Join 2 nested lists

Question

I want to combine two lists

list_1 <- list(LIST1 = list(list("a"), list("b"), list("c")))
list_2 <- list(LIST2 = list(list("1"), list("2"), list("3")))

Desired Output:

combined_list <- list()
combined_list[[1]] <- c("a", "1")
combined_list[[2]] <- c("b", "2")
combined_list[[3]] <- c("c", "3")

I have a nasty for loop way of doing this but I'd like to clean it up using purrr maybe? Any help appreciated!!

Answer 1

Here's a variant that recursively concatenates two nested lists of the same structure and preserves that structure

# Add additional checks if you expect the structures of .x and .y may differ
f <- function(.x, .y)
  if(is.list(.x)) purrr::map2(.x, .y, f) else c(.x, .y)

res <- f( list_1, list_2 )
# ...is identical to...
# list(LIST1 = list(list(c("a","1")), list(c("b","2")), list(c("c","3"))))

You can then unroll the structure as needed. For example, to get the desired output, you can do

purrr::flatten(purrr::flatten(res))
# [[1]]
# [1] "a" "1"
# 
# [[2]]
# [1] "b" "2"
# 
# [[3]]
# [1] "c" "3"