Assign to Logical Indexed Numpy Array

Question

So, I know that you can do this by doing

>>> arr[mask] = value

However, if I want to make the code shorter (and not recompute the mask and index each time), I'd like to do something like this:

>>> sub = arr[mask]
>>> sub[...] = value # This works in other cases, but not this one.

My understanding is that doing Ellipses indexing should allow you to specify that you're not reassigning a given variable, but are rather broadcasting to the actual array.

So, here's the question: why doesn't it work?

My thinking is that it's related to the fact that:

>>> arr[mask] is arr[mask]
False

But surely since the mask indexed versions are just views (not copies of the underlying structure), this shouldn't break assignment.

Maybe...because sub is a value, it can be change to everything. So sub = arr[mask] will change sub to arr[mask](now, sub is a string). But sub = value will change sub to value(sub also is a string), it wouldn't change arr[mask] to value? — Remi Guan
– Remi Guan, Commented Oct 3, 2015 at 10:59
Seems very close if not a duplicate - stackoverflow.com/q/32739828/3293881 — Divakar
– Divakar, Commented Oct 3, 2015 at 11:13
@KevinGuan If you read my question properly, I didn't write sub = arr[mask], I wrote sub[...] = arr[mask] which forces the modification of sub. — cyphar
– cyphar, Commented Oct 3, 2015 at 12:50
@cyphar Oops, I didn't pay attention on it. Sorry about that :P — Remi Guan
– Remi Guan, Commented Oct 3, 2015 at 12:52

das-g · Accepted Answer · 2015-10-03 11:22:35Z

2

But surely since the mask indexed versions are just views (not copies of the underlying structure), this shouldn't break assignment.

The reason why this doesn't work is that indexing with masks will create a copy, not a view:

Advanced indexing always returns a copy of the data (contrast with basic slicing that returns a view).

answered Oct 3, 2015 at 11:22

das-g

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hpaulj · Accepted Answer · 2015-10-03 11:17:16Z

2

arr[mask] is a copy. arr[mask]=... looks the same, but actually is a different assignment operation. Elsewhere I've explained this in terms of calls to __getitem__ and __setitem__.

answered Oct 3, 2015 at 11:17

hpaulj

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