Python | Adding value to sublists

Sometimes, we just have to manipulate a list of lists by appending a similar value to all the sublists. Using a loop for achieving this particular task can be an option but sometimes leads to sacrificing the readability of code. It is always wanted to have a oneliner to perform this particular task. Let’s discuss certain ways in which this can be done. 

Method #1: Using list comprehension can be used to perform this particular task using a similar looping construct but in just a single line. This increases code readability. 

Python3

# Python3 code to demonstrate
# appending single value
# using list comprehension

# initializing list of lists
test_list = [[1, 3], [3, 4], [6, 5], [4, 5]]

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

# declaring element to be inserted
K = "GFG"

# using list comprehension
# appending single value
res = [[i, j, K] for i, j in test_list]

# printing result
print("The list after adding element :  " + str(res))

Output

The original list : [[1, 3], [3, 4], [6, 5], [4, 5]]
The list after adding element :  [[1, 3, 'GFG'], [3, 4, 'GFG'], [6, 5, 'GFG'], [4, 5, 'GFG']]

Time complexity: O(n), where n is the number of elements in the original list (test_list).
Auxiliary space: O(n), as a new list (res) is created with the same number of elements as the original list.

Method #2 : Using list comprehension + “+” operator This method is quite similar to the above method, but the difference is that plus operator is used to add the new element to each sublist. 

Python3

# Python3 code to demonstrate
# appending single value
# using list comprehension + "+" operator

# initializing list of lists
test_list = [[1, 3], [3, 4], [6, 5], [4, 5]]

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

# declaring element to be inserted
K = "GFG"

# using list comprehension + "+" operator
# appending single value
res = [sub + [K] for sub in test_list]

# printing result
print("The list after adding element :  " + str(res))

Output

The original list : [[1, 3], [3, 4], [6, 5], [4, 5]]
The list after adding element :  [[1, 3, 'GFG'], [3, 4, 'GFG'], [6, 5, 'GFG'], [4, 5, 'GFG']]

Time complexity: O(n) where n is the number of sublists in the test_list.
Auxiliary space: O(n) for the new list ‘res’ created using the list comprehension.

Method #3 : Using for loop

Python3

# Python3 code to demonstrate
# appending single value

# initializing list of lists
test_list = [[1, 3], [3, 4], [6, 5], [4, 5]]

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

# declaring element to be inserted
K = "GFG"
res = []
for i in test_list:
    i.append(K)
    res.append(i)


# printing result
print("The list after adding element : " + str(res))

Output

The original list : [[1, 3], [3, 4], [6, 5], [4, 5]]
The list after adding element : [[1, 3, 'GFG'], [3, 4, 'GFG'], [6, 5, 'GFG'], [4, 5, 'GFG']]

Time complexity: O(n), where n is the length of the input list. 
Auxiliary space: O(n), where n is the length of the input list. 

Method #4: Using map() and lambda function

Here is the approach using map() and lambda function

Python3

# Python3 code to demonstrate
# appending single value

# initializing list of lists
test_list = [[1, 3], [3, 4], [6, 5], [4, 5]]
# printing original list
print("The original list : " + str(test_list))
# Declaring element to be inserted
K = "GFG"

# Using map() and lambda function to add element to all sublists
res = list(map(lambda x: x + [K], test_list))
print("The list after adding element :  " + str(res))

#This code is contributed by Edula Vinay Kumar Reddy

Output

The original list : [[1, 3], [3, 4], [6, 5], [4, 5]]
The list after adding element :  [[1, 3, 'GFG'], [3, 4, 'GFG'], [6, 5, 'GFG'], [4, 5, 'GFG']]

Time complexity: O(n)
Auxiliary Space: O(n)

Method #5: Using itertools.chain and zip:

Python3

import itertools  # Import the itertools module
test_list = [[1, 3], [3, 4], [6, 5], [4, 5]]  # Initialize the list of lists
K = "GFG"  # Declare the element to be inserted
# printing original list
print("The original list : " + str(test_list))
res = [list(itertools.chain(i, [K])) for i in test_list]
# Use list comprehension and itertools.chain to add the element K to each sublist in the list of lists
# itertools.chain takes multiple iterables as arguments and returns a single, flatten iterable that contains elements from all the input iterables
# Here, i is each sublist in test_list, and [K] is a list with a single element K
# The result of itertools.chain(i, [K]) is a flatten list that contains all elements in i followed by K
# The list comprehension [list(itertools.chain(i, [K])) for i in test_list] applies this operation to each sublist in test_list and collects the results into a new list of lists
print("The list after adding element :  " + str(res))  # Print the result
# This code is contributed by Jyothi pinjala.

Output

The original list : [[1, 3], [3, 4], [6, 5], [4, 5]]
The list after adding element :  [[1, 3, 'GFG'], [3, 4, 'GFG'], [6, 5, 'GFG'], [4, 5, 'GFG']]

Time complexity: O(n)
Auxiliary Space: O(n*k)

Method #6: Using numpy
To use numpy we need to install it first. You can install it using the following command: pip install numpy

Step by step approach:

• Import numpy module using import numpy as np
• Convert test_list into numpy array using np.array(test_list)
• Append the value ‘K’ to the numpy array using np.append()
• Reshape the numpy array to the original shape using .reshape() method
• Convert the numpy array back to list using .tolist() method

Python3

import numpy as np

# initializing list of lists
test_list = [[1, 3], [3, 4], [6, 5], [4, 5]]

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

# declaring element to be inserted
K = "GFG"

# using numpy
# appending single value
test_np = np.array(test_list)
res_np = np.append(test_np, np.full((len(test_list), 1), K), axis=1).reshape(len(test_list), -1).tolist()

# printing result
print("The list after adding element :  " + str(res_np))

Output:

The original list : [[1, 3], [3, 4], [6, 5], [4, 5]]
The list after adding element :  [['1', '3', 'GFG'], ['3', '4', 'GFG'], ['6', '5', 'GFG'], ['4', '5', 'GFG']]

Time complexity: O(n) (where n is the number of elements in the list)
Auxiliary space: O(n) (for creating a numpy array)

Method #7: Using extend() method inside a for loop

Use a for loop to iterate through each sublist of the original list.
Use the extend() method to append the value to each sublist.

Python3

test_list = [[1, 3], [3, 4], [6, 5], [4, 5]]
K = "GFG"

for sublist in test_list:
    sublist.extend([K])

print("The list after adding element : " + str(test_list))

Output

The list after adding element : [[1, 3, 'GFG'], [3, 4, 'GFG'], [6, 5, 'GFG'], [4, 5, 'GFG']]

Time complexity: O(n), where n is the length of the list.
Auxiliary space: O(1), because no additional data structure is created.