Unnest list of lists of data frames, containing NAs - r

Question

I have a list of nested data, containing lists with two data frames each looking like this:
mylist <- list(
list(
p = data.frame(
id = "01",
stringsAsFactors = F
),
c = data.frame(
text = c("one", "two"),
from = c("A", "B"),
stringsAsFactors = F
)
),
list(
p = data.frame(
id = "02",
stringsAsFactors = F
),
c = data.frame(
text = c("three", "four", "five"),
from = c("C", "D", "E"),
stringsAsFactors = F
)
),
list(
p = data.frame(
id = "03",
stringsAsFactors = F
),
c = data.frame(
text = logical(0),
from = logical(0)
)
)
)
I want to flatten this list into a dataframe, with one row per observation from the "c" data frames, a colum that indicates the "id" of the level above stored in the "p" dataframes, and non-observations filled with NAs. The result should look like this:
df <- data.frame(
p.id = c("01", "01", "02", "02", "02", "03"),
c.text = c("one", "two", "three", "four", "five", NA),
c.from = c("A", "B", "C", "D", "E", NA)
)
#
# p.id c.text c.from
# 01 one A
# 01 two B
# 02 three C
# 02 four D
# 02 five E
# 03 <NA> <NA>
as.data.frame() provides a very simple solution that comes very close to the desired result, but breaks when there are 0 observations in the "c" data frame.
mylist[[1]] %>% as.data.frame()
mylist[[3]] %>% as.data.frame()
I am aware that are similar questions on stackoverflow - but I read several threads closely, have tried different things from bind_rows(), to jsonlite::flatten(), tidr::unnest(), or data.table::rbindlist() - but have not make it work.
Help is highly appreciated!

Answer 1

Here is a tidyverse solution:
library(tidyverse)
map(mylist, ~as_tibble(.)) %>%
enframe() %>%
unnest_longer(value)
Which gives us:
# A tibble: 6 x 2
name value$p$id $c$text $$from
<int> <chr> <chr> <chr>
1 1 01 one A
2 1 01 two B
3 2 02 three C
4 2 02 four D
5 2 02 five E
6 3 NA NA NA

Answer 2

I create a helper function to combine p and c:
foo <- function(x) {
a <- x[[1]]
b <- x[[2]]
if (nrow(b) == 0) b[1, ] <- NA
return(cbind(a, b))
}
Then I run the helper function on each element and bind the rows:
do.call(rbind, lapply(mylist, foo))
The result:
> do.call(rbind, lapply(mylist, foo))
id text from
1 01 one A
2 01 two B
3 02 three C
4 02 four D
5 02 five E
6 03 <NA> <NA>
P.S. The same result using the R base pipe:
lapply(mylist, foo) |> do.call(what = rbind)