The task of concatenate two lists of lists row-wise, meaning we merge corresponding sublists into a single sublist. For example, given a = [[4, 3], [1, 2]] and b = [[7, 5], [9, 6]], we pair elements at the same index: [4, 3] from a is combined with [7, 5] from b, resulting in [4, 3, 7, 5], and [1, 2] from a merges with [9, 6] from b, forming [1, 2, 9, 6]. The final output is [[4, 3, 7, 5], [1, 2, 9, 6]].

Using list comprehension with zip

List comprehension combined with zip() iterates through both lists simultaneously and concatenates corresponding sublists using the + operator. This method is concise and easy to read.

a = [[4, 3, 5], [1, 2, 3], [3, 7, 4]]
b = [[1, 3], [9, 3, 5, 7], [8]]

a = [x + y for x, y in zip(a, b)]
print(a)

[[4, 3, 5, 1, 3], [1, 2, 3, 9, 3, 5, 7], [3, 7, 4, 8]]

Explanation: List comprehension with zip() pairs corresponding sublists from a and b, forming tuples. The + operator concatenates each pair (x, y), appending elements of y to x, resulting in a new list of merged sublists.

Using map with zip

map() function applies a lambda function to each pair of sublists returned by zip(), concatenating them using the + operator. This method is functional and avoids explicit loops.

a = [[4, 3, 5], [1, 2, 3], [3, 7, 4]]
b = [[1, 3], [9, 3, 5, 7], [8]]

a = list(map(lambda x: x[0] + x[1], zip(a, b)))
print(a)

[[4, 3, 5, 1, 3], [1, 2, 3, 9, 3, 5, 7], [3, 7, 4, 8]]

Explanation: map() with zip() pairs corresponding sublists from a and b, merging each pair using + in a lambda function. The result is converted to a list of concatenated sublists.

Using a loop with extend

This method modifies the first list a in place by iterating over its elements and appending elements from the corresponding sublists in b using extend(). It is efficient when working with large datasets.

a = [[4, 3, 5], [1, 2, 3], [3, 7, 4]]
b = [[1, 3], [9, 3, 5, 7], [8]]

for i in range(len(a)):
    a[i].extend(b[i])

print(a)

[[4, 3, 5, 1, 3], [1, 2, 3, 9, 3, 5, 7], [3, 7, 4, 8]]

Explanation: This code iterates through a, using extend() to append corresponding sublists from b, modifying a in place with merged sublists.

Using itertools.chain

chain() function from itertools flattens multiple iterables into a single iterable. Here, it is used inside a list comprehension to concatenate corresponding sublists.

from itertools import chain

a = [[4, 3, 5], [1, 2, 3], [3, 7, 4]]
b = [[1, 3], [9, 3, 5, 7], [8]]

a = [list(chain(x, y)) for x, y in zip(a, b)]
print(a)

[[4, 3, 5, 1, 3], [1, 2, 3, 9, 3, 5, 7], [3, 7, 4, 8]]

Explanation: This code pairs sublists using zip(), merges them with chain() and converts the result into a list, forming concatenated sublists.