Search code examples
listrnull

Prevent unlist to drop NULL values

I have a vector of lists and I use unlist on them. Some of the elements in the vectors are NULL and unlist seems to be dropping them.

How can I prevent this?

Here's a simple (non) working example showing this unwanted feature of unlist

a = c(list("p1"=2, "p2"=5), 
      list("p1"=3, "p2"=4), 
      list("p1"=NULL, "p2"=NULL), 
      list("p1"=4, "p2"=5))
unlist(a)
 p1 p2 p1 p2 p1 p2 
 2  5  3  4  4  5
Solution

  • The issue here is that you can't have NULL in the middle of a vector. For example:

    > c(1,NULL,3)
[1] 1 3

    You can have NA in the middle though. You could could convert it to character and then back to numeric, which automatically converts the NULL values to NA (with a warning):

    > b <- as.numeric(as.character(a))
Warning message:
NAs introduced by coercion

    then put the names back in, because they've been dropped by the previous operation:

    > names(b) <- names(a)
> b
p1 p2 p1 p2 p1 p2 p1 p2 
2  5  3  4 NA NA  4  5 `