| % File src/library/base/man/NULL.Rd |
| % Part of the R package, https://www.R-project.org |
| % Copyright 1995-2017 R Core Team |
| % Distributed under GPL 2 or later |
| |
| \name{NULL} |
| \title{The Null Object} |
| \usage{ |
| NULL |
| as.null(x, \dots) |
| is.null(x) |
| } |
| \alias{NULL} |
| \alias{as.null} |
| \alias{as.null.default} |
| \alias{is.null} |
| \description{ |
| \code{NULL} represents the null object in \R: it is a \link{reserved} |
| word. \code{NULL} is often returned by expressions and functions |
| whose value is undefined. |
| } |
| \arguments{ |
| \item{x}{an object to be tested or coerced.} |
| \item{\dots}{ignored.} |
| } |
| \details{ |
| \code{NULL} can be indexed (see \link{Extract}) in just about any |
| syntactically legal way: whether it makes sense or not, the result is |
| always \code{NULL}. Objects with value \code{NULL} can be changed by |
| replacement operators and will be coerced to the type of the |
| right-hand side. |
| |
| \code{NULL} is also used as the empty \link{pairlist}: see the |
| examples. Because pairlists are often promoted to lists, you may |
| encounter \code{NULL} being promoted to an empty list. |
| |
| Objects with value \code{NULL} cannot have attributes as there is only |
| one null object: attempts to assign them are either an error |
| (\code{\link{attr}}) or promote the object to an empty list with |
| attribute(s) (\code{\link{attributes}} and \code{\link{structure}}). |
| } |
| \note{ |
| \code{is.null} is a \link{primitive} function. |
| } |
| \references{ |
| Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) |
| \emph{The New S Language}. |
| Wadsworth & Brooks/Cole. |
| } |
| \value{ |
| \code{as.null} ignores its argument and returns \code{NULL}. |
| |
| \code{is.null} returns \code{TRUE} if its argument's value |
| is \code{NULL} and \code{FALSE} otherwise. |
| } |
| \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")) |
| } |
| \keyword{attribute} |
| \keyword{manip} |
| \keyword{list} |
| \keyword{sysdata} |