如何对2D列表中的列求和？

Question

说我有一个Python 2D列表如下:

my_list =  [ [1,2,3,4],
             [2,4,5,6] ]

我可以通过列表理解得到行总数:

row_totals = [ sum(x) for x in my_list ]

我可以在没有双for循环的情况下得到列总数吗？即,获取此列表:

[3,6,8,10]

Answer 1

使用拉链

col_totals = [ sum(x) for x in zip(*my_list) ]

Answer 2

>>> map(sum,zip(*my_list))
[3, 6, 8, 10]

或者等效的itertools

>>> from itertools import imap, izip
>>> imap(sum,izip(*my_list))
<itertools.imap object at 0x00D20370>
>>> list(_)
[3, 6, 8, 10]

Answer 3

解决方案map(sum,zip(*my_list))是最快的.但是,如果您需要保留列表,[x + y for x, y in zip(*my_list)]则是最快的.

测试在Python 3.1.2 64位中进行.

>>> import timeit
>>> my_list = [[1, 2, 3, 4], [2, 4, 5, 6]]
>>> t1 = lambda: [sum(x) for x in zip(*my_list)]
>>> timeit.timeit(t1)
2.5090877081503606
>>> t2 = lambda: map(sum,zip(*my_list))
>>> timeit.timeit(t2)
0.9024796603792709
>>> t3 = lambda: list(map(sum,zip(*my_list)))
>>> timeit.timeit(t3)
3.4918002495520284
>>> t4 = lambda: [x + y for x, y in zip(*my_list)]
>>> timeit.timeit(t4)
1.7795929868792655

Answer 4

[x + y for x, y in zip(*my_list)]