Slicing list of lists in Python

Question

I need to slice a list of lists:

A = [[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5]]
idx = slice(0,4)
B = A[:][idx]

The code above isn’t giving me the right output.

What I want is: [[1,2,3],[1,2,3],[1,2,3]]

Asked By: HuckleberryFinn

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Source

Answer 1

you can use a list comprehension such as: [x[0:i] for x in A]
where i is 1,2,3 etc based on how many elements you need.

Answered By: Sid

Answer 2

Either:

>>> [a[slice(0,3)] for a in A]
[[1, 2, 3], [1, 2, 3], [1, 2, 3]]

Or:

>>> [list(filter(lambda x: x<=3, a)) for a in A]
[[1, 2, 3], [1, 2, 3], [1, 2, 3]]

Answered By: Haifeng Zhang

Answer 3

Very rarely using slice objects is easier to read than employing a list comprehension, and this is not one of those cases.

>>> A = [[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5]]
>>> [sublist[:3] for sublist in A]
[[1, 2, 3], [1, 2, 3], [1, 2, 3]]

This is very clear. For every sublist in A, give me the list of the first three elements.

Answered By: timgeb

Answer 4

With numpy it is very simple – you could just perform the slice:

In [1]: import numpy as np

In [2]: A = np.array([[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5]])

In [3]: A[:,:3]
Out[3]: 
array([[1, 2, 3],
       [1, 2, 3],
       [1, 2, 3]])

You could, of course, transform numpy.array back to the list:

In [4]: A[:,:3].tolist()
Out[4]: [[1, 2, 3], [1, 2, 3], [1, 2, 3]]

Answered By: awesoon

Answer 5

A = [[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5]]

print [a[:3] for a in A]

Using list comprehension

Answered By: Rahul Kumar

Answer 6

Here is my code:

A = [[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5]]
print(A[0][0:3],A[1][0:3],A[1][0:3])

Answered By: Technical jangra

Slicing list of lists in Python

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