[R] flatten a list of lists

[Duncan Murdoch]

On 17/10/2013 9:15 AM, Michael Friendly wrote:
> I have functions that generate lists objects of class "foo" and lists of
> lists of these, of class
> "foolist", similar to what is shown below.

You can use c() to join lists.  So in the example below,

c(mfoo$A, mfoo$B)

will give you a list with the components you want, though the class 
won't be set.   More generally, do.call(c, unname(mfoo)) will join any 
number of components.  (Without unname(), the names at the top level 
will be combined with the component names;
maybe you'd actually want that, but your example didn't do it.)

This won't work if your list doesn't have the regular "list of lists" 
structure, e.g. if it mixes foo objects with foolist objects at the same 
level.  Then you probably need a more complicated recursive approach.  
You might be able to do it with rapply().

Duncan Murdoch

>
> How can I flatten something like this to remove the top-level list
> structure, i.e.,
> return a single-level list of "foo" objects, of class "foolist"?
>
> foo <- function(n) {
>       result <- list(x=sample(1:10,n), y=sample(1:10,n))
>       class(result) <- "foo"
>       result
> }
>
> multifoo <- function(vec, label, ...) {
>       result <- lapply(vec, foo, ...)
>       names(result) <- paste0(label, vec)
>       class(result) <- "foolist"
>       result
> }
>
> foo1 <- multifoo(1:2, "A")
> foo2 <- multifoo(1:2, "B")
>
> mfoo <- list(A=foo1, B=foo2)
>
> str(mfoo, 2)
>
>   > str(mfoo, 2)
> List of 2
>    $ A:List of 2
>     ..$ A1:List of 2
>     .. ..- attr(*, "class")= chr "foo"
>     ..$ A2:List of 2
>     .. ..- attr(*, "class")= chr "foo"
>     ..- attr(*, "class")= chr "foolist"
>    $ B:List of 2
>     ..$ B1:List of 2
>     .. ..- attr(*, "class")= chr "foo"
>     ..$ B2:List of 2
>     .. ..- attr(*, "class")= chr "foo"
>     ..- attr(*, "class")= chr "foolist"
>
> In this case, what is wanted is a single-level list, of 4 foo objects,
> A1, A2, B1, B2,
> all of class "foolist"
>