How to change a certain count of numpy matrix elements?

Question

I have a square numpy 2D matrix.

And I need to set a certain count of random matrix values to 0. Let's say it is 5 elements. That means any 5 from 16 matrix values must be set to 0. For example new matrix could be

or

or some else.

How could I do this efficient way?

Do you want a randomized way of setting `x` zeros in the array or always setting first `x` values to 0 would suffice? — Raj, Dec 25 '19 at 10:07

score 5 · Accepted Answer · answered Dec 25 '19 at 10:11

This will do it:

import random
arr1d = arr.ravel()
randidx = random.sample(range(len(arr1d)), 5)
arr1d[randidx] = 0

This modifies arr because ravel() returns a view, not a copy.

For more on how the random numbers can be generated, see: Non-repetitive random number in numpy

mtpelerin · Answer 2 · 2019-12-25T11:27:09.763

-1

lets say your matrix is "matrix"

import random
for i in range(5):
    random1=random.randint(0,size_x_ofmatrix)
    random2=random.randint(0,size_y_ofmatrix)
    matrix[random1,random2]=0

edited Dec 25 '19 at 11:27

answered Dec 25 '19 at 10:15

mtpelerin

9
3

2

This does not check for repetition, so it may set less than five elements to zero. (It also has a typo or three, but that's less of a problem.) – Amadan Dec 25 '19 at 10:16
Indeed :( btw, what do you mean by typo of three? – mtpelerin Dec 25 '19 at 10:33
"or", not "of": `randin`, `random` and `random` instead of `randint`, `random1` and `random2`. – Amadan Dec 25 '19 at 10:34
Thx. I was a little speedy! – mtpelerin Dec 25 '19 at 11:28

How to change a certain count of numpy matrix elements?

2 Answers2