NULL represents the null object in R: it is a reserved
NULL is often returned by expressions and functions
whose value is undefined.
NULL as.null(x, …) is.null(x)
an object to be tested or coerced.
as.null ignores its argument and returns
TRUE if its argument's value
NULL can be indexed (see Extract) in just about any
syntactically legal way: whether it makes sense or not, the result is
NULL. Objects with value
NULL can be changed by
replacement operators and will be coerced to the type of the
NULL is also used as the empty pairlist: see the
examples. Because pairlists are often promoted to lists, you may
NULL being promoted to an empty list.
Objects with value
NULL cannot have attributes as there is only
one null object: attempts to assign them are either an error
attr) or promote the object to an empty list with
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.