Remove NULL elements from list of lists

Question

How do I remove the null elements from a list of lists, like below, in R:

lll <- list(list(NULL),list(1),list("a"))

The object I want would look like:

lll <- list(list(1),list("a"))

I saw a similar answer here: How can I remove an element from a list? but was not able to extend it from simple lists to a list of lists.

EDIT

Bad example above on my part. Both answers work on simpler case (above). What if list is like:

lll <- list(list(NULL),list(1,2,3),list("a","b","c"))

How to get:

lll <- list(list(1,2,3),list("a","b","c"))

Solution 1

This recursive solution has the virtue of working on even more deeply nested lists.

It's closely modeled on Gabor Grothendieck's answer to this quite similar question. My modification of that code is needed if the function is to also remove objects like list(NULL) (not the same as NULL), as you are wanting.

## A helper function that tests whether an object is either NULL _or_ 
## a list of NULLs
is.NullOb <- function(x) is.null(x) | all(sapply(x, is.null))

## Recursively step down into list, removing all such objects 
rmNullObs <- function(x) {
   x <- Filter(Negate(is.NullOb), x)
   lapply(x, function(x) if (is.list(x)) rmNullObs(x) else x)
}

rmNullObs(lll)
# [[1]]
# [[1]][[1]]
# [1] 1
# 
# 
# [[2]]
# [[2]][[1]]
# [1] "a"

Here is an example of its application to a more deeply nested list, on which the other currently proposed solutions variously fail.

LLLL <- list(lll)
rmNullObs(LLLL)
# [[1]]
# [[1]][[1]]
# [[1]][[1]][[1]]
# [[1]][[1]][[1]][[1]]
# [1] 1
# 
# 
# [[1]][[1]][[2]]
# [[1]][[1]][[2]][[1]]
# [1] "a"

Solution 2

Here's an option using Filter and Negate combination

Filter(Negate(function(x) is.null(unlist(x))), lll)
# [[1]]
# [[1]][[1]]
# [1] 1
#
#
# [[2]]
# [[2]][[1]]
# [1] "a"

Solution 3

Using purrr

purrr::map(lll, ~ purrr::compact(.)) %>% purrr::keep(~length(.) != 0)
[[1]]
[[1]][[1]]
[1] 1

[[1]][[2]]
[1] 2

[[1]][[3]]
[1] 3


[[2]]
[[2]][[1]]
[1] "a"

[[2]][[2]]
[1] "b"

[[2]][[3]]
[1] "c"

Solution 4

For this particular example you can also use unlist with its recursive argument.

lll[!sapply(unlist(lll, recursive=FALSE), is.null)]
# [[1]]
# [[1]][[1]]
# [1] 1
#
#
# [[2]]
# [[2]][[1]]
# [1] "a"

Solution 5

Since you have lists in lists, you probably need to run l/sapply twice, like:

lll[!sapply(lll,sapply,is.null)]

#[[1]]
#[[1]][[1]]
#[1] 1
#
#
#[[2]]
#[[2]][[1]]
#[1] "a"

View more solutions

Comments

[mrp]

This is a great function!

[Josh O'Brien]

@mrp -- Thanks! Glad I was able to pass on this handy template to someone else.

[Rafael]

I'm working with lists of S4 objects and this solution was the only one that worked, thanks!

[joran]

fyi, this probably behaves in ways one might not expect if your list elements are primarily data frame, since the is.list will capture and then unwind them. Simply adding an additional && is.data.frame(x) should be enough I think.

[Josh O'Brien]

@joran -- Thanks. Guessing you mean && !is.data.frame(x), right?

[not2qubit]

What's the name of the package? Link?

[Raminsu]

sorry for the delay @not2qubit, the name of the package is RLIST as mentioned above