XPath query with default namespace in XML file

Matlab introduced a MATLAB API for XML Processing (I believe in 2021a).

I am using this API with XPath expressions to extract data from XML files. However, this fails when the XML file has a default namespace registered.

For example with this XML file test.xml:

<books xmlns="http://www.contoso.com/books" xmlns:i="http://www.contoso.com/currency">

<book>

<title>Title</title>

<author>Author Name</author>

<i:price>5.50</i:price>

</book>

</books>

Using the following snippet:

import matlab.io.xml.dom.*
import matlab.io.xml.xpath.*

xmlFilePath = "path\to\test.xml";
xmlDom = parseFile(Parser, xmlFilePath);

xpathPrice = "//i:price";
xpathAuthor = "//author";
res = evaluate(Evaluator, xpathPrice, xmlDom)

works for xpathPrice but fails for xpathAuthor (in the sense that it returns an empty result instead of the author node from the example).

Browsing through the different options, I could not come up with a solution. I am working on Matlab 2021b.

I know that I could use getElementsByTagName but I do require the functionality provided by XPath to identify nodes.

Are there any hints on how I could get that working?

Thanks!

Answer 1

[Sakshay]

Hello Chris,

As per my understanding, you are trying to access elements of an XML file, using XPath. But it seems that the elements with a default namespace having no prefix are not being captured.

This problem here seems to be with how XPath works, rather than in the MATLAB API.

Elements without a prefix, can be accessed using the "local-name()" attribute. As an example, for your case, the query would look like:

xPathAuthor = '//*[local-name()="book"]';
res = evaluate(Evaluator, xPathAuthor, xmlDom);

For information, on the XPath Evaluator Package, you can refer to the following documentation:

XPath expression evaluator - MATLAB (mathworks.com)

[chris]

Hi,

thanks for your reply. After some more research, it looks like you are right and this is some weird behavior of XPath. Personally, I find the solution not very satisfying but that's how it is.

For interested people, another workaround would be to use a default prefix (as described here: Abstract base class for namespace prefix resolvers). Something along these lines

import matlab.io.xml.xpath.*

evalObj = Evaluator();
evalObj.PrefixResolver = myPrefixResolver;
xpathAuthor = "//def:author";
res = evaluate(evalObj, xpathAuthor, xmlDom);

with the custom prefix resolver:

classdef myPrefixResolver < matlab.io.xml.xpath.PrefixResolver
    
    methods
        function url = getNamespaceForPrefix(resolver,prefix)
            if prefix == "i"
                url = "http://www.contoso.com/currency";
            else
                url = "http://www.contoso.com/books";
            end
        end
    end

end

The disadvantage is, that one has to know the namespace URLs beforehand.