The R Project SVN R

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

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

% Copyright 1995-2011 R Core Team

% Distributed under GPL 2 or later

\name{Control}

\alias{Control}

\alias{if}

\alias{else}

\alias{for}

\alias{in}

\alias{while}

\alias{repeat}

\alias{break}

\alias{next}

\title{Control Flow}

\description{

  These are the basic control-flow constructs of the \R language.  They

  function in much the same way as control statements in any Algol-like

  language.  They are all \link{reserved} words.

}

\usage{

if(cond) expr

if(cond) cons.expr  else  alt.expr

for(var in seq) expr

while(cond) expr

repeat expr

break

next

}

\arguments{

  \item{cond}{A length-one logical vector that is not \code{NA}.

    Conditions of length greater than one are accepted with a warning, but

    only the first element is used.  Other types are coerced to logical

    if possible, ignoring any class.

  }

  \item{var}{A syntactical name for a variable.}

  \item{seq}{An expression evaluating to a vector (including a list and

    an \link{expression}) or to a \link{pairlist} or \code{NULL}.  A

    factor value will be coerced to a character vector.}

  \item{expr, cons.expr, alt.expr}{

    An \emph{expression} in a formal sense.  This is either a

    simple expression or a so called \emph{compound expression}, usually

    of the form \code{\{ expr1 ; expr2 \}}.

  }

}

\details{

  \code{break} breaks out of a \code{for}, \code{while} or \code{repeat}

  loop; control is transferred to the first statement outside the

  inner-most loop. \code{next} halts the processing of the current

  iteration and advances the looping index.  Both \code{break} and

  \code{next} apply only to the innermost of nested loops.

  Note that it is a common mistake to forget to put braces (\code{\{ .. \}})

  around your statements, e.g., after \code{if(..)} or \code{for(....)}.

  In particular, you should not have a newline between \code{\}} and

  \code{else} to avoid a syntax error in entering a \code{if ... else}

  construct at the keyboard or via \code{source}.

  For that reason, one (somewhat extreme) attitude of defensive programming

  is to always use braces, e.g., for \code{if} clauses.

  The \code{seq} in a \code{for} loop is evaluated at the start of

  the loop; changing it subsequently does not affect the loop.  If

  \code{seq} has length zero the body of the loop is skipped. Otherwise the

  variable \code{var} is assigned in turn the value of each element of

  \code{seq}. You can assign to \code{var} within the body of the loop,

  but this will not affect the next iteration.  When the loop terminates,

  \code{var} remains as a variable containing its latest value.

}

\value{

  \code{if} returns the value of the expression evaluated, or

  \code{NULL} invisibly if none was (which may happen if there is no

  \code{else}).

  \code{for}, \code{while} and \code{repeat} return \code{NULL} invisibly.

  \code{for} sets \code{var} to the last used element of \code{seq},

  or to \code{NULL} if it was of length zero.

  \code{break} and \code{next} do not return a value as they transfer

  control within the loop.

}

\references{

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

  \emph{The New S Language}.

  Wadsworth & Brooks/Cole.

}

\seealso{

  \code{\link{Syntax}} for the basic \R syntax and operators,

  \code{\link{Paren}} for parentheses and braces.

  \code{\link{ifelse}}, \code{\link{switch}} for other ways to control flow.

}

\examples{

for(i in 1:5) print(1:i)

for(n in c(2,5,10,20,50)) {

   x <- stats::rnorm(n)

   cat(n, ": ", sum(x^2), "\n", sep = "")

}

f <- factor(sample(letters[1:5], 10, replace = TRUE))

for(i in unique(f)) print(i)

}

\keyword{programming}

\keyword{iteration}

\keyword{logic}