python - Transpose list of lists

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Top 5 Answer for python - Transpose list of lists

vote vote

92

Python 3:

# short circuits at shortest nested list if table is jagged: list(map(list, zip(*l)))  # discards no data if jagged and fills short nested lists with None list(map(list, itertools.zip_longest(*l, fillvalue=None))) 

Python 2:

map(list, zip(*l)) 
[[1, 4, 7], [2, 5, 8], [3, 6, 9]] 

Explanation:

There are two things we need to know to understand what's going on:

  1. The signature of zip: zip(*iterables) This means zip expects an arbitrary number of arguments each of which must be iterable. E.g. zip([1, 2], [3, 4], [5, 6]).
  2. Unpacked argument lists: Given a sequence of arguments args, f(*args) will call f such that each element in args is a separate positional argument of f.
  3. itertools.zip_longest does not discard any data if the number of elements of the nested lists are not the same (homogenous), and instead fills in the shorter nested lists then zips them up.

Coming back to the input from the question l = [[1, 2, 3], [4, 5, 6], [7, 8, 9]], zip(*l) would be equivalent to zip([1, 2, 3], [4, 5, 6], [7, 8, 9]). The rest is just making sure the result is a list of lists instead of a list of tuples.

vote vote

83

One way to do it is with NumPy transpose. For a list, a:

>>> import numpy as np >>> np.array(a).T.tolist() [[1, 4, 7], [2, 5, 8], [3, 6, 9]] 

Or another one without zip:

>>> map(list,map(None,*a)) [[1, 4, 7], [2, 5, 8], [3, 6, 9]] 
vote vote

77

Equivalently to Jena's solution:

>>> l=[[1,2,3],[4,5,6],[7,8,9]] >>> [list(i) for i in zip(*l)] ... [[1, 4, 7], [2, 5, 8], [3, 6, 9]] 
vote vote

62

just for fun, valid rectangles and assuming that m[0] exists

>>> m = [[1,2,3],[4,5,6],[7,8,9]] >>> [[row[i] for row in m] for i in range(len(m[0]))] [[1, 4, 7], [2, 5, 8], [3, 6, 9]] 
vote vote

60

Methods 1 and 2 work in Python 2 or 3, and they work on ragged, rectangular 2D lists. That means the inner lists do not need to have the same lengths as each other (ragged) or as the outer lists (rectangular). The other methods, well, it's complicated.

the setup

import itertools import six  list_list = [[1,2,3], [4,5,6, 6.1, 6.2, 6.3], [7,8,9]] 

method 1 — map(), zip_longest()

>>> list(map(list, six.moves.zip_longest(*list_list, fillvalue='-'))) [[1, 4, 7], [2, 5, 8], [3, 6, 9], ['-', 6.1, '-'], ['-', 6.2, '-'], ['-', 6.3, '-']] 

six.moves.zip_longest() becomes

The default fillvalue is None. Thanks to @jena's answer, where map() is changing the inner tuples to lists. Here it is turning iterators into lists. Thanks to @Oregano's and @badp's comments.

In Python 3, pass the result through list() to get the same 2D list as method 2.


method 2 — list comprehension, zip_longest()

>>> [list(row) for row in six.moves.zip_longest(*list_list, fillvalue='-')] [[1, 4, 7], [2, 5, 8], [3, 6, 9], ['-', 6.1, '-'], ['-', 6.2, '-'], ['-', 6.3, '-']] 

The @inspectorG4dget alternative.


method 3 — map() of map()broken in Python 3.6

>>> map(list, map(None, *list_list)) [[1, 4, 7], [2, 5, 8], [3, 6, 9], [None, 6.1, None], [None, 6.2, None], [None, 6.3, None]] 

This extraordinarily compact @SiggyF second alternative works with ragged 2D lists, unlike his first code which uses numpy to transpose and pass through ragged lists. But None has to be the fill value. (No, the None passed to the inner map() is not the fill value. It means there is no function to process each column. The columns are just passed through to the outer map() which converts them from tuples to lists.)

Somewhere in Python 3, map() stopped putting up with all this abuse: the first parameter cannot be None, and ragged iterators are just truncated to the shortest. The other methods still work because this only applies to the inner map().


method 4 — map() of map() revisited

>>> list(map(list, map(lambda *args: args, *list_list))) [[1, 4, 7], [2, 5, 8], [3, 6, 9]]   // Python 2.7 [[1, 4, 7], [2, 5, 8], [3, 6, 9], [None, 6.1, None], [None, 6.2, None], [None, 6.3, None]] // 3.6+ 

Alas the ragged rows do NOT become ragged columns in Python 3, they are just truncated. Boo hoo progress.

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