Python | Adding value to sublists (original) (raw)

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

test_list = [[ 1 , 3 ], [ 3 , 4 ], [ 6 , 5 ], [ 4 , 5 ]]

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

K = "GFG"

res = [[i, j, K] for i, j in test_list]

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

test_list = [[ 1 , 3 ], [ 3 , 4 ], [ 6 , 5 ], [ 4 , 5 ]]

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

K = "GFG"

res = [sub + [K] for sub in test_list]

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

test_list = [[ 1 , 3 ], [ 3 , 4 ], [ 6 , 5 ], [ 4 , 5 ]]

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

K = "GFG"

res = []

for i in test_list:

`` i.append(K)

`` res.append(i)

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

test_list = [[ 1 , 3 ], [ 3 , 4 ], [ 6 , 5 ], [ 4 , 5 ]]

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

K = "GFG"

res = list ( map ( lambda x: x + [K], test_list))

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)
Auxiliary Space: O(n)

Method #5: Using itertools.chain and zip:

Python3

import itertools

test_list = [[ 1 , 3 ], [ 3 , 4 ], [ 6 , 5 ], [ 4 , 5 ]]

K = "GFG"

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

res = [ list (itertools.chain(i, [K])) for i in test_list]

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)
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

test_list = [[ 1 , 3 ], [ 3 , 4 ], [ 6 , 5 ], [ 4 , 5 ]]

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

K = "GFG"

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()

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.