Numpy doesn't change value of an array element after masking

Question

I had a weird behaviour trying to change the value of an element of a numpy array today, and I would like to understand why it didn't work. I have two arrays (a and b), and I want to change the values of b where a > 0.

a = array([[ 1.,  0.,  0.],
           [ 0.,  1.,  0.],
           [ 0.,  0.,  1.]])

b = array([[ 5.,  0.,  0.],
           [ 0.,  5.,  0.],
           [ 0.,  0.,  5.]])

mask = a > 0

print b[mask][0]
   => 5.0

b[mask][0] = 10

print b[mask][0]
   => 5.0

Could someone please explain why the assignment b[mask][0] didn't change my value 5.0?

hpaulj · Accepted Answer · 2015-08-14 05:05:49Z

2

b[mask] is a copy of b. b[mask][0] = 1 is effectively:

 c = b[mask]
 c[0] = 1

The data elements of c are not (in general) a contiguous block of the elements of b.

 b[mask] = 10
 b[mask] = [10, 11, 12]

You can assign values to b[mask] when it is the only thing on the left. You need to change all the masked elements.

If you need to change one or two, then first change the mask so it selects only those elements.

In general

 b[...][...] = ...

is not good practice. Sometimes it works (if the first indexing is a slice that produces a view), but you shouldn't count on it. It takes a while to full grasp the difference between a view and copy.

The [] get translated by the Python interpreter into calls to __getitem__ or __setitem__. The following pairs are equivalent:

 c = b[mask]
 c = b.__getitem__(mask)

 b[mask] = 10
 b.__setitem__(mask, 10)

 b[mask][0] = 10
 b.__getitem__(mask).__setitem__(0, 10)

b[mask][10] is 2 operations, a get followed by a set. The set operates on the result of the get. It modifies b only if the result of the get is a view.

edited Aug 14, 2015 at 5:05

answered Aug 14, 2015 at 3:35

hpaulj

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2 Comments

Romanzo Criminale Over a year ago

Thanks for your answer. I'm not quite sure i understand though. b[mask] = 10 would give b[mask] = [10,10,10] but b[mask][0] = 10 would be [5,5,5]. So is b[mask] a view and b[mask][0] a copy?

hpaulj Over a year ago

Does expressing the indexing in terms of calls to __getitem__ and __setitme__ help?

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