FAQ for the XML package in R/S-Plus

How do I create my own XML content from within R and not just parse other people's XML documents?

There are several facilities for doing this in the XML package. Basically you will be creating a tree, a hierarchical collection of XML nodes. So you need to be able to create the nodes and then arrange them into this hierarachical structure. You can create all the nodes and then do the tree construction but it is typically easier to create the nodes and specify its parent during the creation. The XML package provides low-level functions for creating the nodes and several functions which provide a higher level interface that try to facilitate adding nodes to a 'current' point in the tree. These manage the notion of 'current' for you. These functions share a very similar interface and so it is easy to switch from one to the other. They differ in how they represent the tree which can be complicated in R since there are no references/pointers.

The approach I prefer is the xmlTree() function. This uses the low-level node creation functions (e.g. newXMLNode, newXMLComment, newXMLPINode, etc.) but also allows us to manage a stack of "open" nodes and a default namespace prefix. New nodes are by default added to the most recent "open" node, i.e. that node acts as the parent for new nodes.

Doesn't the use of internal nodes such as with xmlTree() mean that we cannot store the tree across R sessions since they are external pointers to C data structures in memory.

Well, yes and no. We cannot use R's regular serialization to RDA files of the variables holding the tree or the individual nodes. But we can easily create a text representation from the internal nodes by dumping the tree, either to a file or a character vector of length 1, and then we can restore that XML tree by parsing it again:

   tt = xmlTree("top")
   tt$addTag("b", "Some text")
   save(saveXML(tt), file = "tt.rda")
   load("tt.rda")
   tt = xmlTreeParse(tt, asText = TRUE, useInternal = TRUE)

We don't get back the XMLInternalDOM with information about open nodes, etc. from which we could continue to add nodes. But we do get back the exact tree.

We can also convert the nodes from internal nodes to regular R base nodes. And from that

My XML document has attributes that have a namespace prefix (e.g. <node mine:foo="abc" /> ) When I parse this document into S, the namespace prefix on the attribute is dropped. Why and how can I fix it?

The first thing to do is use a value of TRUE for the addAttributeNamespaces argument in the call to xmlTreeParse.
The next thing is to ensure that the namespace (mine, in our example) is defined in the document. In other words, there must be be an xmlns:mine="some url" attribute in some node before or in the node that is being processed. If no definition for the namespace is in the document, the libxml parser drops the prefix on the attribute.
The same applies to namespaces for node names, and not just attributes.

I define a method in the closure, but it never gets called.

The most likely cause is that you omitted to add it to the list of functions returned by the closure. Another possibility is that you have mis-spelled the name of the method. The matching is case-sensitive and exact. If the function corresponds to a particular XML element name, check whether the value of the argument useTagName is T, and also that there really is a tag with this name in the document. Again, the case is important.

When I compile the source code, I get lots of warning messages such as

"RSDTD.c", line 110: warning: argument #2 is incompatible with prototype:
        prototype: pointer to const uchar : "unknown", line 0
        argument : pointer to const char   
      

This is because the XML libraries work on unsigned characters for UniCode. The R and S facilities do not. I am not yet certain which direction to adapt things for this package. The warnings are typically harmless.

When I compile the chapter for Splus5/S4, I get warning messages about SET_CLASS being redefined.

This is ok, in this situation. The warning is left there to remind people that there are some games being played and that if there are problems, to consider these warnings. The SET_CLASS macro being redefined is a local version for S3/R style classes. The one in the Splus5/S4 header files is for the S4 style classes.

On which platfforms does it compile?

I have used gcc on both Linux (RedHat 6.1) (egcs-2.91.66) and Solaris (gcc-2.7.2.3), and the Sun compilers, cc 4.2 on Solaris 2.6/SunOS 5.6 and cc 5.0 on Solaris 2.7/SunOS 5.7.

I can't seem to use conditional DTD segments via the IGNORE/INCLUDE mechanism.

Libxml doesn't support this. Perhaps we will add code for this.

Daneil Veillard might add this.

When I read a relatively simple tree in Splus5 and print it to the terminal/console, I get an error about nested expressions exceeding the limit of 256.

The simple fix is to set the value of the expressions option to a value larger than 256.

 options(expressions=1000)

The main cause of this is that S and R are programming languages not specialized for handling trees. (They are functional languages and have no facilities for pointers or references as in C or Java.)

I get errors when using parameter entities in DTDs?

This was true in version 1.7.3 and 1.8.2 of libxml. Thanks to Daneil Veillard for fixing this quickly when I pointed it out.

Parameters are allowed, but the libxml parsing library is fussy about white-space, etc. The following is is ok

<!ELEMENT legend  (%PlotPrimitives;)* >

but

<!ELEMENT legend  (%PlotPrimitives; )* >

is not. The extra space preceeding the ) causes an error in the parser something like

1: XML Parsing Error: ../Histogram.dtd:80: xmlParseElementChildrenContentDecl : ',' '|' or ')' expected 
2: XML Parsing Error: ../Histogram.dtd:80: xmlParseElementChildrenContentDecl : ',' expected 
3: XML Parsing Error: ../Histogram.dtd:80: xmlParseElementDecl: expected '>' at the end 
4: XML Parsing Error: ../Histogram.dtd:80: Extra content at the end of the document 

This can be fixed by adding a call to SKIP_BLANKS at the end of the loop while(CUR!= ')' { ... } in the routine xmlParseElementChildrenContentDecl() in parser.c The problem lies in the transition between the different input buffers introduced by the entity expansion.

I am trying to use XPath and getNodeSet(). But I am not matching any nodes.

If you are certain that the XPath expression should match what you want, then it is probably a matter of namespaces. If the document in which you are trying to find the nodes has a default namespace (at the top-level node or a sub-node involved in your match), then you have to explicitly identify the namespace. Unfortunately, XPath doesn't use the default namespace of the target document, but requires the namespace to be explicitly mentioned in the XPath expression.

For example, suppose we have a document that looks like

     
       
]]>

and we want to use an XPath expression to find the title node. We might think that "/doc/topic/title" would do the trick. But in fact, we need

  /ns:doc/ns:topci/ns:title    

And then we need to map ns to the URI "http://www.omegahat.org". We do this in a call to getNodeSet() as

  getNodeSet(doc, "/ns:doc/ns:topci/ns:title", c(ns = "http://www.omegahat.org"))

As a simplification, getNodeSet() will create the map between the prefix and the URI of the default namespace of the XML document if you specify a single string with no name as the value of the namespaces argument, e.g.

  getNodeSet(doc, "/ns:doc/ns:topci/ns:title", "ns")

There are some additional comments here.

There is an table of data in HTML that I want to read into R as a data frame. How can I do it?

Well, it is relatively easy, although the technology underlying it is quite powerful and somewhat complex in the general case. There is a document describing the approach(es)

I have a "large" XML file. Can I use DOM parsing or do I have to use SAX style parsing via the more complex xmlEventParse().

Well, I was given a 70 Mb XML file (which when compressed is 6MB) and after uncompressing the file, I can read it into R via xmlTreeParse(, useInternalTrue = TRUE) This file contained 2,895,409 nodes (length(getNodeSet(z, "//*"))) This took 9.4 seconds on Intel MacBook Pro with a 2.33Ghz Dual processor and 3G of RAM, and on a machine with dual core 64bit AMD, it took 20 seconds. To find the nodes of interest took 8.9 seconds on the Mac, and (apparently) 1.1 seconds on the AMD.

I want to include one document inside another. How can I do this?

Firstly, you want to look into XInclude. When processing the document in R, use xinclude = TRUE, which is the default, in calls to xmlTreeParse().

I want to use XInclude to include part of the same document. I can't figure out how to do it. Any ideas?

Yes. Use

 <xi:include xpointer="xpointer(//mynode)"/>

adapting that to what you want. Note that the attribute is named xpointer. There is no href so the XInclude defaults to this document and the expression for the xpointer attribute uses the function xpointer. This is not element.