r - purrr map equivalent of nested for loop

Question

What is the purrr::map equivalent of:

for (i in 1:4) {
  for (j in 1:6) {
    print(paste(i, j, sep = "-"))
  }
}

OR

lapply(1:4, function(i) 
  lapply(1:6, function(j) 
    print(paste(i, j, sep = "-"))))

Conceptually, what I'm not getting is how to refer to the outer loop in the inner map function.

map(1:4, ~ map(1:6, ~ print(paste(.x, ????, sep = "-")))

Answer 1

The use of function formulas (~) is a little limited when trying to nest like this, since it is perfectly unclear which level of map you are attempting to reference. (Well, that's not correct. It's perfectly clear to me that it is referencing inside-out, and since they both use the same nomenclature, the outer variables are being masked by the inner variables.)

I think your best way around it is to not use the formula method, instead using immediate/anonymous (or predefined) functions:

library(purrr)
str(map(1:2, function(x) map(1:3, function(y) paste(x, y, sep = "-"))))
# List of 2
#  $ :List of 3
#   ..$ : chr "1-1"
#   ..$ : chr "1-2"
#   ..$ : chr "1-3"
#  $ :List of 3
#   ..$ : chr "2-1"
#   ..$ : chr "2-2"
#   ..$ : chr "2-3"