NA is for a
logical missing value:
class(NA) # "logical"
This is convenient, since it can easily be coerced to other atomic vector types, and is therefore usually the only
NA you will need:
x <- c(1, NA, 1) class(x) # "numeric"
If you do need a single
NA value of another type, use
NA_complex_. For missing values of fancy classes, subsetting with
NA_integer_ usually works; for example, to get a missing-value Date:
class(Sys.Date()[NA_integer_]) #  "Date"