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