[R] how ot replace the diagonal of a matrix

[roger bos]

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.

Thanks,

Roger