Use numpy array / list as index into a string

Question

I have a string say s='abcd...z', and a multi-dimensional numpy array - for example arr = [[2, 5, 11], [1, 3, 9]]. How can I use the array as an index into the string? The output I want is str_arr = [['b', 'e', 'k'],['a', 'c', 'i']].

I can do this using list comprehensions or for loops, but I am not sure if those are the fastest methods (especially if the array is large). Any vectorized methods? Also looking for something which works not just got 2d arrays but any dimension.

Code Pope · Accepted Answer · 2020-04-29 07:10:10Z

3

You can vectorize it to by creating a mapping array:

import string
import numpy as np
map = np.array(list(string.ascii_lowercase))
arr = np.array([[2, 5, 11], [1, 3, 9]])
map[arr-1]

Result:

array([['b', 'e', 'k'],
       ['a', 'c', 'i']], dtype='<U1')

answered Apr 29, 2020 at 7:10

Code Pope

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Bernardo Costa · Accepted Answer · 2020-04-29 07:15:17Z

0

Just for other's information, using for loops, in a 2D array:

[[s[i] for i in arr[j]] for j in range(len(arr))]

edited Apr 29, 2020 at 7:15

answered Apr 29, 2020 at 7:06

Bernardo Costa

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