Efficiently remove all NULL values in a list and all sublists

Question

Consider the following list:

> l1 <- list(NULL,1,2,list(NULL,3,list(NULL,4)))
> str(l1)
List of 4
 $ : NULL
 $ : num 1
 $ : num 2
 $ :List of 3
  ..         


        

Answer 1

This can be done recursively:

rmNull <- function(x) {
   x <- Filter(Negate(is.null), x)
   lapply(x, function(x) if (is.list(x)) rmNull(x) else x)
}
l2 <- rmNull(l1)

giving:

> str(l2)
List of 3
 $ : num 1
 $ : num 2
 $ :List of 2
  ..$ : num 3
  ..$ :List of 1
  .. ..$ : num 4