[R] Matrix Indexing

Gavin Simpson gavin.simpson at ucl.ac.uk
Sat Apr 12 12:47:37 CEST 2008

On Sat, 2008-04-12 at 10:56 +0100, Rory Winston wrote:
> Hi
> Does anyone know how I might pick out diagonal elements of a matrix using a
> vector?
> If I create a matrix a:
> a <- matrix(c(1:16), 4, byrow=TRUE)

Not sure if this does all that you describe below, but:


and you can subset that to get the first 3 elements say:




> and I want to pick out the elements (1,1),(2,2),(3,3), or another arbitrary
> diagonal (upper or lower), is there any way I can use a vector to do this?
> So if I want a diagonal of size 3, I could create a vector like x <- c(0:2)
> and then pick out a[1+x,1+x]? Currently, using a vector in this way gives me
> a sub-matrix, as I get the cartesian product of the indexes (which is to be
> expected, as I guess I am misusing the vector notion here).
> Anyone know how to do this?
> Cheers
> Rory
> 	[[alternative HTML version deleted]]
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