Re: [xml] XPath with default namespace

[Charlie Bozeman <cbozeman HiWAAY net>]

Daniel Veillard wrote:
> On Wed, Jul 18, 2001 at 10:07:57PM -0500, Charlie Bozeman wrote:
> > I've toyed around using XPath to locate elements but have not had much
> > luck (ver 2.4.0 and CVS). My xml file uses a default namespace; when I
> > remove it XPath works. To test it do:
> > testXPath -i test/SVG/a-wf.xml "//desc"
> >
> > I get
> > Object is a Node Set :
> > Set contains 0 nodes:
> >
> > I had a copy of version 2.3.9 lying around so I tried it and it worked.
>
>   Hum, namespaces are tricky to handle properly in general...
>
>   Well 2.3.9 was broken :-) Someone reported this already suggesting
> it was a bug, it's not. I provided more complete explanation:
>
>   http://bugzilla.gnome.org/show_bug.cgi?id=57644
>
>   basically '//desc' selects only element without namespace, that's the
> semantic of the XPath spec.
>
>   In general the solution is to pass the namespaces in scope to the XPath
> evaluation, for example if you are on element node within document doc:
>
>    xpathCtxt->namespaces = xmlGetNsList(doc, node);
>    xpathCtxt->nsNr = 0;
>    if (xpathCtxt->namespaces != NULL) {
>        while (xpathCtxt->namespaces[xpathCtxt->nsNr] != NULL)
>            xpathCtxt->nsNr++;
>    }
>
>   This works okay but for the default namespace (it will be added to the
> list but cannot be used for the description. Then the simplest is to
> declare a namespace declaration like 'svg:' associated to this namespace
>
>    xmlXPathRegisterNs(xpathCtxt, "svg", "http://www.w3.org/Graphics/SVG";);
>
> and query for //svg:desc (check the real URI for SVG Namespace, I invented it).
> The good point is that this will work whether the given namespace is
> the default one, or mapped to any other prefix. The XPath engine will do
> the matching on the URI resolved using the prefix declared in the XPath
> context and will ignore the actual prefix used in the instance.
>
> Daniel
Thanks for the info, I got it working with the default namespace. One
more thing, neither xmlXPathEval nor xmlXPathCompiledEval return NULL
when they don't find the desired expression. Am I correct to assume this
is because even an empty node set is a valid object? Do I need to check
the node set value's nodeNr to determine if it is empty or is there a
better mechanism.

> --
> Daniel Veillard      | Red Hat Network http://redhat.com/products/network/
> veillard redhat com  | libxml Gnome XML XSLT toolkit  http://xmlsoft.org/
> http://veillard.com/ | Rpmfind RPM search engine http://rpmfind.net/
> Sep 17-18 2001 Brussels Red Hat TechWorld http://www.redhat-techworld.com
>
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