Re: [xsl] Selecting the first node set

["G. Ken Holman" <gkholman@xxxxxxxxxxxxxxxxxxxx>]

At 2006-08-05 12:21 +0100, Fraser Goffin wrote:
> Given the following simplified XML instance, can
> anyone suggest an expression that will return
> ONLY the 'Value' node containing <enum> children
> with values A, B and C. I can't rely on the name
> of the parent node of <Value> hence I have
> deliberately named these as UnknownNameX or any other intervening structure.
>
> <Trade>
>         <UnknownName1>
>                 <Value>
>                         <enum>A</enum>
>                         <enum>B</enum>
>                         <enum>C</enum>
>                 </Value>
>         </UnknownName1>
>         <UnknownName2>
>                 <Value>/Trade//Value[ancestor::Trade]</Value>
>         </UnknownName2>
>         <Treatment>
>                 <UnknownName3>
>                         <Value>
>                                 <enum>D</enum>
>                                 <enum>E</enum>
>                                 <enum>F</enum>
>                         </Value>
>                 </UnknownName3>
>         </Treatment>
> </Trade>
>
>
> /Trade//Value        - returns all 3 <Value> nodes
> /Trade//Value[1]    - also returns all 3 <Value> nodes (why is that ?)
> /Trade//Value[2]    - returns nothing (why is that ?)

A number of people have responded already, but
from what I can tell, the two that have supplied
suggested replacements haven't acknowledged that
the <Value> elements are at different depths of
the tree.  I note that the third <Value> is at
level 4 while the other two are at level 3.

So, I suggest that the following will get you the
first <Value> in the document regardless of the depth of the element:

  (//Value)[1]

Hugh explained why your prior attempts don't work
... my wording for this is as follows: //Value[1]
won't work because the predicate is bound at the
step level so "//" looks through the entire
document and "Value[1]" which is an abbreviation
for "child::Value[1]" returns the first child
named "Value", so the combination returns "the
first child named Value at every level of the document".

This is a trick question I have had in my
hands-on XSLT training class since 1999 when it
was still combined with DSSSL training.  I've
also see it at conferences where vendors post it
on a white board as a "challenge" to stylesheet
writers to determine who knows their XPath and who doesn't.

I hope this helps, Fraser,

. . . . . . . . . . . . . . Ken

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