Select along one of n dimensions in array

Question

I have an array in R, created by a function like this:

A <- array(data=NA, dim=c(2,4,4), dimnames=list(c(\"x\",\"y\"),NULL,NULL))

And I

Answer 1

I spent quite a lot of time figuring out the fastest way to do this for plyr, and the best I could come up with was manually constructing the call to [:

index_array <- function(x, dim, value, drop = FALSE) { 
  # Create list representing arguments supplied to [
  # bquote() creates an object corresponding to a missing argument
  indices <- rep(list(bquote()), length(dim(x)))
  indices[[dim]] <- value

  # Generate the call to [
  call <- as.call(c(
    list(as.name("["), quote(x)),
    indices,
    list(drop = drop)))
  # Print it, just to make it easier to see what's going on
  print(call)

  # Finally, evaluate it
  eval(call)
}

(You can find more information about this technique at https://github.com/hadley/devtools/wiki/Computing-on-the-language)

You can then use it as follows:

A <- array(data=NA, dim=c(2,4,4), dimnames=list(c("x","y"),NULL,NULL))
index_array(A, 2, 2)
index_array(A, 2, 2, drop = TRUE)
index_array(A, 3, 2, drop = TRUE)

It would also generalise in a straightforward way if you want to extract based on more than one dimension, but you'd need to rethink the arguments to the function.