Substitute elements of a matrix at specific coordinates in matlab

Question

I am struggling to write code to substitute particular values in a matrix.

I have a matrix, C, that contains multiple [r,c,v] values. I now want to use these co-ordinates to substitute values from matrix X with those from matrix Y- but only at the coordinates in matrix c. At the coordinates not in matrix c I would like to retain the values from matrix X.

As a simple example of what I am trying to do:

o1= [ 123 123 123; 123 255 123; 255 123 123];
o2= [ 4 4 4; 4 4 4; 4 4 4];
d1= [111 111 111; 111 255 111; 111 111 111];
d2= [5 5 5; 5 5 5; 5 5 5];

o_p= o1;
d_p= d1;

need to find coordinates in o_p and d_p where the values equal 255

[r,c,v] = find(o_p==255);
a= [r, c, v];

[r,c,v] = find(d_p==255);
b= [r, c, v];

c= [a; b];

Then I want to use the coordinates in c to replace the elements at these coordinates in both matrices with o2 and d2 respectively

o1= [ 123 123 123; 123 255 123; 255 123 123]; o2= [ 4 4 4; 4 4 4; 4 4 4]; d1= [111 111 111; 111 255 111; 111 111 111]; d2= [5 5 5; 5 5 5; 5 5 5]; — user3541731
o1= [ 123 123 123; 123 255 123; 255 123 123]; o2= [ 4 4 4; 4 4 4; 4 4 4]; d1= [111 111 111; 111 255 111; 111 111 111]; d2= [5 5 5; 5 5 5; 5 5 5]; % 2 new matrices % values initially set at those from longest exposure time (pixel dwell) o_p= o1; d_p= d1; % need to find coordinates in o_p (o1) and d_p (d1) where the values equal 255 [r,c,v] = find(o_p==255); a= [r, c, v]; [r,c,v] = find(d_p==255); b= [r, c, v]; c= [a; b]; — user3541731
Why not edit your question altogether? It's easier for you and us. — Divakar

excaza excaza · Accepted Answer · 2014-04-17T16:42:35

The question isn't entirely clear, but I'm going to make some assumptions and hopefully the solution is close to what you're looking for.

% Example

x = rand(12,12);
y = ones(12,12);
% Assume c = [x,y]
c = [1 1; 2 2; 3 3; 4 4];  % Pick a few arbitrary points on the diagonal
x(sub2ind(size(x),c(:,1),c(:,2))) = y(sub2ind(size(x),c(:,1),c(:,2)))

In order to do this in one line you can take advantage of how MATLAB stores values in memory. Because MATLAB technically stores matrix in a single column, you can access any value in the matrix with a 1D index instead of nD coordinates. sub2ind allows you to obtain these indices based on the input coordinates and size of the matrix, which you can then use to swap values between matrices.

Substitute elements of a matrix at specific coordinates in matlab

2 Answers