[R] how ot replace the diagonal of a matrix

[Peter Dalgaard]

Duncan Murdoch <murdoch at stats.uwo.ca> writes:

> 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

Or even diag(mps) <- 0.6  

(this *is* displayed quite prominently on help(diag)!) 

-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
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