xmlElementsByTagName: Retrieve the children of an XML node with a specific tag name

In cosmicexplorer/xmlr: Tools for parsing and generating XML within R and S-Plus.

Description

This returns a list of the children or sub-elements of an XML node whose tag name matches the one specified by the user.

Usage

xmlElementsByTagName(el, name, recursive = FALSE)

Arguments

el	the node whose matching children are to be retrieved.
name	a string giving the name of the tag to match in each of el's children.
recursive	a logical value. If this is FALSE, the default, only the direct child nodes are searched. Alternatively, if this is TRUE, all sub-nodes at all levels are searched. In other words, we find all descendants of the node el and return a list with the nodes having the given name. The relationship between the nodes in the resulting list cannot be determined. This is a set of nodes. See the note.

Details

This does a simple matching of names and subsets the XML node's children list. If recursive is TRUE, then the function is applied recursively to the children of the given node and so on.

Value

A list containing those child nodes of el whose tag name matches that specified by the user.

Note

The addition of the recursive argument makes this function behave like the getElementsByTagName in other language APIs such as Java, C\#. However, one should be careful to understand that in those languages, one would get back a set of node objects. These nodes have references to their parents and children. Therefore one can navigate the tree from each node, find its relations, etc. In the current version of this package (and for the forseeable future), the node set is a “copy” of the nodes in the original tree. And these have no facilities for finding their siblings or parent. Additionally, one can consume a large amount of memory by taking a copy of numerous large nodes using this facility. If one does not modify the nodes, the extra memory may be small. But modifying them means that the contents will be copied.

Alternative implementations of the tree, e.g. using unique identifiers for nodes or via internal data structures from libxml can allow us to implement this function with different semantics, more similar to the other APIs.

Author(s)

Duncan Temple Lang

References

http://www.w3.org/XML, http://www.omegahat.org/RSXML,

See Also

xmlChildren xmlTreeParse

Examples

## Not run: 
 doc <- xmlTreeParse("http://www.omegahat.org/Scripts/Data/mtcars.xml")
 xmlElementsByTagName(doc$children[[1]], "variable")

## End(Not run)

 doc <- xmlTreeParse(system.file("exampleData", "mtcars.xml", package="XML"))
 xmlElementsByTagName(xmlRoot(doc)[[1]], "variable")

cosmicexplorer/xmlr documentation built on May 30, 2019, 8:28 a.m.