Iterating through all subsets of a 3d array

Question

Assuming I have created an array by using the code below:

import numpy as np 
array = np.random.randint(0, 255, (3,4,4))

>>> array
array([[[ 72,  11, 158, 252],
        [160,  50, 131, 174],
        [245, 127,  99,   6],
        [152,  25,  58,  96]],

       [[ 29,  37, 211, 215],
        [195,  72, 186,  33],
        [ 12,  68,  44, 241],
        [ 95, 184, 188, 176]],

       [[238,  90, 177,  15],
        [ 48, 221,  41, 236],
        [ 86,  14, 130, 192],
        [ 64,  17,  44, 251]]])

Which created a 3d array of 8×8 matrices I would like to go through all possible adjacent 2×2 sub-matrices. For example, I would like to get the following matrices from the first matrix:

[[72,11],
 [160, 50]]
[[11,158],
 [50, 131]]
[[160,50],
 [245,127]]
etc...

Is there a built-in numpypytorch method I can use or do I have to implement this iteration?

Asked By: Tamir

||

Source

Answer 1

Use sliding_window_view to simply generate the entire matrix:

>>> np.lib.stride_tricks.sliding_window_view(array, (1, 2, 2)).reshape(-1, 2, 2)
array([[[ 72,  11],
        [160,  50]],

       [[ 11, 158],
        [ 50, 131]],

       [[158, 252],
        [131, 174]],

       ...

       [[ 14, 130],
        [ 17,  44]],

       [[130, 192],
        [ 44, 251]]])

Note that the data will be copied in the last step of reshaping, and the memory consumption of the results generated by large arrays may be unacceptable. ~~If you just want to iterate over each sub array, you can reshape the original array to generate a view that is easier to iterate:~~

>>> from itertools import chain >>> view = np.lib.stride_tricks.sliding_window_view( ... array.reshape(-1, array.shape[-1]), (2, 2) ... ) >>> for ar in chain.from_iterable(view): ... print(ar) ... [[ 72 11] [160 50]] [[ 11 158] [ 50 131]] [[158 252] [131 174]] ... [[ 14 130] [ 17 44]] [[130 192] [ 44 251]]

_{The above method has defects, which will cause the data between two 2d sub arrays to be treated as sub views (such as [[152, 25], [29, 37]]). The possible choice is not to use reshape and iterate through multiple loops.}

Answered By: Mechanic Pig

Iterating through all subsets of a 3d array

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