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
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 > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. >
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 >> https://stat.ethz.ch/mailman/listinfo/r-help >> 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