Find empty lists in nested list of lists

Another convenient option to work with nested list is to use data.tree package:

library(data.tree)
nodes <- as.Node(mylist)
any(node$Get(function(node) length(as.list(node))) == 0)
# [1] TRUE

What about a function like this

has_empty_list <- function(x) {
    if(is.list(x)) {
        if (length(x)==0) {
            return(TRUE)
        } else {
            return(any(vapply(x, has_empty_list, logical(1))))
        }
    } else {
        return(FALSE)
    }
}

Basically we create a recursive function to look for lists of length 0.

has_empty_list( list(list("foo", "bar", "baz", list(list()))) )
# TRUE
has_empty_list( list(list("foo", "bar", "baz", list(list(4)))) )
# FALSE

And here's a modification to find the index of the empty list

find_empty_list <- function(x, index=c()) {
    if(is.list(x)) {
        #list
        if (length(x)==0) {
            if (length(index)==0) {
                return(0)
            } else {
                return(index)
            }
        } else {
            m <- Map(find_empty_list, x, lapply(seq_along(x), function(i) append(index,i)))
            # return the most deeply nested
            return( m[[which.max(lengths(m))]] )
        }
    } else {
        return(numeric())
    }
}

This should return a vector of the index that you can use to find the empty list. For example

( i <- find_empty_list(mylist) )
# [1] 1 4 1
mylist[[i]]
# list()

If the first parameter itself is an empty list, it will return 0

find_empty_list(list())
# 0

and if there is no empty list, it should return an empty vector

find_empty_list(list(1:3, list("c", a~b)))
# numeric()

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