[R] how ot replace the diagonal of a matrix

[Charles C. Berry]

On Tue, 3 Oct 2006, Duncan Murdoch wrote:

> 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 just

  	diag(mps) <- 0.6


Charles C. Berry                        (858) 534-2098
                                          Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	         UC San Diego
http://biostat.ucsd.edu/~cberry/         La Jolla, San Diego 92093-0717