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% File src/library/base/man/is.recursive.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2024 R Core Team
% Distributed under GPL 2 or later

\name{is.recursive}
\alias{is.atomic}
\alias{is.recursive}
\title{Is an Object Atomic or Recursive?}
\usage{
is.atomic(x)
is.recursive(x)
}
\description{
  \code{is.atomic} returns \code{TRUE} if \code{x} is of an atomic type
  and \code{FALSE} otherwise.
  \code{is.atomic(NULL)} returns \code{FALSE} since \R version 4.4.0.

  \code{is.recursive} returns \code{TRUE} if \code{x} has a recursive
  (list-like) structure and \code{FALSE} otherwise.
}
\arguments{
  \item{x}{object to be tested.}
}
\details{
  \code{is.atomic} is true for the \link{atomic} types
  (\code{"logical"}, \code{"integer"}, \code{"numeric"},
  \code{"complex"}, \code{"character"} and \code{"raw"}).

  Most types of objects are regarded as recursive.  Exceptions are the atomic
  types, \code{NULL}, symbols (as given by \code{\link{as.name}}),
  \code{S4} objects with slots, external pointers, and---rarely visible
  from \R---weak references and byte code, see \code{\link{typeof}}.

  It is common to call the atomic types \sQuote{atomic vectors}, but
  note that \code{\link{is.vector}} imposes further restrictions: an
  object can be atomic but not a vector (in that sense).

  These are \link{primitive} functions.
}
\references{
  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
  \emph{The New S Language}.
  Wadsworth & Brooks/Cole.
}
\seealso{
  \code{\link{is.list}},
  \code{\link{is.language}}, etc,
  and the \code{demo("is.things")}.
}
\examples{
require(stats)

is.a.r <- function(x) c(is.atomic(x), is.recursive(x))

is.a.r(c(a = 1, b = 3)) # TRUE FALSE
is.a.r(list())          # FALSE TRUE - a list is a list
is.a.r(list(2))         # FALSE TRUE
is.a.r(lm)              # FALSE TRUE
is.a.r(y ~ x)           # FALSE TRUE
is.a.r(expression(x+1)) # FALSE TRUE
is.a.r(quote(exp))      # FALSE FALSE
is.a.r(NULL)            # FALSE FALSE

# Reproduce pre-4.4 behavior of is.atomic()
is.atomicN <- function(x) is.atomic(x) || is.null(x)
is.atomicN(NULL) # TRUE
}
\keyword{programming}
\keyword{classes}