', 'CS']

Time Complexity: O(n*n) where n is the total number of values in the list “test_list”. 
Auxiliary Space: O(n) where n is the total number of values in the list “test_list”.

Method #2: Using list comprehension + enumerate() + split() The combination of above functions can be used to perform this task. In this, we use enumerate to check for the Nth word rather than split(). 

# Python3 code to demonstrate working of 
# Extract Nth words in Strings List
# Using list comprehension + <code>enumerate() + split()
    
# initializing list
test_list = ['Gfg best for', 'All geeks', 'It is for', 'all CS professionals']

# printing original list
print("The original list is : " + str(test_list))

# initializing N 
N = 2

# Extract Nth words in Strings List
# Using list comprehension + <code>enumerate() + split()
res = [ele for sub in test_list for idx, ele in enumerate(sub.split()) if idx == (N - 1)]

# printing result 
print("The Nth words in list are : " + str(res)) 

The original list is : ['Gfg best for', 'All geeks', 'It is for', 'all CS professionals']
The Nth words in list are : ['best', 'geeks', 'is', 'CS']

Time Complexity: O(n*n), where n is the length of the input list. This is because we’re using the list comprehension + enumerate() + split() which has a time complexity of O(n*n) in the worst case.
Auxiliary Space: O(n), as we’re using additional space res other than the input list itself with the same size of input list.

Method #3: Using for loop + append() + split() + indexing

# Python3 code to demonstrate working of
# Extract Nth words in Strings List
# Using append() and for loop

# initializing list
test_list = ['Gfg best for', 'All geeks', 'It is for', 'all CS professionals']

# printing original list
print("The original list is :" + str(test_list))

# initializing N
N = 2
res = []
for i in test_list:
    words = i.split()
    # Extract Nth words in Strings List
    res.append(words[N-1])


# printing result
print("The Nth words in list are : " + str(res))

The original list is :['Gfg best for', 'All geeks', 'It is for', 'all CS professionals']
The Nth words in list are : ['best', 'geeks', 'is', 'CS']

Time Complexity: O(N*M)
Auxiliary Space: O(N*M)

Method #4: Using map() and lambda function

This method uses map() function along with a lambda function to apply the split() function on each string in the list and then extract the Nth word using indexing. The resulting list is converted to a list using list() function.

test_list = ['Gfg best for', 'All geeks', 'It is for', 'all CS professionals']

N = 2
res = list(map(lambda x: x.split()[N-1], test_list))

print("The Nth words in list are : " + str(res))

The Nth words in list are : ['best', 'geeks', 'is', 'CS']

Time complexity: O(n), linear time complexity, where n is the length of the list.
Auxiliary space: O(n), the space required to store the result list is proportional to the length of the list.

Method #5: Using reduce() 

In this method, we reduce() with a lambda function to accumulate the Nth word from each string in the input list. If the string has less than N words, it is skipped. The accumulator acc is initialized to an empty list. The final result is a list of all Nth words in the input list.

# Python3 code to demonstrate working of 
# Extract Nth words in Strings List
# Using reduce() + split()

# importing reduce() function from functools module
from functools import reduce

# initializing list
test_list = ['Gfg best for', 'All geeks', 'It is for', 'all CS professionals']

# printing original list
print("The original list is :" + str(test_list))

# initializing N 
N = 2

# Extract Nth words in Strings List
# Using reduce() + split()
res = reduce(lambda acc, sub: acc + [sub.split()[N-1]] if len(sub.split()) > N-1 else acc, test_list, [])

# printing result 
print("The Nth words in list are : " + str(res))

Output:

The original list is :['Gfg best for', 'All geeks', 'It is for', 'all CS professionals']
The Nth words in list are : ['best', 'geeks', 'is', 'CS']

Time complexity: O(n), where n is the length of the input list. This is because we need to iterate through every string in the list and perform a split operation on it.
Auxiliary space: O(n), where n is the length of the input list. This is because we create a new list to store the Nth word extracted from each string that satisfies the condition.

Method#6:

Using the re.findall() function to extract words from each string in the list and then selects the Nth word from each list if it exists. The result is a list containing the Nth words from each string.

Step-by-Step Approach

• Import the re module.
• Create a list named test_list containing strings.
• Print the original list to the console.
• Set the value of N to 2.
• Use a list comprehension to iterate over each string in test_list.
• For each string, extract all words using re.findall(r’\b\w+\b’, sub).
• Select the Nth word ([N-1]) if it exists (if len(…) > N-1).
• Store the results in the res list.
• Print the list of Nth words obtained in the previous step.

import re

# initializing list
test_list = ['Gfg best for', 'All geeks', 'It is for', 'all CS professionals']

# printing original list
print("The original list is :" + str(test_list))

# initializing N 
N = 2

# Extract Nth words in Strings List
# Using re.findall()
res = [re.findall(r'\b\w+\b', sub)[N-1] for sub in test_list if len(re.findall(r'\b\w+\b', sub)) > N-1]

# printing result 
print("The Nth words in list are : " + str(res))

The original list is :['Gfg best for', 'All geeks', 'It is for', 'all CS professionals']
The Nth words in list are : ['best', 'geeks', 'is', 'CS']

Time Complexity:

The time complexity is O(N * M), where N is the number of strings in test_list and M is the average number of words in each string. This is because, in the worst case, we iterate over each character of each word in each string.

Auxiliary Space Complexity:

The auxiliary space complexity is O(K), where K is the total number of words across all strings in test_list. This is because we store the Nth words in the res list. The space required is proportional to the number of words we extract and store in the result list.

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