\name{getNodeSet}
\alias{getNodeSet}
\title{Find matching nodes in an internal XML tree/DOM}
\description{
  This function provides a way to find XML nodes that match a particular
  criterion. It uses the XPath syntax and allows quite powerful
  expressions for identifying nodes.  The XPath language requires some
  knowledge, but tutorials are available.
}
\usage{
getNodeSet(doc, path, namespaces = character())
}
\arguments{
  \item{doc}{an object of class \code{XMLInternalDocument}}
  \item{path}{a string (character vector of length 1) giving the
    XPath expression to evaluate.}
  \item{namespaces}{a named character vector giving
    the name-URI pairs for the namespaces of interest in the
    XPath query.}
}
\details{
  This calls the libxml routine \code{xmlXPathEval}.
}
\value{
  The results can currently be different
  based on the returned value from the XPath expression evaluation:
  \item{list}{a node set}
  \item{numeric}{a number}
  \item{logical}{a boolean}
  \item{character}{a string, i.e. a single character element.}
}

\references{\url{http://xmlsoft.org}, 
  \url{http://www.w3.org/xml}
  \url{http://www.w3.org/TR/xpath}
  \url{http://www.omegahat.org/RSXML}
}
\author{Duncan Temple Lang <duncan@wald.ucdavis.edu>}

\note{
  More of the XPath functionality provided by libxml can and may be
  made available to the R package.
  Facilities such as compiled XPath expressions, functions, ordered node
  information,
   
  Please send requests to the maintainer.
}

\seealso{
 \code{\link{xmlTreeParse}} with \code{useInternalNodes} as \code{TRUE}.
}
\examples{
 doc = xmlTreeParse(system.file("exampleData", "tagnames.xml", package = "XML"), useInternalNodes = TRUE)
 getNodeSet(doc, "/doc//b[@status]")
 getNodeSet(doc, "/doc//b[@status='foo']")

 
 els = getNodeSet(doc, "/doc//a[@status]")
 sapply(els, function(el) xmlGetAttr(el, "status"))
}
\keyword{file}
\keyword{IO}