在二维列表中交换两列

Question

what would be the quickest way to get to :

[[0, 2, 0, 0],\
[0, 2, 0, 0],\
[0, 2, 0, 0],\
[0, 2, 0, 0]]

from:

[[0, 0, 2, 0],\
[0, 0, 2, 0],\
[0, 0, 2, 0],\
[0, 0, 2, 0]]

without using numpy or any other external library

Answer 1

Let's say we have a list l , which is [0, 0, 2, 0] , and we want to shift all the elements one place to left.

Firstly, we need to get all the elements to the right except the first one. List slicing l[1:] would work here, which would get [0, 2, 0] .

Secondly, we need to get the remaining elements on the left with l[1:] , which would get [0] .

You can now probably see that we can shift the elements one place to the left with adding the above 2 lists together:

>>> lst = [0, 0, 2, 0]
>>> first = lst[1:]
>>> second = lst[:1]
>>> first + second
[0, 2, 0, 0]

Which can be summarized in this function:

def shift(lst, n):
    return lst[n:] + lst[:n]

Since this can shift one lists position, it can applied to all lists in a nested list and shift their positions to left by 1:

nested_lst = [shift(sublist, 1) for sublist in nested_lst]

Answer 2

For your specific task:

l = [[0, 2, 0, 0], [0, 2, 0, 0], [0, 2, 0, 0], [0, 2, 0, 0]]

for arr in l:
    arr[1], arr[2] = arr[2], arr[1]