Taking from index array multiple times

Question

here is my situation. Suppose you have an array A of some elements, A is np.ndarray, and B - also np.ndarray, where B[i] is how many times I'd like to take element A[i] for the output. Example: A = np.array([1, 2, 3, 4, 5]), B = np.array([0, 0, 2, 1, 3]). The result should be np.array([3, 3, 4, 5, 5, 5]).

So how to make it in numpy fast, without for-loops etc.

Thanks.

Tried to find something appropriate in numpy doc, but no success.

Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. — Community
– Community Bot, Commented Dec 10, 2022 at 10:39

mathfux · Accepted Answer · 2022-12-09 23:28:23Z

2

Try np.repeat: np.repeat(A, B)

answered Dec 9, 2022 at 23:28

mathfux

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2 Comments

Денис Стафилов Over a year ago

what if B is matrix, and I want to make it along rows of B? np repeat won't work this time

mathfux Over a year ago

@ДенисСтафилов Can you give a minimal example? I'm not sure about an output you expect. It might be array with multiple sizes of rows which is not supported in numpy

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