Why does an assignment for double-sliced numpy arrays not work?

Question

Hello everybody,

why do the following lines not work as I expect?

import numpy as np
a = np.array([0,1,2,1,1])
a[a==1][1:] = 3
print a
>>> [0 1 2 1 1]
# I would expect [0 1 2 3 3]

Is this a 'bug' or is there another recommended way to this?

On the other hand, the following works:

a[a==1] = 3
print a
>>> [0 3 2 3 3]

Cheers, Philipp

Answer 1

Because the a[a==1] part isn't actually a slice. It creates a new array. It makes sense when you think about it-- you're only taking the elements that satisfy the boolean condition (like a filter operation).

Answer 2

It appears you simply can't do an assignment through a double-slice like that.

This works though:

a[numpy.where(a==1)[0][1:]] = 3

Answer 3

This does what you want

a[2:][a[2:]==1]=3

Answer 4

It's related to how fancy indexing works. There is a thorough explanation here. It is done this way to allow inplace modification with fancy indexing (ie a[x>3] *= 2). A consequence of this is that you can't assign to a double index as you have found. Fancy indexing always returns a copy rather than a view.