Python | Merge elements of sublists

Given two lists containing sublists, the task is to merge elements of sublist of two lists in a single list. 

Examples:

Input:
list1 = [[1, 20, 30],
         [40, 29, 72], 
         [119, 123, 115]]

list2 = [[29, 57, 64, 22],
         [33, 66, 88, 15], 
         [121, 100, 15, 117]]

Output: [[1, 20, 30, 29, 57, 64, 22],
         [40, 29, 72, 33, 66, 88, 15],
         [119, 123, 115, 121, 100, 15, 117]]

Method #1: Using Map + lambda

Python3

# Python code to merge elements of sublists

# Initialisation of first list
list1 = [[1, 20, 30],
         [40, 29, 72],
         [119, 123, 115]]

# Initialisation of second list
list2 = [[29, 57, 64, 22],
         [33, 66, 88, 15],
         [121, 100, 15, 117]]

# Using map + lambda to merge lists
Output = list(map(lambda x, y:x + y, list1, list2))

# Printing output
print(Output)

Output:

[[1, 20, 30, 29, 57, 64, 22],
 [40, 29, 72, 33, 66, 88, 15], 
 [119, 123, 115, 121, 100, 15, 117]]

Time Complexity: O(n*n), where n is the length of the list test_list 
Auxiliary Space: O(n) additional space of size n is created where n is the number of elements in the res list 

Method #2: Using Zip()

Python3

# Python code to merge elements of sublists

# Initialisation of first list
list1 = [[1, 20, 30],
         [40, 29, 72],
         [119, 123, 115]]

# Initialisation of second list
list2 = [[29, 57, 64, 22],
         [33, 66, 88, 15],
         [121, 100, 15, 117]]

# Using zip to merge lists
Output = [x + y for x, y in zip(list1, list2)]

# Printing output
print(Output)

Output:

[[1, 20, 30, 29, 57, 64, 22],
 [40, 29, 72, 33, 66, 88, 15],
 [119, 123, 115, 121, 100, 15, 117]]

Method #3: Using starmap() and concat()

Python3

# Python code to merge elements of sublists

from operator import concat
from itertools import starmap

# Initialisation of first list
list1 = [[1, 20, 30],
         [40, 29, 72],
         [119, 123, 115]]

# Initialisation of second list
list2 = [[29, 57, 64, 22],
         [33, 66, 88, 15],
         [121, 100, 15, 117]]

# Using starmap() and concat to merge list
Output = list(starmap(concat, zip(list1, list2)))

# Printing output
print(Output)

Output:

[[1, 20, 30, 29, 57, 64, 22],
 [40, 29, 72, 33, 66, 88, 15],
 [119, 123, 115, 121, 100, 15, 117]]

Method #4: Using inbuilt extend()

This code uses the zip function to iterate through list1 and list2 simultaneously. For each sublist in list1 and list2, it uses the extend method to add the elements of sublist2 to sublist1. The modified sublist1 is then appended to merged_list.

Python3

# Initialisation of first list
list1 = [[1, 20, 30],
         [40, 29, 72],
         [119, 123, 115]]

# Initialisation of second list
list2 = [[29, 57, 64, 22],
         [33, 66, 88, 15],
         [121, 100, 15, 117]]

# Merge elements of sublists using extend()
merged_list = []
for sublist1, sublist2 in zip(list1, list2):
    sublist1.extend(sublist2)
    merged_list.append(sublist1)

# Print merged list
print(merged_list)
#This code is contributed by Edula Vinay Kumar Reddy

Output

[[1, 20, 30, 29, 57, 64, 22], [40, 29, 72, 33, 66, 88, 15], [119, 123, 115, 121, 100, 15, 117]]

The time complexity of this code is O(n), where n is the total number of elements in list1 and list2. The space complexity is also O(n), as the merged_list will also have n elements.