XML (version 1.2-0)

getNodeSet: Find matching nodes in an internal XML tree/DOM

Description

These functions provide 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 on the Web and in books. XPath queries can result in different types of values such as numbers, strings, and node sets.

These sets of matching nodes are returned in R as a list. And then one can iterate over these elements to process the nodes in whatever way one wants. Unfortunately, this involves two loops - one in the XPath query over the entire tree, and another in R. Typically, this is fine as the number of matching nodes is reasonably small. However, if repeating this on numerous files, speed may become an issue. We can avoid the second loop (i.e. the one in R) by applying a function to each node before it is returned to R as part of the node set. The result of the function call is then returned, rather than the node itself.

One can provide an expression rather than a function. This is expected to be a call and the first argument of the call will be replaced with the node.

Usage

getNodeSet(doc, path, namespaces = getDefaultNamespace(xmlRoot(doc)), fun = NULL, ...)
xpathApply(doc, path, fun, ... , namespaces = getDefaultNamespace(xmlRoot(doc)))

Value

• The results can currently be different based on the returned value from the XPath expression evaluation:
• list
  a node set
• numeric
  a number
• logical
  a boolean
• character
  a string, i.e. a single character element.
• If fun is supplied and the result of the XPath query is a node set, the result in R is a list.

Details

This calls the libxml routine xmlXPathEval.

References

http://xmlsoft.org, http://www.w3.org/xml http://www.w3.org/TR/xpath http://www.omegahat.org/RSXML

See Also

xmlTreeParse with useInternalNodes as 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"))

  # Using a namespace
 f = system.file("exampleData", "SOAPNamespaces.xml", package = "XML") 
 z = xmlTreeParse(f, useInternal = TRUE)
 getNodeSet(z, "/a:Envelope/a:Body", c("a" = "http://schemas.xmlsoap.org/soap/envelope/"))
 getNodeSet(z, "//a:Body", c("a" = "http://schemas.xmlsoap.org/soap/envelope/"))

  # Get two items back with namespaces
 f = system.file("exampleData", "gnumeric.xml", package = "XML") 
 z = xmlTreeParse(f, useInternal = TRUE)
 getNodeSet(z, "//gmr:Item/gmr:name", c(gmr="http://www.gnome.org/gnumeric/v2"))


 #####
 # European Central Bank (ECB) exchange rate data

  # Data is available from "http://www.ecb.int/stats/eurofxref/eurofxref-hist.xml"
  # or locally.

 uri = system.file("exampleData", "eurofxref-hist.xml.gz", package = "XML")
 doc = xmlTreeParse(uri, useInternalNodes = TRUE)

   # The default namespace for all elements is given by
 namespaces <- c(ns="http://www.ecb.int/vocabulary/2002-08-01/eurofxref")


     # Get the data for Slovenian currency for all time periods.
     # Find all the nodes of the form <Cube currency="SIT"...>

 slovenia = getNodeSet(doc, "//ns:Cube[@currency='SIT']", namespaces )

    # Now we have a list of such nodes, loop over them 
    # and get the rate attribute
 rates = as.numeric( sapply(slovenia, xmlGetAttr, "rate") )
    # Now put the date on each element
    # find nodes of the form <Cube time=".." ... >
    # and extract the time attribute
 names(rates) = sapply(getNodeSet(doc, "//ns:Cube[@time]", namespaces ), 
                      xmlGetAttr, "time")

    #  Or we could turn these into dates with strptime()
 strptime(names(rates), "%Y-%m-%d")


   #  Using xpathApply, we can do
 rates = xpathApply(doc, "//ns:Cube[@currency='SIT']", xmlGetAttr, "rate", namespaces = namespaces )
 rates = as.numeric(unlist(rates))

   # Using an expression rather than  a function and ...
 rates = xpathApply(doc, "//ns:Cube[@currency='SIT']", quote(xmlGetAttr(x, "rate")), namespaces = namespaces )

   #
  uri = system.file("exampleData", "namespaces.xml", package = "XML")
  d = xmlTreeParse(uri, useInternalNodes = TRUE)
  getNodeSet(d, "//c:c", c(c="http://www.c.org"))

   # the following, perhaps unexpectedly but correctly, returns an empty
   # with no matches
   
  getNodeSet(d, "//defaultNs", "http://www.omegahat.org")

   # But if we create our own prefix for the evaluation of the XPath
   # expression and use this in the expression, things work as one
   # might hope.
  getNodeSet(d, "//dummy:defaultNs", c(dummy = "http://www.omegahat.org"))

   # And since the default value for the namespaces argument is the
   # default namespace of the document with the prefix 'd', we can use
  getNodeSet(d, "//d:defaultNs")


   # And the syntactic sugar is 
  d["//d:defaultNs"]