Find empty lists in nested list of lists

Question

Given an arbitrarily nested list, how can I find if a list contains empty lists? Consider the following example:

mylist <- list(list("foo", "bar", "baz", list(list())))

I tried rapply, but that skips through lists. While I could use lapply, I'd need to know the level of nesting beforehand. For this exercise, I don't need to know where the list is (although that would be a bonus), I just need a way to detect if there is one.

Answer 1

Another approach is to use rrapply in the rrapply-package (an extension of base-rrapply):

library(rrapply)

## check if any empty list exists
any(
  rrapply(mylist,
          classes = "list",
          condition = function(x) length(x) < 1,
          f = function(x) TRUE, 
          deflt = FALSE,
          how = "unlist"
  )
)
#> [1] TRUE

It is straightforward to update the above call to return the index vectors of any empty lists:

## return flat list with position vectors of empty list
rrapply(mylist,
        classes = "list",
        condition = function(x) length(x) < 1,
        f = function(x, .xpos) .xpos, 
        how = "flatten"
)
#> [[1]]
#> [1] 1 4 1

Here, we make use of the .xpos argument which evaluates to the position of the current list element under evaluation.

---

Note that this automatically returns all empty list positions instead of only one:

mylist2 <- list(list("foo", list(), "baz", list(list())))

rrapply(mylist2,
        classes = "list",
        condition = function(x) length(x) < 1,
        f = function(x, .xpos) .xpos, 
        how = "flatten"
)
#> [[1]]
#> [1] 1 2
#> 
#> [[2]]
#> [1] 1 4 1

## using MrFlick's find_empty_list function
find_empty_list(mylist2)
#> [1] 1 4 1