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% File src/library/base/man/parse.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2014 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.\sspace{}\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.

  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. 
}
\section{Partial parsing}{
  When a syntax error occurs during parsing, \code{parse} 
  signals an error.  The partial parse data will be stored in the
  \code{srcfile} argument if it is a \code{\link{srcfile}} object
  and the \code{text} argument was used to supply the text.  In other
  cases it will be lost when the error is triggered.
  
  The partial parse data can be retrieved using
  \code{\link{getParseData}} applied to the \code{srcfile} object.
  Because parsing was incomplete, it will typically include references
  to \code{"parent"} entries that are not present.  
}
\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")

# A partial parse with a syntax error
txt <- "
x <- 1
an error
"
sf <- srcfile("txt")
try(parse(text = txt, srcfile = sf))
getParseData(sf)
}
\keyword{file}
\keyword{programming}
\keyword{connection}