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RE: XPath brain teaser

• To: "Roger L. Costello" <costello@m...>,<xml-dev@l...>
• Subject: RE: XPath brain teaser
• From: "Dare Obasanjo" <dareo@m...>
• Date: Tue, 26 Aug 2003 09:58:07 -0700
• Thread-index: AcNr8hq3PlvMU1zKSN2HDTywL1DqGAAAFzug
• Thread-topic: XPath brain teaser

> -----Original Message-----
> From: Roger L. Costello [mailto:costello@m...] 
> Sent: Tuesday, August 26, 2003 9:46 AM
> To: xml-dev@l...
> Subject:  XPath brain teaser
> 
> These two XPath expressions are equivalent:
> 
>      //para[1]
>      /descendant::para[1]
> 
> (a) True
> (b) False
> 
<snip /> 

> Am I the only one who answered this: (a) True?
> This doesn't seem intuitive to me. Comments?  /Roger
> 

This is how XPath has always worked. From my article "Things To Know and
Avoid When Querying XML Documents with XPath" at
http://msdn.microsoft.com/library/en-us/dnexxml/html/xml06172002.asp 

"Given the information above, we can go back to my original problem
query and see why it gave the unexpected results. 

	//*[position() = 1]/@on-loan 

is actually an abbreviation for 

	/descendent-or-self::node()/child::*[position() = 1]/@on-loan

which selects every node in the document and retrieves the on-loan
attribute of the first child of each of the selected nodes. Judicious
use of parenthesis quickly fixes the problem and 

	(//*)[position() = 1]/@on-loan

 which is short for 

	(/descendent-or-self::node()/child::*)[position() = 1]/@on-loan

 is actually what I wanted. 


ANOTHER EXAMPLE

Abbreviation: //*[1]
Full Query:  /descendent-or-self::node()/child::*[position()=1] 
Query Results: Select the first child of every node in the document.  


Abbreviation: (//*)[1] 
Full Query: (/descendent-or-self::node()/child::*)[position()=1] 
Query Results: Select the first node in the document.  

-- 
PITHY WORDS OF WISDOM 
To succeed in politics, it is often necessary to rise above your
principles.                                                         

This posting is provided "AS IS" with no warranties, and confers no
rights. 

 

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