R: How to extract sets of multidimensional array values according to another matrix

Question

Suppose I have a 4 dimensional n x n x n x n array A. A is a distance matrix, such that A[i,j,l,k] is the distance from location i,j to location pair l,k. Suppose I have an nxn

Answer 1

My guess is that there is a more elegant way of doing this. Its inspired by DWin's solution but takes care of all potential combinations of getting from a type 0 to a type 1 cell. Let me know if you guys think of a better way

> type
     [,1] [,2] [,3]
[1,]    0    0    1
[2,]    0    1    0



type.0 = which(type == 0, arr.ind=T) #locations of type 0 cells
type.1 = which(type == 1, arr.ind=T) #locations of type 1 cells

nlocs0 = length(type.0[,1]) #number of locations of type 0
nlocs1 = length(type.1[,1]) #number of locations of type 1

reploc0 = rbind( do.call(rbind, rep(list(type.0), nlocs1)) ) #rbinded on top of itself nloc1 times
reploc1 = rbind( do.call(rbind, rep(list(type.1[1,]), nlocs0)) ) #rbinding the first location of type.1 on top of itself nlocs0 times


if(nlocs1>1){
  for(i in 2:nlocs1){
    reploc1 = rbind( rbind( do.call(rbind, rep(list(type.1[i,]), nlocs0)) ), reploc1)
  } 
}

d0_1 = A[cbind(reploc0,reploc1)]

Answer 2

I guess you have changed the request from the one stated in the first paragraph. See if this answers the second version.

 A [ cbind( which(T == 0, arr.ind=TRUE), which(T == 1, arr.ind=TRUE) )]
#[1] 1 1

(At first I thought you might need abind but cbind works fine.)