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% File src/library/base/man/Syntax.Rd
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% Part of the R package, https://www.R-project.org
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% Copyright 1995-2015 R Core Team
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% Distributed under GPL 2 or later
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% = is treated specially in the parser: see PR#14595
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\name{Syntax}
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\alias{Syntax}
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\title{Operator Syntax and Precedence}
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\description{
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  Outlines \R syntax and gives the precedence of operators.
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}
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\details{
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  The following unary and binary operators are defined.  They are listed
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  in precedence groups, from highest to lowest.
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  \tabular{ll}{
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    \code{:: :::}\tab access variables in a namespace\cr
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    \code{$ @}\tab component / slot extraction\cr
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    \code{[ [[}\tab indexing\cr
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    \code{^}\tab exponentiation (right to left)\cr
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    \code{- +}\tab unary minus and plus\cr
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    \code{:}\tab sequence operator\cr
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    \code{\%any\% |>}\tab special operators (including \code{\%\%} and \code{\%/\%})\cr
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    \code{* /}\tab multiply, divide\cr
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    \code{+ -}\tab (binary) add, subtract\cr
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    \code{< > <= >= == !=}\tab ordering and comparison\cr
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    \code{!}\tab negation\cr
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    \code{&  &&}\tab and\cr
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    \code{| ||}\tab or\cr
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    \code{~}\tab as in formulae\cr
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    \code{-> ->>}\tab rightwards assignment\cr
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    \code{<- <<-}\tab assignment (right to left)\cr
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    \code{=}\tab assignment (right to left)\cr
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    \code{?}\tab help (unary and binary)\cr
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  }
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  Within an expression operators of equal precedence are evaluated
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  from left to right except where indicated.  (Note that \code{=} is not
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  necessarily an operator.)
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  The binary operators \code{::}, \code{:::}, \code{$} and \code{@} require
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  names or string constants on the right hand side, and the first two
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  also require them on the left.
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  The links in the \bold{See Also} section cover most other aspects of
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  the basic syntax.
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}
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\note{
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  There are substantial precedence differences between \R and S.  In
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  particular, in S \code{?} has the same precedence as (binary) \code{+ -}
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  and \code{& && | ||} have equal precedence.
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}
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\references{
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  \bibshow{R:Becker+Chambers+Wilks:1988}
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}
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\seealso{
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  \code{\link{Arithmetic}}, \code{\link{Comparison}}, \code{\link{Control}},
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  \code{\link{Extract}}, \code{\link{Logic}},
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  \code{\link{NumericConstants}}, \code{\link{Paren}},
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  \code{\link{Quotes}}, \code{\link{Reserved}}.
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  Section \manual{R-lang}{Expressions}.
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}
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\examples{
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## Logical AND ("&&") has higher precedence than OR ("||"):
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TRUE || TRUE && FALSE   # is the same as
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TRUE || (TRUE && FALSE) # and different from
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(TRUE || TRUE) && FALSE
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## Special operators have higher precedence than "!" (logical NOT).
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## You can use this for \%in\% :
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! 1:10 \%in\% c(2, 3, 5, 7) # same as !(1:10 \%in\% c(2, 3, 5, 7))
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## but we strongly advise to use the "!( ... )" form in this case!
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## '=' has lower precedence than '<-' ... so you should not mix them
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##     (and '<-' is considered better style anyway):
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\dontrun{## Consequently, this gives a ("non-catchable") error
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 x <- y = 5  #->  Error in (x <- y) = 5 : ....
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}
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}
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\keyword{documentation}
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\keyword{programming}