How to add one to each entry of the diagonal [Matlab]

Question

I have a matrix such as

[1     1     1
 1     1     1
 1     1     1]

I want it to be

[2     1     1
 1     2     1
 1     1     2]

How do I do that?

Answer 1

Use eye function to get an identity matrix and add to original matrix

result = A+eye(3,3) ; % A the original matrix 

Answer 2

Another possibility, which requires less operations (may be better for large matrices):

A(1:size(A,1)+1:end) = A(1:size(A,1)+1:end) + 1;

This uses the concept of linear indexing to address the diagonal elements.

Answer 3

Yet another way via logical indexing:

idx = eye(size(A))>0;
A(idx)= A(idx)+1;

This can easily be used for other things as well:

A(~idx)=2*A(~idx); %Multiply all non diagonal elements by two
A(eye(size(A))>0)=1:min(size(A)); %Set the diagonal to 1:n