xmlTreeParse: XML Parser

Description

Parses an XML file or string, and generates an R structure representing the XML tree.

Usage

xmlTreeParse(file, ignoreBlanks=T, handlers=NULL, replaceEntities=F,asText=F, trim=T, validate=F, getDTD=F, isURL=F)

Arguments

file

The name of the file containing the XML contents. This can contain ~ which is expanded to the user's home directory. It can also be a URL. See isURL. Additionally, the file can be compressed (gzip) and is read directly without the user having

ignoreBlanks

logical value indicating whether text elements made up entirely of white space should be included in the resulting `tree'.

handlers

Optional collection of functions used to map the different XML nodes to R objects. This is a named list of functions, and a closure can be used to provide local data.

replaceEntities

logical value indicating whether to substitute entity references with their text directly. This should be left as False. The text still appears as the value of the node, but there is more information about its source, allowing the parse to be

asText

logical value indicating that the first argument, `file', should be treated as the XML text to parse, not the name of a file. This allows the contents of documents to be retrieved from different sources (e.g. HTTP servers, XML-RPC, e

trim

whether to strip white space from the beginning and end of text strings.

validate

logical indicating whether to use a validating parser or not, or in other words check the contents against the DTD specification. If this is true, warning messages will be displayed about errors in the DTD and/or document, but the parsing will proceed ex

getDTD

logical flag indicating whether the DTD (both internal and external) should be returned along with the document nodes. This changes the return type.

isURL

indicates whether the file argument refers to a URL (accessible via ftp or http) or a regular file on the system. If asText is TRUE, this should not be specified. The function attempts to determine whether the data sour

Value

By default, an object of class XML doc is returned, which contains fields/slots named file, version and children.
fileThe (expanded) name of the file containing the XML.
versionA string identifying the version of XML used by the document.
childrenA list of the XML nodes at the top of the document. Each of these is of class XMLNode. These are made up of 4 fields. name{The name of the element.} attributes{For regular elements, a named list of XML attributes converted from the } children{List of sub-nodes.} value{Used only for text entries.} Some nodes specializations of XMLNode, such as XMLComment, XMLProcessingInstruction, XMLEntityRef are used.
If the value of the argument getDTD is TRUE, the return value is a list of length 2. The first element is as the document as described above. The second element is a list containing the external and internal DTDs. Each of these contains 2 lists - one for elements and another for entities. See parseDTD.

Details

The handlers argument is used similarly to those specifid in xmlEventParse. When an XML tag (element) is processed, we look for a function in this collection with the same name as the tag's name. If this is not found, we look for one named startElement. If this is not found, we use the default built in converter. The same works for comments, entity references, etc. The default entries should be named comment, startElement, externalEntity, processingInstruction text. They should take the XMLnode as their first argument. In the future, other information may be passed via ..., for example, the depth in the tree, etc. Specifically, the second argument will be the parent node into which they are being added, but this is not currently implemented, so should have a default value (NULL).

Each of these functions can return arbitrary values that are then entered into the tree in place of the default node passed to the function as the first argument. This allows the caller to generate the nodes of the resulting document tree exactly as they wish. If the function returns NULL, in the future, we will drop this node from the tree.

References

http://xmlsoft.org, http://www.w3.org/xml

Examples

Run this code

fileName <- system.file("data/test.xml", pkg="XML")
   # parse the document and return it in its standard format.
 xmlTreeParse(fileName)

   # parse the document, discarding comments.
  
 xmlTreeParse(fileName, handlers=list("comment"=function(x, parent){NULL}))

 invisible(xmlTreeParse(fileName,
            handlers=list(entity=function(x) {
                                    cat("In entity",x$name, x$value,"")}
                                  )
                          )
         )

 # Parse some XML text.
 # Read the text from the file
 xmlText <- paste(scan(fileName, what="",sep=""),"", collapse="")
 xmlTreeParse(xmlText, asText=T)

 # Read a MathML document and convert each node
 # so that the primary class is 
 #   <name of tag>MathML
 # so that we can use method  dispatching when processing
 # it rather than conditional statements on the tag name.
 # See plotMathML() in examples/.
 fileName <- system.file("data/mathml.xml",pkg="XML")
m <- xmlTreeParse(fileName, 
                  handlers=list(startElement=function(node){
                              cname <- paste(xmlName(node),"MathML",sep="",collapse="")
                              class(node) <- c(cname, class(node)); 
                              node
                }))

# Parse an XML document directly from a URL.
 # Requires Internet access.
 xmlTreeParse("http://www.omegahat.org/Scripts/Data/mtcars.xml", asText=T)

Run the code above in your browser using DataLab

The Learning Leader's Guide to AI Literacy