Numpy: Replace random elements in an array

Question

I already googled a bit and didn't find any good answers.

The thing is, I have a 2d numpy array and I'd like to replace some of its values at random positions.

I found some answers using numpy.random.choice to create a mask for the array. Unfortunately this does not create a view on the original array so I can not replace its values.

So here is an example of what I'd like to do.

Imagine I have 2d array with float values.

[[ 1., 2., 3.],
 [ 4., 5., 6.],
 [ 7., 8., 9.]]

And then I'd like to replace an arbitrary amount of elements. It would be nice if I could tune with a parameter how many elements are going to be replaced. A possible result could look like this:

[[ 3.234, 2., 3.],
 [ 4., 5., 6.],
 [ 7., 8., 2.234]]

I couldn't think of nice way to accomplish this. Help is appreciated.

Answer 1

Just mask your input array with a random one of the same shape.

import numpy as np

# input array
x = np.array([[ 1., 2., 3.], [ 4., 5., 6.], [ 7., 8., 9.]])

# random boolean mask for which values will be changed
mask = np.random.randint(0,2,size=x.shape).astype(np.bool)

# random matrix the same shape of your data
r = np.random.rand(*x.shape)*np.max(x)

# use your mask to replace values in your input array
x[mask] = r[mask]

Produces something like this:

[[ 1.          2.          3.        ]
 [ 4.          5.          8.54749399]
 [ 7.57749917  8.          4.22590641]]