Index matrix to pick elements from 3-dimensional matrix
Hello Juhana
try this (but there must be a better way!)
stratified.select <- function(A,J){
out <- sapply(J,function(i){sample(A[,,i],1)})
attributes(out) <- attributes(J)
return(out)
}
A <- array(letters[1:12],c(2,2,3))
J <- matrix(c(1,2,3,3),2,2)
R> stratified.select(A,J)
[,1] [,2]
[1,] "b" "i"
[2,] "g" "k"
R> stratified.select(A,J)
[,1] [,2]
[1,] "d" "j"
[2,] "f" "l"
R>
best wishes
Robin
On Apr 26, 2005, at 05:16 am, juhana vartiainen wrote:
Hi all Suppose I have a dim=c(2,2,3) matrix A, say: A[,,1]= a b c d A[,,2]= e f g h A[,,3]= i j k l Suppose that I want to create a 2x2 matrix X, which picks elements from the above-mentioned submatrices according to an index matrix J referring to the "depth" dimension: J= 1 3 2 3 In other words, I want X to be X= a j g l since the matrix J says that the (1,1)-element should be picked from A[,,1], the (1,2)-element should be picked from A[,,3], etc. I have A and I have J. Is there an expression in A and J that creates X? Thanks Juhana juhana at fief.se -- Juhana Vartiainen docent in economics Director, FIEF (Trade Union Foundation for Economic Research, Stockholm), http://www.fief.se gsm +46 70 360 9915 office +46 8 696 9915 email juhana at fief.se homepage http://www.fief.se/staff/Juhana/index.html
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