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

The correct way to indicate a missing value is NA (not NULL). Here is another version that is working:

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

p1 p2 p1 p2 p1 p2 p1 p2 
 2  5  3  4 NA NA  4  5