[R] blockwise matrix multiplication

[Gabor Grothendieck]

Try this:

   A * M[as.matrix(expand.grid(x,x))[,2:1]]


On 2/9/07, Robin Hankin <r.hankin at noc.soton.ac.uk> wrote:
> Hi
>
> Given an n-by-n  matrix A, say n=10 and
>
> A <- matrix(1:100,10,10)
>
> and a vector x of length n where 1 <=x[i] <= n for i=1..n
> say
>
> x <- c(1,1,1,2,4,3,3,3,4,4)
>
> and a matrix M of size max(x)-by-max(x), say
>
> M <- matrix(c(1, 0.1, 0, 0.2, 0.1, 1, 0, 0, 0, 0, 1, 0.2, 0.2,
> 0, 0.2, 1),4,4)
>
> how do I partition A according to the equivalence classes
> of the elements  of x and  "block multiply"  by M?
>
> I want
>
> for(i in 1:4){for(j in 1:4){
>   A[which(x==i),which(x==j)] <-  A[which(x==i),which(x==j)]*M[i,j]
> }}
>
>
> Is there a better way than this ghastly for() loop?
>
>
> --
> Robin Hankin
> Uncertainty Analyst
> National Oceanography Centre, Southampton
> European Way, Southampton SO14 3ZH, UK
>  tel  023-8059-7743
>
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