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Firstly, //b|c and //c|b don't necessarily return the same nodes. The
expression parses as (//b)|c, not as //(b|c).
Secondly, the XPath 1.0 specification defines that a path expression (or a
union expression) returns a node-set, that is, an unordered set of nodes.
Some host languages, for example XSLT 1.0, specify that node-sets are always
processed in document order. But you appear (as far as I can tell) to be
invoking XPath from some Microsoft API, and I've no idea what that API says
about the processing order: it's up to the XPath host language to define it,
or it could choose to leave it undefined.
This changes in XPath 2.0, which specifies that path expressions and union
expressions return a sequence of distinct nodes in document order.
> -----Original Message-----
> From: Tor Helland [mailto:email@example.com]
> Sent: 02 October 2004 09:17
> To: firstname.lastname@example.org
> Subject: [xml-dev] xpath union and node set order
> Hi, I'm looking for an answer to this question:
> Am I right in saying that both these paths should return nodes in the
> same order?
> on this document:
> and that the order should be the document order:
> Or is the order something I can't count on?
> I'm using a DOM, and xpath through the msxml-like interface method
> IDomNodeSelect.selectNodes. And that particular DOM returns
> the node set for
> "//c|b" in this order:
> I found a couple of things in the spec that may be relevant,
> listed below.
>  says the node set is unordered, so it is implementation
> dependent. 
> and  are very specific on document order -  when used with a
> predicate,  regarding proximity position.
> Is there a difference between the finally resulting node set
> and the node
> set in the expression? I would think the resulting node set
> should be like
> the second part of the example in , using the child axis,
> and in document
>  says I have to specify an ordered node set, or I will get
> an unordered
> on. IDomNodeSelect.selectNodes does not follow , but it
> does suggest that
> I can't rely on document order.
> With  and  being very specific about document order, I get the
> impression that it it allowed to scramble the node set order before
> delivering the results ;-)
> I based some code on the assumption that I would always get
> document order.
> If I'm right, I'll file a bug report for the DOM, and maybe
> fix it. If not,
> my code is rather flawed. I'll have to fix either the DOM or
> my code...
> Bits from the spec (http://www.w3.org/TR/xpath):
>  Section 3.3
> The meaning of a Predicate depends crucially on which axis
> applies. For
> example, preceding::foo returns the first foo element in
> reverse document
> order, because the axis that applies to the  predicate is
> the preceding
> axis; by contrast, (preceding::foo) returns the first foo
> element in
> document order, because the axis that applies to the 
> predicate is the
> child axis.
>  Section 1
> node-set (an unordered collection of nodes without duplicates)
>  Section 2.4
> The proximity position of a member of a node-set with respect
> to an axis is
> defined to be the position of the node in the node-set
> ordered in document
> order if the axis is a forward axis and ordered in reverse
> document order if
> the axis is a reverse axis.
> And from DOM Level 3 XPath
>  Section 1.4
> If the natural result is a node set when ANY_TYPE was requested, then
> UNORDERED_NODE_ITERATOR_TYPE is always the resulting type. Any other
> representation of a node set must be explicitly requested.
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