Sum of corresponding nested list elements in R

Question

I have a nested list like this:

foo <- list(
      a = list("a1" = c(1,2,3), "a2" = c(4,5,6), "a3" = c(7,8,9)),
      b = list("b1" = c(10, 20, 30), "b2" = c(40, 50, 60), "b3" = c(70, 80, 90)),
      ...
      ...
      ...
    )

I want to compute the sum of corresponding nested list elements: i.e a1+b1+..., a2+b2+..., a3+b3+... If there are just two nested list elements, a and b, they can be done like this

 mapply(function(x,y) x+y,  foo[[1]], foo[[2]], SIMPLIFY = FALSE)

But if there is a variable number of nested list elements like a, b, c, d, .. etc. I would like to write a generic script that can do this

Answer 1

You could do:

l = min(lengths(foo))
lapply(seq(l), \(x) Reduce(`+`, unlist(lapply(foo, `[`, x), recursive = FALSE)))
# [[1]]
# [1] 11 22 33
# 
# [[2]]
# [1] 44 55 66
# 
# [[3]]
# [1] 77 88 99

Or, if you want the sum in each:

sapply(1:3, \(x) Reduce(`+`, unlist(lapply(foo, `[`, x))))
#[1]  66 165 264

Answer 2

We can use Reduce + Map + list2DF like below

> unname(as.list(Reduce(`+`, Map(list2DF, foo))))
[[1]]
[1] 11 22 33

[[2]]
[1] 44 55 66

[[3]]
[1] 77 88 99