purrr - 如果一个输入是列表列表，如何使用 map2？

Question

Consider first a basic case of map2 usage like this:

listA <- list(1, 2, 3)
listB <- list(4, 5, 6)

map2(listA, listB, sum)

However, what if we have a situation where one input is a list of lists, and the other input is just a list:

listA <- list(list(1, 2, 3),
              list(4, 5, 6))

listB <- list(7, 8, 9)

How can I achieve this desired output with purrr ?:

list(list(8, 10, 12),
     list(11, 13, 15))

Answer 1

You can use map twice.

library(purrr)

map(listA, map2, listB, sum)

# this version might show it a bit more clearly
map(listA, ~ map2(.x, listB, sum))

[[1]]
[[1]][[1]]
[1] 8

[[1]][[2]]
[1] 10

[[1]][[3]]
[1] 12


[[2]]
[[2]][[1]]
[1] 11

[[2]][[2]]
[1] 13

[[2]][[3]]
[1] 15