[R] How to make XML support Expat?

Duncan Temple Lang duncan at wald.ucdavis.edu
Sat Oct 24 23:31:46 CEST 2009

Johannes Graumann wrote:
> Hi,
> I had heard that Expat is was faster. Your mail actually made me go check 
> google for some comparisons and that does not seem the case ... do you have 
> any insight into this?

A couple of points..

i) At this point, I don't have any data about which of libxml2 and expat are faster
   C-level parsers

ii) Since you are calling the parser from R and then presumably working the resluting
   content via manipulation in R, these R-level operations are likely to be the
   slower parts of the overall process.

iii) I tend to use XPath for processing the resulting XML DOM/tree. That makes things
     quite fast (and also easy to express if you know XPath).
     expat is a parser and doesn't provide XPath facilities. So you would lose out
     big time in terms of speed here.

iv)  Xerces is an alternative, but again doesn't have a full XPath implementation
     by itself, AFAIK.

So basically, I wouldn't prematurely worry about speed.
If you have a test case, you can profile the code and see
where the bottlenecks are.


> Thanks, Joh
> On Saturday 24 October 2009 20:38:23 Duncan Temple Lang wrote:
>> Hi Joh.
>> What particular aspects of expat do you want that libxml2 and
>> the XML package currently cannot provide?
>> The early versions of the XML package (for the first few years)
>> could support expat and libxml2 as the C++/C-level parsers.
>> However, the support for expat was not maintained, so while
>> it could be resurrected and I have thought about it at several
>> times, I doubt it would compile out of the box now as
>> expat has most likely changed significantly.
>> If you wanted to experiment with the expat support in the package,
>> use
>>   R CMD INSTALL --configure-args='--with-expat'  XML
>> and that will endeavor to find the expat libraries, etc.
>> HTH,
>>   D.
>> Johannes Graumann wrote:
>>> Hi,
>>> How can I make the result of the following lines "TRUE"?
>>>> install.packages("XML")
>>>> library(XML)
>>>> supportsExpat()
>>> [1] FALSE
>>> I'm on linux, looked into the actual package, but don't seem to be able
>>> to wrap my head around how to compile this in ...
>>> Any pointers are welcome,
>>> Thanks Joh
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