Block-diagonal matrix from array in R

Question

How can we construct a block-diagonal matrix from a three-dimensional array in R? There are several possibilities when starting from a list of matrices (e.g., Reduce(magic::adiag, list_of_matrices)) or individual matrices (e.g., magic::adiag(matrix1, matrix2)). However, I could not find anything when we start with an array:

matrices      <- array(NA, c(3,3,2))
matrices[,,1] <- diag(1,3)
matrices[,,2] <- matrix(rnorm(9), 3, 3)

Are there any efficient solutions for constructing the corresponding 9x9 block matrix or is it a better idea to just convert to a list and use magic::adiag? The latter seems relatively inefficient, especially when the number of matrices is large.

Answer 1

I guess converting to a list and using magic::adiag is the fastest way. Try the following lines of code which is rather short and I use frequently:

library(magic)
arr <- array(1:8, c(2,2,3))
do.call("adiag", lapply(seq(dim(arr)[3]), function(x) arr[ , , x]))

This essentially reduces to a one-liner but uses lists.