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Re: [xsl] Are these equivalent XPath expressions for determining that there are no duplicates in a list of items?


Subject: Re: [xsl] Are these equivalent XPath expressions for determining that there are no duplicates in a list of items?
From: Michael Kay <mike@xxxxxxxxxxxx>
Date: Thu, 01 Nov 2012 21:26:11 +0000

As David says: there's the "//" difference, and there's the fact that index-of() has slightly different comparison semantics - "=" throws an error if you compare a number to a string, index-of() effectively returns false.

Both are O(n^2), and are therefore poor solutions given that there's a perfectly good O(n log n) solution, namely

count(Websites/*) = count(distinct-values(Websites/*))

Michael Kay
Saxonica

On 01/11/2012 13:01, Costello, Roger L. wrote:
Hi Folks,

Are there no duplicate websites in this list:

     <Websites>
         <Website id="Amazon"> http://www.amazon.com </Website>
         <Website id="Apple"> http://www.apple.com </Website>
         <Website id="Ebay"> http://www.ebay.com </Website>
         <Website id="Google"> http://www.google.com </Website>
         <Website id="Microsoft"> http://www.microsoft.com </Website>
         <Website id="VirginAtlantic"> http://www.virgin-atlantic.com </Website>
     </Websites>

Here is an XPath 2.0 expression to answer that question (thanks to Dimitre):

empty(Websites/*[index-of(../*,.)[2]])

Here is an XPath 1.0 expression to answer that question:

not(//Websites/*[. = preceding-sibling::*])

Are there any situations where those two XPath expressions differ in their answer?

/Roger


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