Transform List of List of Vectors into Dataframe in R

This is about transforming lists in R.

My data looks as follows:

>data = list(list(c('a', 'b'), c('c', 'd')), list(c('a', 'b', 'c'), c('d', 'e', 'f'), c('g', 'h', 'i')))
>data
[[1]]
[[1]][[1]]
[1] "a" "b"

[[1]][[2]]
[1] "c" "d"


[[2]]
[[2]][[1]]
[1] "a" "b" "c"

[[2]][[2]]
[1] "d" "e" "f"

[[2]][[3]]
[1] "g" "h" "i"

I would like to concatenate every vector of the sublists, e.g. paste("a", "b") = "a b"), so that the whole list can be transformed into a dataframe, that looks as follows:

> df <- data.frame(col1 = c("a b", "c d", NA), col2 = c("a b c", "d e f", "g h i"))
> df
  col1  col2
1  a b a b c
2  d e d e f
3 <NA> g h i

I tried to solve this problem now with the apply-functions in R, but this is really not my territory... Thx in advance for your help!

Solution

Here is something. We use two helper functions from data.table transpose() and setDF().

library(data.table)
nrs <- max(lengths(data))

df <- setDF(
  lapply(lapply(data, transpose), function(x) do.call(paste, x)[1:nrs])
)
df
#     V1    V2
# 1  a b a b c
# 2  c d d e f
# 3 <NA> g h i

EDIT:

Dependency-free solution:

as.data.frame(sapply(data, function(x) sapply(x, paste, collapse = " ")[1:nrs]))