Ways to filter specific value of array in numpy

Question

I haven't use numpy to his full potential yet. I have this pretty huge 3d array (700000 x 32 x 32). All of the values are int between 1 and 4. I want to be able to filter the array so that the I get a same shape array, but only with 1 for values of 4 and 0 for everything else.

For example,

[[4, 2, 2, 3],      [[1, 0, 0, 0],
 [1, 4, 4, 2],       [0, 1, 1, 0],
 [2, 4, 1, 3],   ->  [0, 1, 0, 0],
 [2, 3, 2, 4]]       [0, 0, 0, 1]]

It works with np.where(array==4). I get a huge 3d array that I can reshape, but would there more numpy way to do it ? Thanks.

Just (array==4).astype(int)?

Quang Hoang
– Quang Hoang

2020-06-10 13:31:46 +00:00
Commented Jun 10, 2020 at 13:31 — Quang Hoang
– Quang Hoang, Commented Jun 10, 2020 at 13:31
Yes simple as that.

Mathieu Châteauvert
– Mathieu Châteauvert

2020-06-10 13:41:23 +00:00
Commented Jun 10, 2020 at 13:41 — Mathieu Châteauvert
– Mathieu Châteauvert, Commented Jun 10, 2020 at 13:41

AMH · Accepted Answer · 2020-06-10 13:32:39Z

2

arr = np.array([[4, 2, 2, 3],      
                [1, 4, 4, 2],       
                [2, 4, 1, 3],  
                [2, 3, 2, 4]])

In[58]: (arr==4).astype(int)

Out[58]: array([[1, 0, 0, 0],
                [0, 1, 1, 0],
                [0, 1, 0, 0],
                [0, 0, 0, 1]])

answered Jun 10, 2020 at 13:32

AMH

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Comments

IMB · Accepted Answer · 2020-06-12 12:04:44Z

1

You can also get it with np.where without having to reshape it with the next expression:

arr = np.array([[4, 2, 2, 3],      
                [1, 4, 4, 2],       
                [2, 4, 1, 3],  
                [2, 3, 2, 4]])

np.where(arr == 4, 1, 0)

Output:

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

answered Jun 12, 2020 at 12:04

IMB

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Mathieu Châteauvert Over a year ago

This is also good and fast, but the accepted answer is like 5% faster

Mehul Gupta · Accepted Answer · 2020-06-10 13:35:43Z

0

import numpy as np
matrix = np.zeros((5,5))
matrix[0]=[1,-2,3,4,-5]
matrix[2]=[0,1,4,0,4]
matrix=np.array([[1 if value==4 else 0 for value in row ] for row in matrix])

The above snippet uses nested list comprehension & than converts list back to np array

answered Jun 10, 2020 at 13:35

Mehul Gupta

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Mathieu Châteauvert Over a year ago

Yes this gets the job done well for a 2d or 3d matrix, but the use of for loops take a long time to compute. My array is pretty big. The other answer is better in my case.

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