How do I get value of one node's attribute based on value of another nodes attribute?

Question

Using Saxon parser freebie.

This is a snip of a larger XML file.

<osmChange>
  <modify>
    <node id="28377390">
      <tag k="name" v="Holywell"/>
      <tag k="public_transport" v="station"/>
      <tag k="railway" v="station"/>
    </node>
        <node id="168325631">
      <tag k="name" v="Whitecraigs"/>
      <tag k="public_transport" v="station"/>
      <tag k="railway" v="station"/>
    </node>
  </modify>
</osmChange>

This Xpath:

<xsl:sequence select="$OSM_OSC/osmChange/./modify/(node|way)[tag[@k='railway'][@v='station']]/tag[@k='name']"/>

Returns this, as expected.

<tag k="name" v="Whitecraigs"/>
<tag k="name" v="Holywell"/>

How do I get it to return just the actual names?

Whitecraigs
Holywell

I thought adding /@v to the end would be the solution:

<xsl:sequence select="$OSM_OSC/osmChange/./modify/(node|way)[tag[@k='railway'][@v='station']]/tag[@k='name']/@v"/>

However it returns a long-winded error message:

java.lang.UnsupportedOperationException: copy() applied to attribute node
at net.sf.saxon.tree.tiny.TinyAttributeImpl.copy(TinyAttributeImpl.java:259)
at net.sf.saxon.serialize.UncommittedSerializer.append(UncommittedSerializer.java:207)
at net.sf.saxon.event.ProxyReceiver.append(ProxyReceiver.java:200)
at net.sf.saxon.serialize.PrincipalOutputGatekeeper.append(PrincipalOutputGatekeeper.java:101)

Etc...

If you want a sequence of string values I would try <xsl:sequence select="osmChange/modify/(node|way)[tag[@k='railway'][@v='station']]/tag[@k='name']/@v/string()"/>. The error seems odd but it is hard to tell whether it is caused by trying to output the attribute nodes outside of a parent element container or by some bug in Saxon. — Martin Honnen
– Martin Honnen, Commented Jun 10 at 15:26
Trying to output attribute node(s) as part of a document should raise w3.org/TR/xslt-30/#err-XTDE0420 so something is wrong in Saxon, I would say, with the error you get, even if you haven't shown the context of the xsl:sequence selecting the two attribute nodes. But try to access the string values with the shown /@v/string() and you should get your two values Whitecraigs Holywell, I think, even with that bug/quirk in Saxon. — Martin Honnen
– Martin Honnen, Commented Jun 10 at 15:36
If you want to serialize (=write to the output) the values of those attributes, then use xsl:value-of instead of xsl:sequence. — michael.hor257k
– michael.hor257k, Commented Jun 10 at 15:40
BTW, I believe your xsl:sequence instruction would work if it were a child of some element (well, sort of work: an element cannot have two attributes with the same name, so the result would be something like <elem v="Whitecraigs"/>). Trying to output attributes without a parent element should result in an error, though I am not convinced that's the best error message. — michael.hor257k
– michael.hor257k, Commented Jun 10 at 16:39

Michael Kay · Accepted Answer · 2025-06-10 20:48:02Z

2

Well, yes, it's a lousy error message - I will see if I can reproduce it.

The actual error is that you are trying to serialize attribute nodes without a containing element. I'm not sure what output method you are using, or exactly where the xsl:sequence instruction fits into the overall code, but I would suggest extracing the string value of the attributes directly by a call on string - use /string(@v) in place of /@v.

I have raised this as a Saxon issue at https://saxonica.plan.io/issues/6818 and the next maintenance release will give you a proper error message.

edited Jun 10 at 20:48

answered Jun 10 at 17:37

Michael Kay

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