Transpose list of lists

问题

Let\'s take:

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

The result I\'m looking for is

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

and not

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

Much appreciated

回答1:

How about

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

For python 3.x users can use

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

回答2:

One way to do it is with the 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]]

回答3:

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]]

回答4:

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]]

回答5:

The first two methods work in Python 2 or 3, and they work on "ragged" rectangular 2D lists. That is, the inner lists don't need to have the same lengths. 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

• itertools.izip_longest() in Python 2
• itertools.zip_longest() in Python 3

The default fillvalue is None. Thanks to @jena's answer, where map() is changing the inner tuples to lists. Here it's 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 transpose and passes 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's no function to process each column. They're 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're just truncated. Boo hoo progress.

回答6:

Three options to choose from:

1. Map with Zip

solution1 = map(list, zip(*l))

2. List Comprehension

solution2 = [list(i) for i in zip(*l)]

3. For Loop Appending

solution3 = []
for i in zip(*l):
    solution3.append((list(i)))

And to view the results:

print(*solution1)
print(*solution2)
print(*solution3)

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

回答7:

Maybe not the most elegant solution, but here's a solution using nested while loops:

def transpose(lst):
    newlist = []
    i = 0
    while i < len(lst):
        j = 0
        colvec = []
        while j < len(lst):
            colvec.append(lst[j][i])
            j = j + 1
        newlist.append(colvec)
        i = i + 1
    return newlist

回答8:

import numpy as np
r = list(map(list, np.transpose(l)))

回答9:

Here is a solution for transposing a list of lists that is not necessarily square:

maxCol = len(l[0])
for row in l:
    rowLength = len(row)
    if rowLength > maxCol:
        maxCol = rowLength
lTrans = []
for colIndex in range(maxCol):
    lTrans.append([])
    for row in l:
        if colIndex < len(row):
            lTrans[colIndex].append(row[colIndex])

回答10:

    #Import functions from library
    from numpy import size, array
    #Transpose a 2D list
    def transpose_list_2d(list_in_mat):
        list_out_mat = []
        array_in_mat = array(list_in_mat)
        array_out_mat = array_in_mat.T
        nb_lines = size(array_out_mat, 0)
        for i_line_out in range(0, nb_lines):
            array_out_line = array_out_mat[i_line_out]
            list_out_line = list(array_out_line)
            list_out_mat.append(list_out_line)
        return list_out_mat

来源：https://stackoverflow.com/questions/6473679/transpose-list-of-lists