R - how to get a value of a multi-dimensional array by a vector of indices

Question

Let\'s say I have a multi-dimensional array called pi, and its number of dimensions isn\'t known until the runtime:

dims <- rep(3, dim_count)         


        

Answer 1

do.call() is an option:

dim_count <- 5
indexes <- c(1, 2, 2, 2, 3)
dims <- rep(3, dim_count)
pi <- array(seq_len(prod(dims)), dims)

do.call(`[`, c(list(x = pi), as.list(indexes)))

Which gives:

> do.call(`[`, c(list(x = pi), as.list(indexes)))
[1] 202
> pi[1, 2, 2, 2, 3]
[1] 202

The tricky bit is getting the list of arguments in the right format. pi should be the first argument to "[" (or named as argument x, see ?"["), whilst we want each element of indexes itself to be a component of the supplied list, not a vector within that list. Hence the convoluted c(list(x = pi), as.list(indexes)).

An alternative way to construct the argument list which might be easier to follow is:

ARGS <- vector("list", length = dim_count + 1)
ARGS[[1]] <- pi
ARGS[2:length(ARGS)] <- indexes
do.call("[", ARGS)

which gives

> do.call("[", ARGS)
[1] 202
> pi[1, 2, 2, 2, 3]
[1] 202

Answer 2

Making use of a little known usage of [:

When indexing arrays by [ a single argument i can be a matrix with as many columns as there are dimensions of x; the result is then a vector with elements corresponding to the sets of indices in each row of i.

you can simply do:

pi[matrix(indexes, 1)]

Answer 3

do.call("[",...) seems to work.

indexes <- c(1,2,3,3,3)
pi[1,2,3,3,3] <- 17  ## so we know if we succeeded or not
do.call("[",c(list(pi),as.list(indexes)))

Note that your example wouldn't work -- your dimensions were all 3, but some of your index elements were >3 ...