[R] how ot replace the diagonal of a matrix

[Duncan Murdoch]

On 10/3/2006 4:59 PM, roger bos wrote:
> Dear useRs,
> 
> Trying to replace the diagonal of a matrix is not working for me.  I
> want a matrix with .6 on the diag and .4 elsewhere.  The following
> code looks like it should work--when I lookk at mps and idx they look
> how I want them too--but it only replaces the first element, not each
> element on the diagonal.
> 
> mps <- matrix(rep(.4, 3*3), nrow=n, byrow=TRUE)
> idx <- diag(3)
> mps
> idx
> mps[idx] <- rep(.6,3)
> 
> I also tried something along the lines of diag(mps=.6, ...) but it
> didn't know what mps was.

Matrix indexing can use a two column matrix, giving row and column 
numbers.  So you could get what you want by

mps[cbind(1:n,1:n)] <- 0.6