[R] Matrix Indexing

[Duncan Murdoch]

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?
>   
diag() is what you want, as others have said.

For more general cases, you can also consider matrix indexing using two 
columns of indices:

 > diag(a)
[1]  1  6 11 16
 > x <- 0:2
 >  a[cbind(1 + x, 1 + x)]
[1]  1  6 11

Duncan Murdoch
> Cheers
> Rory
>
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