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

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