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]]
6 Answers
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.
Comments
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]]
Comments
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
Comments
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.
4 Comments
Sid
Why? he needs the first idx elements from each list.
Sid
@WayneWerner correct but I changed the meaning of idx...instead of the slice it's a raw int
Wayne Werner
the OP is defining
idx = slice(0,4), not 4.Sid
fixed to change the name of idx to i
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]]
1 Comment
Wayne Werner
While that does solve the OPs problem, I doubt it was what they were going for.
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])
1 Comment
greybeard
Welcome to StackOverflow! While this prints what HuckleberryFinn wrote he wanted, he wanted it without mentioning the indices of A, using a slice object for the 2nd dimension. There's an error in your third argument to
print().
slice(0,4)give you three elements per sublist if it did happen to work?