The R Project SVN R

% File src/library/base/man/Syntax.Rd

% Part of the R package, https://www.R-project.org

% Copyright 1995-2015 R Core Team

% Distributed under GPL 2 or later

% = is treated specially in the parser: see PR#14595

\name{Syntax}

\alias{Syntax}

\title{Operator Syntax and Precedence}

\description{

  Outlines \R syntax and gives the precedence of operators.

}

\details{

  The following unary and binary operators are defined.  They are listed

  in precedence groups, from highest to lowest.

  \tabular{ll}{

    \code{:: :::}\tab access variables in a namespace\cr

    \code{$ @}\tab component / slot extraction\cr

    \code{[ [[}\tab indexing\cr

    \code{^}\tab exponentiation (right to left)\cr

    \code{- +}\tab unary minus and plus\cr

    \code{:}\tab sequence operator\cr

    \code{\%any\%}\tab special operators (including \code{\%\%} and \code{\%/\%})\cr

    \code{* /}\tab multiply, divide\cr

    \code{+ -}\tab (binary) add, subtract\cr

    \code{< > <= >= == !=}\tab ordering and comparison\cr

    \code{!}\tab negation\cr

    \code{&  &&}\tab and\cr

    \code{| ||}\tab or\cr

    \code{~}\tab as in formulae\cr

    \code{-> ->>}\tab rightwards assignment\cr

    \code{<- <<-}\tab assignment (right to left)\cr

    \code{=}\tab assignment (right to left)\cr

    \code{?}\tab help (unary and binary)\cr

  }

  Within an expression operators of equal precedence are evaluated

  from left to right except where indicated.  (Note that \code{=} is not

  necessarily an operator.)

  The binary operators \code{::}, \code{:::}, \code{$} and \code{@} require

  names or string constants on the right hand side, and the first two

  also require them on the left.

  The links in the \bold{See Also} section cover most other aspects of

  the basic syntax.

}

\note{

  There are substantial precedence differences between \R and S.  In

  particular, in S \code{?} has the same precedence as (binary) \code{+ -}

  and \code{& && | ||} have equal precedence.

}

\references{

  Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)

  \emph{The New S Language}.

  Wadsworth & Brooks/Cole.

}

\seealso{

  \code{\link{Arithmetic}}, \code{\link{Comparison}}, \code{\link{Control}},

  \code{\link{Extract}}, \code{\link{Logic}},

  \code{\link{NumericConstants}}, \code{\link{Paren}},

  \code{\link{Quotes}}, \code{\link{Reserved}}.

  The \sQuote{R Language Definition} manual.

}

\examples{

## Logical AND ("&&") has higher precedence than OR ("||"):

TRUE || TRUE && FALSE   # is the same as

TRUE || (TRUE && FALSE) # and different from

(TRUE || TRUE) && FALSE

## Special operators have higher precedence than "!" (logical NOT).

## You can use this for \%in\% :

! 1:10 \%in\% c(2, 3, 5, 7) # same as !(1:10 \%in\% c(2, 3, 5, 7))

## but we strongly advise to use the "!( ... )" form in this case!

## '=' has lower precedence than '<-' ... so you should not mix them

##     (and '<-' is considered better style anyway):

\donttest{## Consequently, this gives a ("non-catchable") error

 x <- y = 5  #->  Error in (x <- y) = 5 : ....

}

}

\keyword{documentation}

\keyword{programming}