Rebuild a matrix from indices

Hi everyone! After I extracted the indices from a matrix, I´d like to rebuild it and store other data in the places marked by the indices. For example, if I had

A = [0 2 2 0 0 ; 2 0 0 2 2 ; 0 2 0 2 2 ; 0 0 0 2 2 ; 2 2 0 2 2];
[i,j] = find(A==2);

I get the indices where the 2's are in the matrix (i for rows and j for columns). What I'm trying to do is change the value of the 2's and store data in that positions. That data has the same indices i and j (they are coordinates UTM). I'm working on a big matrix, around 5000x5000. Thank you.

Answer 1

[Fabio Freschi]

Try linear indexing

% the data
A = [0 2 2 0 0 ; 2 0 0 2 2 ; 0 2 0 2 2 ; 0 0 0 2 2 ; 2 2 0 2 2];
[i,j] = find(A==2);

% new value
newVal = 3;
% replace
A(sub2ind(size(A),i,j)) = newVal;

[Alyssa Webb]

Thank you so much!

[Guillaume]

Well, if you're going to use linear indexing, get a linear index directly from find:

idx = find(A == 2);
A(idx) = newval;

But, even simpler, don't bother with find and use logical indexing:

A(A == 2) = newval;

[Fabio Freschi]

yes, this is definitely cleaner!