How to Transpose a List of Lists in Python?

Transpose a List of Lists

Given a list of lists, the goal is to transpose it, reversing the rows and columns. For instance, for the input:

l = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

the desired output is:

r = [[1, 4, 7], [2, 5, 8], [3, 6, 9]]

Python 3 Solutions:

• Using zip and map:

list(map(list, zip(*l)))

This solution uses zip to iterate over the columns of the list and map to convert each column to a list.

• Using zip_longest from itertools:

list(map(list, itertools.zip_longest(*l, fillvalue=None)))

This solution uses itertools.zip_longest, which works similarly to zip but includes a fillvalue parameter to fill in missing values when the lists are jagged.

Python 2 Solution:

map(list, zip(*l))

Explanation:

The zip function creates an iterator of tuples, where each tuple represents a column in the input list. By unpacking the argument list with *l, zip iterates over each column in parallel. The map function then converts each tuple to a list, producing the desired transposed list.

Note: Python 2's zip does not have the fillvalue parameter, so it will discard data if the lists are jagged.

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