Repeating numpy array elements according to another index array

Question

I have the following placeholder array:

placeholder = np.zeros(10)

A contains the values that I want to go in to the placeholder

A = np.array([25, 40, 65,50])

idx has the indices of the placeholder ( note that this has the same shape as A)

idx = np.array([0, 5, 6, 8])

Question:

I want placeholder to be populated with the elements of A. The amount to which an element in A to be repeated is defined by the length of the intervals of the idx array.

For example, 25 is repeated 5 times because the corresponding index range is [0, 5). 65 is repeated twice because the corresponding index range is [6,8)

Expected output:

np.array([25, 25, 25, 25, 25, 40, 65, 65, 50, 50])

cs95 · Accepted Answer · 2018-02-19 08:47:08Z

3

Quick and easy, using np.diff + np.repeat:

repeats = np.diff(np.append(idx, len(placeholder)))

A.repeat(repeats)
array([25, 25, 25, 25, 25, 40, 65, 65, 50, 50])

answered Feb 19, 2018 at 8:47

cs95

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HYRY · Accepted Answer · 2018-02-19 09:02:28Z

3

If you want to use placeholder array:

import numpy as np

placeholder = np.zeros(10)
A = np.array([25, 40, 65,50])
idx = np.array([0, 5, 6, 8])

placeholder[idx[0]] = A[0]
placeholder[idx[1:]] = np.diff(A)    
np.cumsum(placeholder, out=placeholder)

answered Feb 19, 2018 at 9:02

HYRY

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1 Comment

akilat90 Over a year ago

This solution is out of this world! Unfortunately, I had to accept the other one because of it's readability.

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