CODEWITHSUNDEEP
pythonlist
user3088454
user3088454

Reputation:

python select range from list of list

Is there a pythonic (please no numpy!) way to select a subset list-of-lists from a list-of-lists? In MATLAB, the behavior I'm trying to mimic can be exemplified with something like A(2:7, 5:8).

This isn't a homework problem, just looking for a clean solution.

Here's a concrete example:

A = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
select(A, (1, 3), (0, 3))

should yield

[[2, 3], [5, 6], [8, 9]]

Upvotes: 1

Views: 2368

Answers (2)

Martijn Pieters
Martijn Pieters

Reputation: 1121406

You can pass arguments to slices:

def select(lst, subselect, select):
    return [sublst[slice(*subselect)] for sublst in lst[slice(*select)]]

Demo:

>>> def select(lst, subselect, select):
...     return [sublst[slice(*subselect)] for sublst in lst[slice(*select)]]
... 
>>> A = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
>>> select(A, (1, 3), (0, 3))
[[2, 3], [5, 6], [8, 9]]

You can even use None to indicate default values for slicing (so 0 for start and the length of the input list for the stop argument) or pass in just one argument to specify a stop argument:

>>> select(A, (1, None), (3,))
[[2, 3], [5, 6], [8, 9]]

and a third argument gives you the option to specify a stride. If the stride is negative, the list is reversed:

>>> select(A, (1, None), (None, 3, 2))
[[2, 3], [8, 9]]
>>> select(A, (1, None), (None, None, -1))
[[8, 9], [5, 6], [2, 3]]

Upvotes: 4

gefei
gefei

Reputation: 19766

select(A, (1, 3), (0, 3)) can be implemented as [x[1:3] for x in A[0:3]]

>>> A = [1, 2, 3],[4, 5, 6],[7, 8, 9]
>>> [x[1:3] for x in A[0:3]]
[[2, 3], [5, 6], [8, 9]]
>>>

Upvotes: 0

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