[R] Matrix Indexing

[Chuck Cleland]

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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