R: Using the unlist() function for a list that has some elements being integer(0)?

Question

In R, I am calling unlist on a list to convert to a vector. However, some of my list elements are empty, and hence returns integer(0). The unlist() function by default drops this. How can I return a vector that has NA where the list is empty? I have created a reproducible example:

> ex.list <- list()
> ex.list[[1]] <- integer(0)
> ex.list[[2]] <- c(1,2,3)
> ex.list
[[1]]
integer(0)

[[2]]
[1] 1 2 3

> unlist(ex.list)
[1] 1 2 3

Thanks.

Answer 1

A way of doing it is to use function is.na<-.

is.na(ex.list) <- lengths(ex.list) == 0

ex.list
#[[1]]
#[1] NA
#
#[[2]]
#[1] 1 2 3

Then you will have a NA where the list had a length of 0.

unlist(ex.list)
#[1] NA  1  2  3