XPath fails on an XML document in R

问题

I'll be glad to find some help with XML manipulation with R.

I'm trying to proceed XPath on my XML/TEI file. Here's its structure :

<?xml version="1.0" encoding="UTF-8"?>
<TEI xmlns="http://www.tei-c.org/ns/1.0">
<text>
    <body>
        <div>
            <p>
                <seg>
                        <name ref="Actr1235">Jen B.</name>frate M. <name ref="Actr1234">Léard B.</name> rhoncus orci quis luctus ultrices <note place="margin-left">1713 &amp; 1714</note>, a été
                        vehicula cursus nunc, at sagittis lorem aliquet sed <name ref="Actr1236"> Jaes L.</name>
                        aeman graeca <name type="place">Digo</name> iaculis volutpat risu <name ref="Cole14">la
                        Charias</name>. M. <name ref="Actr1236">Laure</name> bibendum augue erat, fermentum semper. M. <name ref="Actr1235">B.</name> bibendum augue erat, fermentum semper
                </seg>
            </p>
        </div>
    </body>
</text>
</TEI>

I'd like to extract all the attribute's values beginning with "Actr" inside <name> tags. I've tried this XPath in an XMLeditor //tei:name/@ref[starts-with(., 'Actr')] and it's working.

Now, I'm trying to do it with R to put the query's results in a dataframe, using XML package to parse the document

library(XML)
data1715<-xmlParse("My_document.xml")
name_query<-xpathSApply(data1715, "data(//tei:name/@ref[starts-with(., 'Actr')])", xmlValue)

It returns following error:

XPath error : Undefined namespace prefix xmlXPathCompOpEval: parameter error XPath error : Invalid expression Erreur dans xpathApply.XMLInternalDocument(doc, path, fun, ..., namespaces = namespaces, : error evaluating xpath expression data(//tei:name/@ref[starts-with(., 'Actr')])

How do you define namespace in this case?

回答1:

The XML package doesn't handle default namespaces very well. You need to be explicit about assigning a name to the namespace before you can use xpath style expressions. How about something like

xpathSApply(data1715, 
     "//tei:name/@ref[starts-with(.,'Actr')]", 
     unname,
     namespaces=c(tei=getDefaultNamespace(data1715)[[1]]$uri))

Note I also removed data() and changed xmlValue. I'm not sure what you were trying to do with data(), but here we are returning attributes and xmlValue doesn't appear to like working with attributes.

来源：https://stackoverflow.com/questions/30984420/xpath-fails-on-an-xml-document-in-r

标签

xpath

xml-parsing