The R Project SVN R

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

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

% Copyright 1995-2011 R Core Team

% Distributed under GPL 2 or later

\name{parse}

\alias{parse}

\title{Parse Expressions}

\description{

  \code{parse} returns the parsed but unevaluated expressions in a

  list.

}

\usage{

parse(file = "", n = NULL, text = NULL, prompt = "?",

      keep.source = getOption("keep.source"), srcfile,

      encoding = "unknown")

}

\arguments{

  \item{file}{a \link{connection}, or a character string giving the name of a

    file or a URL to read the expressions from.

    If \code{file} is \code{""} and \code{text} is missing or \code{NULL}

    then input is taken from the console.}

  \item{n}{integer (or coerced to integer).  The maximum number of

    expressions to parse.  If \code{n} is \code{NULL} or negative or

    \code{NA} the input is parsed in its entirety.}

  \item{text}{character vector.  The text to parse.  Elements are treated

    as if they were lines of a file.  Other \R objects will be coerced

    to character if possible.}

  \item{prompt}{the prompt to print when parsing from the keyboard.

    \code{NULL} means to use \R's prompt, \code{getOption("prompt")}.}

  \item{keep.source}{a logical value; if \code{TRUE}, keep 

    source reference information.}

  \item{srcfile}{\code{NULL}, a character vector, or a

    \code{\link{srcfile}} object.  See the \sQuote{Details} section.}

  \item{encoding}{encoding to be assumed for input strings.  If the

    value is \code{"latin1"} or \code{"UTF-8"} it is used to mark

    character strings as known to be in Latin-1 or UTF-8: it is not used

    to re-encode the input.  To do the latter, specify the encoding as

    part of the connection \code{con} or \emph{via}

    \code{\link{options}(encoding=)}: see the example under

    \code{\link{file}}.}

}

\references{

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

  \emph{The New S Language}.

  Wadsworth & Brooks/Cole.

  Murdoch, D. (2010).  

  \href{http://journal.r-project.org/archive/2010-2/RJournal_2010-2_Murdoch.pdf}{Source 

  References}.  \emph{The R Journal} 2/2, 16-19.

}

\seealso{

  \code{\link{scan}}, \code{\link{source}}, \code{\link{eval}},

  \code{\link{deparse}}.

  The source reference information can be used for debugging (see e.g.

  \code{\link{setBreakpoint}}) and profiling (see

  \code{\link{Rprof}}). It can be examined by \code{\link{getSrcref}}

  and related functions.  More detailed information is available through

  \code{\link{getParseData}}.  

}

\details{

  If \code{text} has length greater than zero (after coercion) it is used in

  preference to \code{file}.

  All versions of \R accept input from a connection with end of line

  marked by LF (as used on Unix), CRLF (as used on DOS/Windows)

  or CR (as used on classic Mac OS).  The final line can be incomplete,

  that is missing the final EOL marker.

  See \code{\link{source}} for the limits on the size of functions

  that can be parsed (by default).

  When input is taken from the console, \code{n = NULL} is equivalent to

  \code{n = 1}, and \code{n < 0} will read until an EOF character is

  read.  (The EOF character is Ctrl-Z for the Windows front-ends.)  The

  line-length limit is 4095 bytes when reading from the console (which

  may impose a lower limit: see \sQuote{An Introduction to R}).

  The default for \code{srcfile} is set as follows.  If

  \code{keep.source} is not \code{TRUE}, \code{srcfile}

  defaults to a character string, either \code{"<text>"} or one

  derived from \code{file}.  When \code{keep.source} is

  \code{TRUE}, if \code{text} is used, \code{srcfile} will be set to a

  \code{\link{srcfilecopy}} containing the text.  If a character

  string is used for \code{file}, a \code{\link{srcfile}} object

  referring to that file will be used.

  When \code{srcfile} is a character string, error messages will

  include the name, but source reference information will not be added

  to the result.  When \code{srcfile} is a \code{\link{srcfile}}

  object, source reference information will be retained. 

}

\value{

  An object of type \code{"\link{expression}"}, with up to \code{n}

  elements if specified as a non-negative integer.

  When \code{srcfile} is non-\code{NULL}, a \code{"srcref"} attribute

  will be attached to the result containing a list of

  \code{\link{srcref}} records corresponding to each element, a

  \code{"srcfile"} attribute will be attached containing a copy of

  \code{srcfile}, and a \code{"wholeSrcref"} attribute will be

  attached containing a \code{\link{srcref}} record corresponding to

  all of the parsed text. Detailed parse information will be stored in

  the \code{"srcfile"} attribute, to be retrieved by

  \code{\link{getParseData}}.

  A syntax error (including an incomplete expression) will throw an error.

  Character strings in the result will have a declared encoding if

  \code{encoding} is \code{"latin1"} or \code{"UTF-8"}, or if

  \code{text} is supplied with every element of known encoding in a

  Latin-1 or UTF-8 locale.

}

\examples{

cat("x <- c(1, 4)\n  x ^ 3 -10 ; outer(1:7, 5:9)\n", file = "xyz.Rdmped")

# parse 3 statements from the file "xyz.Rdmped"

parse(file = "xyz.Rdmped", n = 3)

unlink("xyz.Rdmped")

}

\keyword{file}

\keyword{programming}

\keyword{connection}