[R] blockwise matrix multiplication

[Robin Hankin]

Wow.


It generalizes nicely to arbitrary dimensional arrays too.

thanks a lot!


rksh


On 9 Feb 2007, at 14:19, Gabor Grothendieck wrote:

> 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
>>
>> ______________________________________________
>> R-help at stat.math.ethz.ch mailing list
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>> PLEASE do read the posting guide http://www.R-project.org/posting- 
>> guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>

--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743