How to assign values to a MATLAB matrix on the diagonal?

Question

Suppose I have an NxN matrix A, an index vector V consisting of a subset of the numbers 1:N, and a value K, and I want to do this:

 for i = V
     A(i,i) = K         


        

Answer 1

Suppose K is the value. The command

A=A-diag(K-diag(A))

may be a bit faster

>> A=randn(10000,10000);

>> tic;A(logical(eye(size(A))))=12;toc

Elapsed time is 0.517575 seconds.

>> tic;A=A+diag((99-diag(A)));toc

Elapsed time is 0.353408 seconds.

But it consumes more memory.