base (version 3.4.1)

NULL: The Null Object

Description

NULL represents the null object in R: it is a reserved word. NULL is often returned by expressions and functions whose value is undefined.

Usage

NULL
as.null(x, …)
is.null(x)

Arguments

x

an object to be tested or coerced.

…

ignored.

Value

as.null ignores its argument and returns NULL.

is.null returns TRUE if its argument's value is NULL and FALSE otherwise.

Details

NULL can be indexed (see Extract) in just about any syntactically legal way: whether it makes sense or not, the result is always NULL. Objects with value NULL can be changed by replacement operators and will be coerced to the type of the right-hand side.

NULL is also used as the empty pairlist: see the examples. Because pairlists are often promoted to lists, you may encounter 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 attribute(s) (attributes and structure).

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

Examples

is.null(list())     # FALSE (on purpose!)
is.null(pairlist()) # TRUE
is.null(integer(0)) # FALSE
is.null(logical(0)) # FALSE
as.null(list(a = 1, b = "c"))