Python numpy: Selecting array entries based on input array

Question

Assume I have an array:

a = np.array([1,2,3,4,5])

Now I want to find the indices of elements in this array corresponding to the values given by another array input:

input = np.array([2,4,5])

The expected result should be:

result = [1,3,4]

A boolean mask, which is true for element indices 1,3,4 would also be fine.

I do not want to use looping to solve this. I assume that a possible solution has to do with the numpy where() function, but using this one, I am only able to compare the entries of array a with one element of array input at a time. Because the length of input might differ, I cannot really use this approach. Do you have any other ideas?

Thanks in advance.

Dupe of [Getting the indices of several elements in a NumPy array at once](https://stackoverflow.com/questions/32191029/getting-the-indices-of-several-elements-in-a-numpy-array-at-once) — user3483203, Jul 17 '20 at 18:27

Ehsan · Answer 1 · 2020-07-17T18:36:39.997

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np.where(np.in1d(a, inp))[0]

or:

np.isin(a, inp).nonzero()[0]

or as suggested here:

sorter = np.argsort(a)
sorter[np.searchsorted(a, inp, sorter=sorter)]

output:

[1 3 4]

edited Jul 17 '20 at 18:36

answered Jul 17 '20 at 18:28

Ehsan

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score 0 · Answer 2 · answered Jul 17 '20 at 19:13

0

np.where(np.in1d(a, inp))[0] np.where(np.in1d(a, inp))[0]

answered Jul 17 '20 at 19:13

Shalitha Jayamal

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The answer from the other thread worked perfectly. Thank you very much, you helped a lot. – Herr_von_Bö Jul 18 '20 at 18:05
please up the answer.thank u happy learning – Shalitha Jayamal Jul 18 '20 at 18:57

Python numpy: Selecting array entries based on input array

2 Answers2