Numpy Array and axis

Question

I have an array A which has the shape (2, n) and second array B with the shape (n, 2) and I want to create an array C with the shape (n, 2, 2) by multiplying axis=0 of the first array A and axis=1 of the second array B to receive 10 "arrays" of the shape (2, 2) which are stored in the array C

I dont know how to do this... hope someone can help, thanks in advance!

Here some test data with n=10:

A = [array([1, 2, 3, 4, 4, 4, 4, 4, 4, 2]), array([2, 3, 2, 4, 3, 6, 8, 5, 2, 1])]
B = [array([1, 2]), array([3, 2]), array([1, 1]), array([2, 2]), array([6, 1]), array([4, 5]), array([1, 2]), array([1, 2]), array([1, 2]), array([1, 2])]

Mad Physicist · Accepted Answer · 2020-01-04 13:39:15Z

1

You can use moveaxis to change the array shapes to line up, then insert additional axes as necessary:

C = np.moveaxis(A, 1, 0)[..., None] * B[:, None, ...]

Another way would be to apply it after the multiplication, but that would run the risk of creating a non-contiguous memory layout, and is therefore generally less desirable:

C = np.moveaxis(A[..., None] * B[None, ...], 1, 0)

Similar results can be achieved with transpose and swapaxes.

answered Jan 4, 2020 at 13:39

Mad Physicist

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