R: Apply function to nodes in an XML tree/DOM.

xmlDOMApply {XML} | R Documentation

## Apply function to nodes in an XML tree/DOM.

### Description

This recursively applies the specified function to each node in an XML tree, creating a new tree, parallel to the original input tree. Each element in the new tree is the return value obtained from invoking the specified function on the corresponding element of the original tree. The order in which the function is recursively applied is "bottom-up". In other words, function is first applied to each of the children nodes first and then to the parent node containing the newly computed results for the children.

### Usage

    xmlDOMApply(dom, func)

### Arguments

`dom` | a node in the XML tree or DOM on which to recursively apply the given function. This should not be the `XMLDocument` itself returned from `xmlTreeParse` but an object of class `XMLNode`. This is typically obtained by calling `xmlRoot` on the return value from `xmlTreeParse`.
`func` | the function to be applied to each node in the XML tree. This is passed the node object for the and the return value is inserted into the new tree that is to be returned in the corresponding position as the node being processed. If the return value is `NULL`, this node is dropped from the tree.

### Details

This is a native (C code) implementation that understands the structure of an XML DOM returned from `xmlTreeParse` and iterates over the nodes in that tree.

### Value

A tree that parallels the structure in the `dom` object passed to it.

### Author(s)

Duncan Temple Lang

### References

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

### See Also

xmlTreeParse

### Examples

     dom <- xmlTreeParse(system.file("exampleData","mtcars.xml", package="XML"))
     tagNames <- function() {
        tags <- character(0)
        add <- function(x) {
          if(inherits(x, "XMLNode")) {
            if(is.na(match(xmlName(x), tags)))
               tags <<- c(tags, xmlName(x))
          }

          NULL
        }

        return(list(add=add, tagNames = function() {return(tags)}))
     }

     h <- tagNames()
     xmlDOMApply(xmlRoot(dom), h$add)
     h$tagNames()

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