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I already know that Numpy "double-slice" with fancy indexing creates copies instead of views, and the solution seems to be to convert them to one single slice (e.g. This question). However, I am facing this particular problem where i need to deal with an integer indexing followed by boolean indexing and I am at a loss what to do. The problem (simplified) is as follows:

a = np.random.randn(2, 3, 4, 4)
idx_x = np.array([[1, 2], [1, 2], [1, 2]])
idx_y = np.array([[0, 0], [1, 1], [2, 2]])
print(a[..., idx_y, idx_x].shape) # (2, 3, 3, 2)
mask = (np.random.randn(2, 3, 3, 2) > 0)
a[..., idx_y, idx_x][mask] = 1 # assignment doesn't work

How can I make the assignment work?

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3 Answers 3

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Not sure, but an idea is to do the broadcasting manually and adding the mask respectively just like Tim suggests. idx_x and idx_y both have the same shape (3,2) which will be broadcasted to the shape (6,6) from the cartesian product (3*2)^2.

x = np.broadcast_to(idx_x.ravel(), (6,6))
y = np.broadcast_to(idx_y.ravel(), (6,6))

# this should be the same as
x,y = np.meshgrid(idx_x, idx_y)

Now reshape the mask to the broadcasted indices and use it to select

mask = mask.reshape(6,6)
a[..., x[mask], y[mask]] = 1

The assignment now works, but I am not sure if this is the exact assignment you wanted.

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7 Comments

This is what I was trying to figure out just now - I was getting myself mixed up with the dimensions! I was going to suggest a solution using np.ravel and np.tile but this is much neater. I'll leave yours as the correct answer ;)
Just tried again with a simple test case - a few notes python a = np.arange(27).reshape([3, 3, 3]); ind_x = np.array([[0, 0], [1, 2]]); ind_y = np.array([[1, 2], [1, 1]]); # x = np.broadcast_to(ind_x.ravel(), (4, 4)); # y = np.broadcast_to(ind_y.ravel(), (4, 4)).T; x, y = np.meshgrid(ind_x, ind_y); mask = a[:, ind_y, ind_x] % 2 == 0; 1. I think y from meshgrid is actually y.T 2. In this 3D example, how should we reshape mask? It shouldn't be hard but I can't get my head around it at the moment!
@TimJim you're right the y from meshgrid is transposed - but the principle remains the same. I am not sure on what the exact input and output should be, maybe OP can provide an example? The code snippet you pasted here is a bit hard to read, maybe you could edit your post with it formatted?
The assignment works but its not the same assignment. You can check with a[..., idx_y, idx_x][mask] (where mask is the original mask, not modified) and not all of them has changed to 1.
oh god i missed the apparent solution. thanks to all your efforts!
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Ok apparently I am making things complicated. No need to combine the indexing. The following code solves the problem elegantly:

b = a[..., idx_y, idx_x]
b[mask] = 1
a[..., idx_y, idx_x] = b
print(a[..., idx_y, idx_x][mask]) # all 1s
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2 Comments

D'oh! I'm sure I tried this out when wrapping my head around the re-indexing! I'm glad it works out - typical we all make our lives harder than we need to... time for bed! Nice one.
Nice. a[..., idx_x, idx_y] *= ~mask; a[..., idx_x, idx_y] += mask * 1 should also work if you want to do it arithmetically :)
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EDIT: Use @Kevin's solution which actually gets the dimensions correct!

I haven't tried it specifically on your sample code but I had a similar issue before. I think I solved it by applying the mask to the indices instead, something like:

a[..., idx_y[mask], idx_x[mask]] = 1

-that way, numpy can assign the values to the a array correctly.

EDIT2: Post some test code as comments remove formatting.

a = np.arange(27).reshape([3, 3, 3])
ind_x = np.array([[0, 0], [1, 2]])
ind_y = np.array([[1, 2], [1, 1]])
x = np.broadcast_to(ind_x.ravel(), (4, 4))
y = np.broadcast_to(ind_y.ravel(), (4, 4)).T
# x1, y2 = np.meshgrid(ind_x, ind_y)  # above should be the same as this
mask = a[:, ind_y, ind_x] % 2 == 0  # what should this reshape to?
# a[..., x[mask], y[mask]] = 1  # Then you can mask away (may also need to reshape a or the masked x or y)
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3 Comments

Apparently this does not work. mask is 4-dimensional but idx_y is only 2-dimensional.
@Jerry Sorry, I hadn't checked the dimensions - you should use Kevin's answer. The idea is to apply your second mask to the first and index with that.
I'll try to work through this example, but for now i will stick with the easy solution :) thanks

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