[R] Matrix Indexing
Chuck Cleland
ccleland at optonline.net
Sat Apr 12 12:44:13 CEST 2008
On 4/12/2008 5:56 AM, 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)
>
> 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?
a <- matrix(c(1:16), 4, byrow=TRUE)
a
[,1] [,2] [,3] [,4]
[1,] 1 2 3 4
[2,] 5 6 7 8
[3,] 9 10 11 12
[4,] 13 14 15 16
diag(a)
[1] 1 6 11 16
diag(a[1:3,2:4])
[1] 2 7 12
?diag
> Cheers
> Rory
>
> [[alternative HTML version deleted]]
>
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--
Chuck Cleland, Ph.D.
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