如何将嵌套列表切片两次？

Question

With a nested list like:

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

I need to be able to slice this list for:

[[1, 2], [4, 5]]

I've been trying:

list(ex_list[:2][:2])

but this isn't working. I'm obviously doing something very wrong but haven't been able to find a solution as using commas doesn't work either for some reason.

Answer 1

You need to slice the elements separately to the outer list; it's better to do the outer list first to avoid unnecessary inner slices.

[inner[:2] for inner in ex_list[:2]]

Answer 2

You should try using comprehension: Try:

[i[:2] for i in ex_list[:2]]

Code:

ex_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
print([i[:2] for i in ex_list[:2]])

Output:

[[1, 2], [4, 5]]

Answer 3

Is using numpy an option?

import numpy as np

ex_list = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
print(ex_list[:2,:2].tolist()) # [[1, 2], [4, 5]]

The first :2 slice the outer list, the second slice each one of the inner lists.

Answer 4

You can try map

l = list(map(lambda lst: lst[:2], ex_list))

print(l)

[[1, 2], [4, 5], [7, 8]]

Or with itemgetter

from operator import itemgetter

l = list(map(itemgetter(slice(0,2)), ex_list))

print(l)

[[1, 2], [4, 5], [7, 8]]