xmlEventParse {XML} | R Documentation

## XML Event/Callback element-wise Parser

### Description

Reads and processes the contents of an XML file or string by invoking user-level functions associated with different components of the XML tree. These include beginning and end of XML elements, comments, CDATA (escaped character data), entities, processing instructions, etc. This allows the caller to create the appropriate data structure from the XML document contents rather than the default tree (see xmlTreeParse). Functions for specific tags/elements can be used in addition to the standard callback names.

### Usage

    xmlEventParse(file, handlers=xmlEventHandler(), ignoreBlanks = FALSE, addContext=TRUE,
                   useTagName=TRUE, asText = FALSE, trim=TRUE, useExpat=FALSE, isURL = FALSE,
                    state = NULL, replaceEntities = TRUE, validate = FALSE,
                     saxVersion = 1)

### Arguments

`file` | the source of the XML content. This can be a string givinging the name of a file or remote URL, the XML itself, a connection object, or a function. If this is a string, and `asText` is `TRUE`, the value is the XML content. This allows one to read the content separately from parsing without having to write it to a file. If `asText` is `FALSE` and a string is passed for `file`, this is taken as the name of a file or remote URI. If one is using the libxml parser (i.e. not expat), this can be a URI accessed via HTTP or FTP or a compressed local file. If it is the name of a local file, it can include `~`, environment variables, etc. which will be expanded by R. (Note this is not the case in S-Plus, as far as I know.) If a connection is given, the parser incrementally reads one line at a time by calling the function `readLines` with the connection as the first argument (and `1` as the number of lines to read). The parser calls this function each time it needs more input. If invoking the `readLines` function to get each line is excessively slow or is inappropriate, one can provide a function as the value of `fileName`. Again, when the XML parser needs more content to process, it invokes this function to get a string. This function is called with a single argument, the maximum size of the string that can be returned. The function is responsible for accessing the correct connection(s), etc. which is typically done via lexical scoping/environments. This mechanism allows the user to control how the XML content is retrieved in very general ways. For example, one might read from a set of files, starting one when the contents of the previous file have been consumed. This allows for the use of hybrid connection objects. Support for connections and functions in this form is only provided if one is using libxml2 and not libxml version 1.
`handlers` | a closure object that contains functions which will be invoked as the XML components in the document are encountered by the parser. The standard functions are `startElement()`, `endElement()` `comment()`, `externalEntity()`, `entityDeclaration()`, `processingInstruction()`, `text()`, `cdata()`, `startDocument()`, and `endDocument()`.
`ignoreBlanks` | a logical value indicating whether text elements made up entirely of white space should be included in the resulting `tree'.
`addContext` | logical value indicating whether the callback functions in `handlers' should be invoked with contextual information about the parser and the position in the tree, such as node depth, path indices for the node relative the root, etc. If this is True, each callback function should support ....
`useTagName` | logical value indicating whether the callback mechanism should look for a function matching the tag name in the startElement and endElement events, before calling the default handler functions. This allows the caller to handle different element types for a particular DTD with their own functions directly, rather than performing a second dispatch in `startElement()`.
`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, etc.) and still use this parser.
`trim` | whether to strip white space from the beginning and end of text strings.
`useExpat` | a logical value indicating whether to use the expat SAX parser, or to default to the libxml. If this is TRUE, the library must have been compiled with support for expat. See supportsExpat.
`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.
`state` | an optional S object that is passed to the callbacks and can be modified to communicate state between the callbacks. If this is given, the callbacks should accept an argument named `.state` and it should return an object that will be used as the updated value of this state object. The new value can be any S object and will be passed to the next callback where again it will be updated by that functions return value, and so on. If this not specified in the call to `xmlEventParse`, no `.state` argument is passed to the callbacks. This makes the interface compatible with previous releases.
`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 reversed with full reference information.
`saxVersion` | an integer value which should be either 1 or 2. This specifies which SAX interface to use in the C code. The essential difference is the number of arguments passed to the `startElement` handler function(s). In addition to the name of the element and the named-attributes vector, two additional arguments are provided. The first identifies the namespace of the element. This is a named character vector of length 1, with the value being the URI of the namespace and the name being the prefix that identifies that namespace within the document. For example, `xmlns:r="http://www.r-project.org"` would be passed as `c(r = "http://www.r-project.org")`. If there is no prefix because the namespace is being used as the default, the result of calling `names` on the string is `""`. The fourth argument gives the collection of all the namespaces defined within this element. Again, this is a named character vector.
`validate` | Currently, this has no effect as the libxml2 parser uses a document structure to do validation. a 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 except for the presence of terminal errors.

### Details

This is now implemented using the libxml parser. Originally, this was implemented via the Expat XML parser by Jim Clark (http://www.jclark.com).

### Value

The return value is the `handlers' argument. It is assumed that this is a closure and that the callback functions have manipulated variables local to it and that the caller knows how to extract this.

### Note

The libxml parser can read URLs via http or ftp. It does not require the support of textbf{wget} as used in other parts of R, but uses its own facilities to connect to remote servers.

### Author(s)

Duncan Temple Lang

### References

http://www.w3.org/XML, http://www.jclark.com/xml

### See Also

xmlTreeParse

### Examples

     fileName <- system.file("exampleData", "mtcars.xml", package="XML")

       # Print the name of each XML tag encountered at the beginning of each
       # tag.
       # Uses the libxml SAX parser.
     xmlEventParse(fileName,
                    list(startElement=function(name, attrs){
                                        cat(name,"\n")
                                      }),
                    useTagName=FALSE, addContext = FALSE)

    ## Not run:
      # Parse the text rather than a file or URL by reading the URL's contents
      # and making it a single string. Then call xmlEventParse
    xmlURL <- "http://www.omegahat.org/Scripts/Data/mtcars.xml"
    xmlText <- paste(scan(xmlURL, what="",sep="\n"),"\n",collapse="\n")
    xmlEventParse(xmlText, asText=TRUE)
    ## End(Not run)

        # Using a state object to share mutable data across callbacks
    f <- system.file("exampleData", "gnumeric.xml", package = "XML")
    zz <- xmlEventParse(f,
                        handlers = list(startElement=function(name, atts, .state) {
                                                         .state = .state + 1
                                                         print(.state)
                                                         .state
                                                     }), state = 0)
    print(zz)

        # Illustrate the startDocument and endDocument handlers.
    xmlEventParse(fileName,
                   handlers = list(startDocument = function() {
                                                     cat("Starting document\n")
                                                   },
                                   endDocument = function() {
                                                     cat("ending document\n")
                                                 }),
                   saxVersion = 2)

    if(libxmlVersion()$major >= 2) {

     startElement = function(x, ...) cat(x, "\n")

     xmlEventParse(file(f), handlers = list(startElement = startElement))

     # Parse with a function providing the input as needed.
     xmlConnection =
      function(con) {

       if(is.character(con))
         con = file(con, "r")

       if(isOpen(con, "r"))
         open(con, "r")

       function(len) {

         if(len < 0) {
            close(con)
            return(character(0))
         }

          x = character(0)
          tmp = ""
        while(length(tmp) > 0 && nchar(tmp) == 0) {
          tmp = readLines(con, 1)
          if(length(tmp) == 0)
            break
          if(nchar(tmp) == 0)
            x = append(x, "\n")
          else
            x = tmp
        }
        if(length(tmp) == 0)
          return(tmp)

        x = paste(x, collapse="")

        x
      }
     }

     ff = xmlConnection(f)
     xmlEventParse(ff, handlers = list(startElement = startElement))

      # Parse from a connection. Each time the parser needs more input, it
      # calls readLines(<con>, 1)
     xmlEventParse(file(f),  handlers = list(startElement = startElement))

      # using SAX 2
     h = list(startElement = function(name, attrs, namespace, allNamespaces){
                                     cat("Starting", name,"\n")
                                     if(length(attrs))
                                         print(attrs)
                                     print(namespace)
                                     print(allNamespaces)
                             },
              endElement = function(name, uri) {
                              cat("Finishing", name, "\n")
                })
     xmlEventParse(system.file("exampleData", "namespaces.xml", package="XML"), handlers = h, saxVersion = 2)

    }

[Package XML version 1.2-0 Index]