Prevent unlist to drop NULL values

Question

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.<

Answer 1

In this case (one level depth list) this should works too:

a[sapply(a, is.null)] <- NA
unlist(a)
# p1 p2 p1 p2 p1 p2 p1 p2 
#  2  5  3  4 NA NA  4  5