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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