[R] how ot replace the diagonal of a matrix

[Tony Plate]

You are indexing with numeric 0's and 1's, which will refer to only the 
matrix element 1,1 (multiple times), cf:

 > matrix(1:9,3)[diag(3)]
[1] 1 1 1
 >

Try one of these:

 > idx <- diag(3) > 0
 > idx <- which(diag(3)>0)
 > idx <- cbind(seq(len=n), seq(len=n))

(For very large matrices, the third will be more efficient, I believe.)

-- Tony Plate

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.
> 
> Thanks,
> 
> Roger
> 
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