Split an array in all possible combinations (not regular splitting)

Question

Please read this question carefully before down voting. I could not find my problem in other questions here.

Suppose I have an array,

>>> import numpy as np
>>> array  = np.linspace(1,4,4, dtype=np.int)
>>> array
array([1, 2, 3, 4])

I want a function that will split this array in all possible parts, such that,

No split :

([1,2,3,4])

Split in 2 parts :

([1], [2,3,4])
([1,2], [3,4])
([1,2,3] ,[4])

Split in 3 parts :

([1], [2], [3,4])
([1,2]), [3], [4])
([1], [2,3], [4])

Split in len(array) parts :

([1],[2],[3],[4])

I know there is np.split(array, r), but it will not give all possible splits. e.g. np.split(array, 2) will give,

[array([0, 1]), array([2, 3])]

As you can see this is not what I need. How to achieve my need?

Asked By: ddas

||

Source

Answer 1

not familiar with numpy, but you can do it in pure python using divide and conquer(whether split on this position or not ):

def split(a):
    if not a:
        return [[]]
    elif len(a) == 1:
        return [[a]]
    else:
        result = []
        for i in range(1, len(a) + 1):
            result += [(a[:i], *sub_split) for sub_split in split(a[i:])]
        return result

split([1,2,3])
# output => [([1], [2], [3]), ([1], [2, 3]), ([1, 2], [3]), ([1, 2, 3],)]

Answered By: CtheSky

Answer 2

You could use itertools.combinations to generate the indices where to split inside a loop over the number of splits:

>>> from itertools import combinations
>>> [np.split(array, idx) 
...  for n_splits in range(5) 
...  for idx in combinations(range(1, len(array)), n_splits)]
[[array([1, 2, 3, 4])],
 [array([1]), array([2, 3, 4])],
 [array([1, 2]), array([3, 4])],
 [array([1, 2, 3]), array([4])],
 [array([1]), array([2]), array([3, 4])],
 [array([1]), array([2, 3]), array([4])],
 [array([1, 2]), array([3]), array([4])],
 [array([1]), array([2]), array([3]), array([4])]]

Answered By: MSeifert

Answer 3

Looking for a clue for the task, I came here. It’s my offer.

from more_itertools import partitions

def partitions_as_string(string):
    return [part for part in partitions(string) if len(part) > 1]

print(partitions_as_string([1,2,3,4]))

Result:

[[[1], [2, 3, 4]], [[1, 2], [3, 4]], [[1, 2, 3], [4]], [[1], [2], [3, 4]], [[1], [2, 3], [4]], [[1, 2], [3], [4]], [[1], [2], [3], [4]]]

Answered By: Szymon83

Split an array in all possible combinations (not regular splitting)

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