CODEWITHSUNDEEP
pythonarraysnumpymask
Dschoni
Dschoni

Reputation: 3862

Difference in numpy array masking

I have three numpy arrays, one the source array, one destination array and one mask array. I want to replace the values in the destination with the same values from the source only at the places where the mask is equal to one.

My naiive try was:

import numpy as np
destination = np.arange(9).reshape((3,3))
source = np.ones((3,3))
mask = np.zeros((3,3)).astype(np.uint8)
mask[1,1]=1

destination[mask] = source[mask]

which leads me to destination being

[[1, 1, 1],
 [1, 1, 1],
 [6, 7, 8]]

whereas I expect it to be

[[0, 1, 2],
 [3, 1, 5],
 [6, 7, 8]].

I do get the correct result, when I do

destination[mask==1] = source[mask==1].

My question is: Why are these two commands not identical, or what does the first even do?

Upvotes: 0

Views: 1256

Answers (1)

Alperen Tahta
Alperen Tahta

Reputation: 476

First you must check inside the matrices and which matrix gives you what you want.

mask

Output

[[0, 0, 0],
 [0, 1, 0],
 [0, 0, 0]]

but destination[mask == 1] gives you a boolean matrix

mask == 1

Output

[[False, False, False],
 [False,  True, False],
 [False, False, False]]

whereas:

destination[mask]

Output

[[[0, 1, 2],
  [0, 1, 2],
  [0, 1, 2]]

[[0, 1, 2],
 [3, 4, 5],
 [0, 1, 2]],

[[0, 1, 2],
 [0, 1, 2],
 [0, 1, 2]]]

but using destination[mask == 1] gives you a single value which is 4. It's the same for the source[mask == 1] which gives you the single value 1.

and if you use destination[mask==1] = source[mask==1] instead of destination[mask] = source[mask] you will only change the value 4 in the destination matrix.

I hope my explanation is clear.

Edit:

I hope I understand your question correct:

The simple integer indexing structure x[[i]] gives you the i'th row of the matrix.

So destination[0,1,2] gives:

[[0, 1, 2],
 [3, 4, 5],
 [6, 7, 8]]

and for an understandable example the input destination[1,2,0] leads to

[[3, 4, 5],
 [6, 7, 8],
 [0, 1, 2]]

Upvotes: 1

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