Question

I need a function that subsets a multidimensional array in R; the catch is I don’t know which dimension or the length along that dimension until the function is called.

# subset a 3-d array; leave dims 1 and 2, but start 3rd dim at its 11th value
mydim <- dim(myarr)
myarr[, , 11:mydim[3]]

# subset a 4-d array; leave dims 1, 3 and 4, but start 2rd dim at its 8th value
mydim <- dim(myarr)
myarr[, 8:mydim[2], , ]

I always need to subset along exactly one dimension, and the subsetting is always to start at some value other than 1. I also need to preserve the array structure, so matrix indexing into arrays does not look appealing. Thanks in advance.

Answer 1

Here’s an option that takes advantage of the possibility of subsetting an array based on a matrix:

myarr <- array(1:(2*3*4), dim = c(2, 3, 4))

myfun <- function(arr, from, len, Dim){
    dimArr <- dim(arr)
    if(missing(len)){
        subIdx <- from:dimArr[Dim]
    } else {
        subIdx <- from:(from + len - 1)
    }
    arrD <- lapply(as.list(dimArr), seq_len)
    arrD[[Dim]] <- subIdx
    subMat <- as.matrix(do.call(expand.grid, arrD))
    array(arr[subMat], dim = lapply(arrD, length))
}

> myfun(myarr, 2, 1, 3)
, , 1

     [,1] [,2] [,3]
[1,]    7    9   11
[2,]    8   10   12

> myfun(myarr, 2, Dim = 3)
, , 1

     [,1] [,2] [,3]
[1,]    7    9   11
[2,]    8   10   12

, , 2

     [,1] [,2] [,3]
[1,]   13   15   17
[2,]   14   16   18

, , 3

     [,1] [,2] [,3]
[1,]   19   21   23
[2,]   20   22   24