lapply with nested list

Question

I have a nested list and I'd like to lapply as.data.frame on the deepest nesting level and then rbindlist (from data.table) everything. Here's what my data looks like:

a <- list(date="2017-01-01",ret=1:5)
b <- list(date="2017-01-02",ret=7:9)
lvl3 <- list(a,b) 
lvl2 <- list(lvl3,lvl3)
lvl1 <- list(lvl2,lvl2,lvl2)

If I only had lvl3, I would to this to tranform into a data.frame and rbind the data:

rbindlist(lapply(lvl3,as.data.frame))
         date ret
1: 2017-01-01   1
2: 2017-01-01   2
3: 2017-01-01   3
4: 2017-01-01   4
5: 2017-01-01   5
6: 2017-01-02   7
7: 2017-01-02   8
8: 2017-01-02   9

How would I do that from lvl1 and rbind all nested data.frames? This does not work:

rbindlist(lapply(lvl1,as.data.frame))

Desired result contains 48 rows:

         date ret
 1: 2017-01-01   1
 2: 2017-01-01   2
 3: 2017-01-01   3
 4: 2017-01-01   4
 5: 2017-01-01   5
 6: 2017-01-02   7
 7: 2017-01-02   8
 8: 2017-01-02   9
 9: 2017-01-01   1
10: 2017-01-01   2
11: 2017-01-01   3
12: 2017-01-01   4
13: 2017-01-01   5
14: 2017-01-02   7
15: 2017-01-02   8
16: 2017-01-02   9
17: 2017-01-01   1
18: 2017-01-01   2
19: 2017-01-01   3
20: 2017-01-01   4
21: 2017-01-01   5
22: 2017-01-02   7
23: 2017-01-02   8
24: 2017-01-02   9
25: 2017-01-01   1
26: 2017-01-01   2
27: 2017-01-01   3
28: 2017-01-01   4
29: 2017-01-01   5
30: 2017-01-02   7
31: 2017-01-02   8
32: 2017-01-02   9
33: 2017-01-01   1
34: 2017-01-01   2
35: 2017-01-01   3
36: 2017-01-01   4
37: 2017-01-01   5
38: 2017-01-02   7
39: 2017-01-02   8
40: 2017-01-02   9
41: 2017-01-01   1
42: 2017-01-01   2
43: 2017-01-01   3
44: 2017-01-01   4
45: 2017-01-01   5
46: 2017-01-02   7
47: 2017-01-02   8
48: 2017-01-02   9

Answer 1

You can build your own recursive function, à la

f <- function(l) {
  data.table::rbindlist(lapply(l, function(x) {
    if(all(sapply(x, is.atomic))) as.data.table(x) else f(x)
  }))
}
f(lvl1)

This returns an ordinary data.table of 48 rows and 2 columns.

Also note that this works with lvl1, lvl2, and lvl3 without modifications.

Answer 2

@docendo's general solution is best, in my opinion, but if you know that it is only nested two-deep...

library(magrittr)

lvl1 %>% 
  unlist(recursive=FALSE) %>% 
  unlist(recursive=FALSE) %>% 
  lapply(as.data.table) %>% 
  rbindlist

From @lmo, here's the pipeless analogue (that doesn't require magrittr):

do.call(
  rbind, 
  lapply(
    unlist(unlist(lvl1, recursive=FALSE), recursive=FALSE), 
    as.data.frame
  )
)