How to duplicate a specific value in a list/array?

Question

Any advice on how to repeat a certain value in an array in Python? For instance, I want to repeat only 2 in array_a:

array_a = [1, 2, 1, 2, 1, 1, 2]

Wanted outcome is: I repeat each 2 and leave the 1:

array_a = [1, 2, 2, 1, 2, 2, 1, 1, 2, 2]  # only the `2` should be repeated

I tried numpy and I could duplicate the entire array but not a certain value.

Answer 1

If you're interested in a numpy solution, you can repeat an array on itself using np.repeat.

>>> import numpy as np
>>> np.repeat(array_a, array_a)
array([1, 2, 2, 1, 2, 2, 1, 1, 2, 2])

This works only if you haves 1s and 2s in your data. For a generic solution, consider

>>> n_repeats = 2
>>> temp = np.where(np.array(array_a) == 2, n_repeats, 1)
>>> np.repeat(array_a, temp)
array([1, 2, 2, 1, 2, 2, 1, 1, 2, 2])