[R] Matrix Indexing
murdoch at stats.uwo.ca
Sat Apr 12 13:01:05 CEST 2008
Rory Winston wrote:
> Does anyone know how I might pick out diagonal elements of a matrix using a
> If I create a matrix a:
> a <- matrix(c(1:16), 4, byrow=TRUE)
> 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?
diag() is what you want, as others have said.
For more general cases, you can also consider matrix indexing using two
columns of indices:
 1 6 11 16
> x <- 0:2
> a[cbind(1 + x, 1 + x)]
 1 6 11
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