I want to fix the zeros values in their same indices in certain two matrices A and B (to not be updated) in a cost function?

I want to build a cost function, I want to fix the zeros values in their same indices in certain two matrices A and B (to not be updated) .

for example this is matrix A:

A = [ 0.1 0.2 0

0.2 0.8 0

0 0 0.7 ]

and I want to update this matrix by some computions, but I want to keep the zero values in their same indices and not update them.

Is there any sugestion to do that?

and in general is there a method to fix values in their same indices ( I mean do the calculation on the all values in this matrix but dont change these values; in my case the zero values)

Answer 1

[Voss]

A = [0.1 0.2 0; 0.2 0.8 0; 0 0 0.7] 

A = 3×3

0.1000 0.2000 0 0.2000 0.8000 0 0 0 0.7000

newA = [1 2 3; 4 5 6; 7 8 9]

newA = 3×3

1 2 3 4 5 6 7 8 9

idx = A == 0;
A(~idx) = newA(~idx)

A = 3×3

1 2 0 4 5 0 0 0 9