Altova Mailing List Archives


RE: [xsl] Problem using Position() in .NET environment.

From: Wendell Piez <wapiez@---------------->
To:
Date: 10/14/2004 1:41:00 PM
At 09:38 AM 10/14/2004, it was written:
I would guess that giving a "//" in a match attribute, which we know now is
actually not necessary, makes the pattern somehow invalid - don't know if
this should happen or not, but now that we know, we can at least work around
it.

It doesn't make it invalid: it works fine. Or almost fine.



The '//' operator is explicitly allowed in match patterns. Remember its 
semantics are as though it expands to this:



/descendent-or-self::node()/



so match="//tile" would match as if it were 
"/descendent-or-self::node()/child::title" (although the 
descendant-or-self:: axis itself can't appear in a match pattern).



If you work out how match patterns work -- a pattern returns a Boolean true 
(i.e. it matches) if the node against which it is tested can be reached, 
using the pattern as an XPath expression, from some other place in the 
document -- you'll see this is just the same as match="title". While a 
title element, say, could be selected from its parent using the XPath 
"title" (i.e. "child::title"), it could likewise be selected from anywhere 
at all using the XPath "/descendent-or-self::node()/child::title".



Accordingly the two match patterns are effectively the same; the "//" is 
not wrong, but just unnecessary and probably slower to execute. (The "//" 
operator is very useful in such patterns as "front//title", however.)



Except -- as David C. often has to remind us -- "//title" does *not* have 
the same template assignment priority, according to the rules for 
establishing priority, but is considered a "better match" than the plain 
"title".



It's primarily this last reason, which can lead to unpredictable results 
when such a pattern is used in a stylesheet in which other templates also 
appear, that leads us purists to recommend against it.



My guess is that newbies write "//title" either because they're trying to 
be extra-safe, and think "//title" to be more exact than "title", or 
because they're under the mistaken impression that a match pattern "reaches 
into" the tree and selects a node for processing.



But it isn't, and doesn't, and they need not worry.



I think what the OP is encountering is bugginess in new software, and 
perhaps a misunderstanding on the part of its implementors.



XPath 2.0 has some new rules, but I don't think it should affect this.



Cheers,
Wendell





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Wendell Piez                            mailto:wapiez@xxxxxxxxxxxxxxxx
Mulberry Technologies, Inc.                http://www.mulberrytech.com
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