Retrieve index of random item in numpy array with a specific value

Question

I have a 2d numpy array with a small range of different values. I want to retrieve a random index, but with a specific value. For example say this is the array:

arr = [[1,2,1,1,3],
       [4,3,4,1,1],
       [2,3,1,2,1],
       [1,2,1,1,2],
       [3,1,3,4,2]]

I want a random index of any value which is a 1, for example (2,2). The only way I have been able to do this so far is to take any random index, and then check afterwards if it contains the value I want and then redraw if it isn't. This would be ok for the above example, but I will need to retrieve many random indices from a large array.

Is there a method or algorithm for the procedure I am describing?

The application of this is to provide random pixels of a classified image to perform an accuracy assessment.

np.argwhere(arr==1)?

Divakar
– Divakar

2017-01-12 21:33:19 +00:00
Commented Jan 12, 2017 at 21:33 — Divakar
– Divakar, Commented Jan 12, 2017 at 21:33

MSeifert · Accepted Answer · 2017-01-12 21:36:32Z

6

You convert your list to a numpy.array, then randomly pick one coordinate from a np.argwhere result:

import numpy as np
import random
random.choice(np.argwhere(np.array(arr)==value))

answered Jan 12, 2017 at 21:36

MSeifert

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