Custom extractor function

my_extractor.R

# my_extractor.R
# 
# Given an M dimensional array, extract the last index based on prescribed
# values of the first M-1 values.
#
# For example, for a 3D array with indices [x, y, z] write a function that will
# extract z as function of x and y.

set.seed(42)

################################################################################
# 2D example
Amat <- matrix(rnorm(25), nrow = 5)
Amat

# etract a different column value for each row
v <- cbind(1:5, c(3, 2, 1, 5, 5))
v

# use apply to extract the elements as wanted
apply(v, 1, function(x, mat) {mat[x[1], x[2]]}, mat = Amat)

# note that `[` is really a function.
apply(v, 1, function(x, mat) {`[`(mat, x[1], x[2])}, mat = Amat)


################################################################################
# A general function
my_extractor <- function(x, dat) {
  apply(x, 1,
        function(xx) { 
          do.call(`[`, c(list(dat), split(xx, seq_along(xx)))) 
        })
} 

# check
my_extractor(v, Amat)

all.equal(my_extractor(v, Amat),
          apply(v, 1, function(x, mat) {`[`(mat, x[1], x[2])}, mat = Amat)
          )

################################################################################
# set up an example 3D Array, elements are three digit numbers of the form xyz
# with x representing the row, y the column, and z the depth
A <- expand.grid(a = 1:5, b = 1:8, c = 1:3)
A <- apply(A, 1, paste, collapse = "")
A <- array(A, dim = c(5, 8, 3))
A

# example vector for which element of z is selected for each xy com
v <- expand.grid(x = 1:2, y = 1:3)
v <- cbind(v, z = sample(1:3, nrow(v), replace = TRUE))

v
my_extractor(v, A)