Select along one of n dimensions in array - r

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 would like to select along one dimension, so for the example above I would have:
A["x",,]
dim(A["x",,]) #[1] 4 4
Is there a way to generalize if I do not know in advance how many dimensions (in addition to the named one I want to select by) my array might have? I would like to write a function that takes input that might formatted as A above, or as:
B <- c(1,2)
names(B) <- c("x", "y")
C <- matrix(1, 2, 2, dimnames=list(c("x","y"),NULL))
Background
The general background is that I am working on an ODE model, so for deSolve's ODE function it must take a single named vector with my current state. For some other functions, like calculating phase-planes/direction fields, it would be more practical to have a higher-dimensional array to apply the differential equation to, and I would like to avoid having many copies of the same function, simply with different numbers of commas after the dimension I want to select.

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.

Answer 2

I wrote this general function. Not necessarily super fast but a nice application for arrayInd and matrix indexing:
extract <- function(A, .dim, .value) {
val.idx <- match(.value, dimnames(A)[[.dim]])
all.idx <- arrayInd(seq_along(A), dim(A))
keep.idx <- all.idx[all.idx[, .dim] == val.idx, , drop = FALSE]
array(A[keep.idx], dim = dim(A)[-.dim], dimnames = dimnames(A)[-.dim])
}
Example:
A <- array(data=1:32, dim=c(2,4,4),
dimnames=list(c("x","y"), LETTERS[1:4], letters[1:4]))
extract(A, 1, "x")
extract(A, 2, "D")
extract(A, 3, "b")

Answer 3

The abind package has a function, asub, to do this in addition to other very useful array manipulation functions:
library(abind)
A <- array(data=1:32, dim=c(2,4,4),
dimnames=list(c("x","y"), LETTERS[1:4], letters[1:4]))
asub(A, 'x', 1)
asub(A, 'D', 2)
asub(A, 'b', 3)
And it allows indexing in multiple dimensions:
asub(A, list('x', c('C', 'D')), c(1,2))

Answer 4

Perhaps there is an easier way, but this works:
do.call("[",c(list(A,"x"),lapply(dim(A)[-1],seq)))
[,1] [,2] [,3] [,4]
[1,] NA NA NA NA
[2,] NA NA NA NA
[3,] NA NA NA NA
[4,] NA NA NA NA
Let's generalize it into a function that can extract from any dimension, not necessarily the first one:
extract <- function(A, .dim, .value) {
idx.list <- lapply(dim(A), seq_len)
idx.list[[.dim]] <- .value
do.call(`[`, c(list(A), idx.list))
}
Example:
A <- array(data=1:32, dim=c(2,4,4),
dimnames=list(c("x","y"), LETTERS[1:4], letters[1:4]))
extract(A, 1, "x")
extract(A, 2, "D")
extract(A, 3, "b")