How to index a multidimensional R array dynamically?

Question

I am working on some data in R that consist of four-dimensional arrays composed of three spatial dimensions and a time dimension: x, y, z, t. For some of my analyses, I woul

Answer 1

I'm probably missing something, but don't you simply want a4d[indices[,1],indices[,2],indices[,3],]?

Answer 2

I think you are looking for:

apply(a4d, 4, `[`, indices)

And to check that our results match:

result1 <- matrix(result[,5], ncol = 10)
result2 <- apply(a4d, 4, `[`, indices)
identical(result1, result2)
# [1] TRUE

Answer 3

Accessing by each dimension individually does not work as @tilo-wiklund or I expect. Instead of 23 rows across 10 time steps, the result is a 23x23x23 cube across the 10 time steps.

r.idvdim <- a4d[indices[,1],indices[,2],indices[,3],]
r.apply  <- apply(a4d, 4, `[`, indices)
r.cbind  <- matrix(a4d[lookup],ncol=nt)

dim(r.idvdim)     # [1] 23 23 23 10
dim(r.apply)      # [1] 23 10
dim(r.cbind)      # [1] 23 10