pyigal - 2 years ago 191

Python 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 choose, for example, 3 (must be less than 8 in this case) random elements out of the 8 that exist and get as an output an array with the same length as x with Trues at 3 random places.

A possible result can be:

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

How to implement it?

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Answer Source

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)
```

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