Choose random elements from specific elements of an array

Question

I have a 1D (numpy) array with boolean values. for example:

x = [True, True, False, False, False, True, False, True, True, True, False, True, True, False]

The array contains 8 True values. I would like to keep, for example, exactly 3 (must be less than 8 in this case) as True values randomly from the 8 that exist. In other words I would like to randomly set 5 of those 8 True values as False.

A possible result can be:

x = [True, True, False, False, False, False, False, False, False, False, False, False, True, False]

How to implement it?

What have you done to try and solve the problem yourself so far? Where is the difficulty? Could you show us your code that you've tried to implement this with? — Derek, Aug 17 '17 at 11:01
What exactly should be random? The number of elements (in your case 3) or the positions in the new array? Or which elements to pick from your array `x`? — MSeifert, Aug 17 '17 at 11:03

Divakar · Accepted Answer · 2017-08-17T11:05:59.873

5

One approach would be -

# Get the indices of True values
idx = np.flatnonzero(x)

# Get unique indices of length 3 less than the number of indices and 
# set those in x as False
x[np.random.choice(idx, len(idx)-3, replace=0)] = 0

Sample run -

# Input array
In [79]: x
Out[79]: 
array([ True,  True, False, False, False,  True, False,  True,  True,
        True, False,  True,  True, False], dtype=bool)

# Get indices
In [80]: idx = np.flatnonzero(x)

# Set 3 minus number of True indices as False
In [81]: x[np.random.choice(idx, len(idx)-3, replace=0)] = 0

# Verify output to have exactly three True values
In [82]: x
Out[82]: 
array([ True, False, False, False, False, False, False,  True, False,
       False, False,  True, False, False], dtype=bool)

edited Aug 17 '17 at 11:05

answered Aug 17 '17 at 11:03

Divakar

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@Divakar You understood my perfectly, Thanks! – pyigal Aug 17 '17 at 11:10
Yes, that makes sense. Given that it's accepted I must have misunderstood the question. Removing the comments now :) – MSeifert Aug 17 '17 at 11:28

score 0 · Answer 2 · answered Aug 17 '17 at 11:04

0

Build an array with the number of desired True and False, then just shuffle it

import random
def buildRandomArray(size, numberOfTrues):
    res = [False]*(size-numberOfTrues) + [True]*numberOfTrues
    random.shuffle(res)
    return res

Live example

answered Aug 17 '17 at 11:04

Netwave

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Choose random elements from specific elements of an array

2 Answers2