Convert complex list of lists to list of data frames

Question

I have a complex list that consists of other lists and data frames. I need to simplify this list to consist only from data frames - so each second level list should be made into separate first level data frames.

Here is reproducible example:

 dd<-data.frame(x=1:3,y=4:6)
 l1<-list(dd,list(dd,dd))
 #original list
 l1
[[1]]
  x y
1 1 4
2 2 5
3 3 6

[[2]]
[[2]][[1]]
  x y
1 1 4
2 2 5
3 3 6

[[2]][[2]]
  x y
1 1 4
2 2 5
3 3 6

Result I need to get

 l2<-list(dd,dd,dd)
 l2
[[1]]
  x y
1 1 4
2 2 5
3 3 6

[[2]]
  x y
1 1 4
2 2 5
3 3 6

[[3]]
  x y
1 1 4
2 2 5
3 3 6

I tried with the function unlist() and argument recursive=FALSE but in this case first level data frame is converted to two vectors.

 unlist(l1,recursive=FALSE)
$x
[1] 1 2 3

$y
[1] 4 5 6

[[3]]
  x y
1 1 4
2 2 5
3 3 6

[[4]]
  x y
1 1 4
2 2 5
3 3 6

Answer 1

One way I could think of is to check if the class of input (while running lapply) is data.frame or list, and if it's a data.frame convert it to a list of data.frame. This'll result the entire list becoming list of list of data.frames. Then you can use unlist with recursive=FALSE as follows:

unlist(lapply(l1, function(x) 
      if (class(x) == "data.frame") list(x) else x), recursive=FALSE)
[[1]]
  x y
1 1 4
2 2 5
3 3 6

[[2]]
  x y
1 1 4
2 2 5
3 3 6

[[3]]
  x y
1 1 4
2 2 5
3 3 6

This works of course when the data is as you mention exactly. They are either list of data.frames or list of list of data.frames. Oh and welcome to SO (first question)! :)