Slice array and give particular name to every small array by using loop in Python

Question

I want to slice a big array into small ones and give different names to every small array.

I used 2 for loop to slice big 10 * 10 array into 25 2 * 2 arrays, I can print them out in every loop, but I failed to give every array particular name. I also tried combine '.format()' function into the loop but still failed.

Code below worked fine and print out sliced 2 * 2 matrix that I want:

import numpy as np

a = np.arange(100).reshape(10, 10)

for p in range(0,a.shape[0],2):
    for q in range(0,a.shape[1],2):
        print(a[p:p+2,q:q+2])

But code below failed when I tried to generate particular name for every array generated:

import numpy as np

a = np.arange(100).reshape(10, 10)

for p in range(0,a.shape[0],2):
    for q in range(0,a.shape[1],2):
        'slice_{0}_{1}'.format(p,q) = a[p:p+2,q:q+2]

I expect to get arrays with format below:

slice_0_0 = array([[ 0, 1],[10,11]])
slice_0_2 = array([[ 2, 3],[12,13]])
...
slice_8_8 = array([[88,89],[98,99]])

So that I can apply these arrays with names of array instead of load the original big array directly.

I was mentioned that this question could be a duplicate of this one, however I really didn't get what "variable variables" means, though we were both recommended using dictionary.

Possible duplicate of How do I create a variable number of variables? — Faboor
– Faboor, Commented May 14, 2019 at 8:47
Hi @Chunpin, you can either use a dictionary, or a 2D list to simplify this problem. Check my answer below :) — Devesh Kumar Singh
– Devesh Kumar Singh, Commented May 14, 2019 at 11:02

marc_s · Accepted Answer · 2019-11-05 22:13:51Z

6

The best way to do this is to create a dictionary :

d={}

for p in range(0,a.shape[0],2):
    for q in range(0,a.shape[1],2):
        d['slice_{0}_{1}'.format(p,q)] = a[p:p+2,q:q+2]

In [7] : d['slice_0_0']
Out[7] : array([[ 0,  1], [10, 11]])

edited Nov 5, 2019 at 22:13

marc_s

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answered May 14, 2019 at 8:48

vlemaistre

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Devesh Kumar Singh · Accepted Answer · 2019-05-14 09:06:49Z

Why do you want to create so many variables, when you can simply create a two-dimensional list of 9x9 and use it's indexes in order to represent the variable names

import numpy as np

a = np.arange(100).reshape(10, 10)

#2D list of 9x9
slice = [ [0 for _ in range(9)] for _ in range(9)]
for p in range(0,a.shape[0],2):
    for q in range(0,a.shape[1],2):
        #Assign elements to the 2D list
        slice[p][q] = a[p:p+2,q:q+2]

print(slice)

You can then access any variable using the same indexes you were using in your variable names

#slice_0_0
print(slice[0][0])
#slice_0_2
print(slice[0][2])
#slice_8_8
print(slice[8][8])

The output will be

[[ 0  1]
 [10 11]]
[[ 2  3]
 [12 13]]
[[88 89]
 [98 99]]

Or another way to solve this problem is to use a dictionary with key as slice variable names

import numpy as np

a = np.arange(100).reshape(10, 10)

#Dictionary for slice variables
slice = {}

for p in range(0,a.shape[0],2):
    for q in range(0,a.shape[1],2):
        #Assign values to appropriate keys
        key = 'slice_{0}_{1}'.format(p,q)
        slice[key] = a[p:p+2,q:q+2]

print(slice)

The output will be

{'slice_0_0': array([[ 0,  1],
       [10, 11]]), 'slice_0_2': array([[ 2,  3],
       [12, 13]]), 'slice_0_4': array([[ 4,  5],
       [14, 15]]), 'slice_0_6': array([[ 6,  7],
       [16, 17]]), 'slice_0_8': array([[ 8,  9],
       [18, 19]]), 'slice_2_0': array([[20, 21],
       [30, 31]]), 'slice_2_2': array([[22, 23],
       [32, 33]]), 'slice_2_4': array([[24, 25],
       [34, 35]]), 'slice_2_6': array([[26, 27],
       [36, 37]]), 'slice_2_8': array([[28, 29],
       [38, 39]]), 'slice_4_0': array([[40, 41],
       [50, 51]]), 'slice_4_2': array([[42, 43],
       [52, 53]]), 'slice_4_4': array([[44, 45],
       [54, 55]]), 'slice_4_6': array([[46, 47],
       [56, 57]]), 'slice_4_8': array([[48, 49],
       [58, 59]]), 'slice_6_0': array([[60, 61],
       [70, 71]]), 'slice_6_2': array([[62, 63],
       [72, 73]]), 'slice_6_4': array([[64, 65],
       [74, 75]]), 'slice_6_6': array([[66, 67],
       [76, 77]]), 'slice_6_8': array([[68, 69],
       [78, 79]]), 'slice_8_0': array([[80, 81],
       [90, 91]]), 'slice_8_2': array([[82, 83],
       [92, 93]]), 'slice_8_4': array([[84, 85],
       [94, 95]]), 'slice_8_6': array([[86, 87],
       [96, 97]]), 'slice_8_8': array([[88, 89],
       [98, 99]])}

Faboor · Accepted Answer · 2019-05-14 08:46:07Z

0

Take a look at the many possible answers here, but I think that is generally a bad idea to do. You might be better of just doing slice[p][q].

answered May 14, 2019 at 8:46

Faboor

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1 Comment

Chunpin Over a year ago

So it's better to create a dictionary, 2D list, or just keep the original array and get small array I want with command like a[p:p+2,q:q+2] ? I am new to coding and can't actually figure out the advantages of these methods.

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