Re: [xsl] Stuck with select distinct

[Wendell Piez <wapiez@xxxxxxxxxxxxxxxx>]

Mark,

One can frequently use raw XPath to achieve the same end as using a 
key. It's just that for certain applications in certain processors, 
this can give very poor performance.

In order to get all 'colour' values, as you know, you can traverse "//colour".

In order to exclude Red and Blue, use a predicate:

//colour[not(.='Red' or 'Blue')]

You have also stipulated that "rgb value is not FFFFFF". But 'colour' 
sometimes has several 'rgb' element siblings. Your source code sample 
suggests it's the 'rgb' value directly following each 'colour' that 
is a concern, so:

//colour[not(.='Red' or 'Blue')][not(following-sibling::rgb[1]='FFFFFF']

which returns all colour elements whose value is not 'Red' or 'Blue' 
and whose directly succeeding 'rgb' element's value is not 'FFFFFF'.

To pull the distinct values from this set without a key, the easiest 
thing is to perform a test within your for-each (or your matched 
template), as in

<xsl:for-each select="$colours">
  <xsl:variable name="pos" select="position()"/>
  <xsl:if test="not(. = $colours[position &lt; $pos])">
    ...

... which is very direct and useful for many purposes.

Not for all, however. (For example, if positions are going to be 
important within the for-each.) Then you have to filter $colours 
itself to keep only distinct values, which means brute-force testing 
using the predicates again:

$colours[not(. = preceding::colour[not(.='Red' or 'Blue')]
                  [not(following-sibling::rgb[1]='FFFFFF'])]

... which is so outlandish (and potentially so poor in performance) 
that one wonders why you'd "prefer to avoid using keys if possible", 
since a key is so straightforward:

<xsl:key name="c" use="."
  match="colour[not(.='Red' or 'Blue')]
               [not(following-sibling::rgb[1]='FFFFFF']"/>

and then select="$colours[generate-id()=generate-id(key('c',.)[1])]"/>

Cheers,
Wendell

At 11:36 AM 11/8/2008, you wrote:
> Hi All
>
> I'm trying to get a list of distinct items from an XML. I've done 
> this many times using a predicate containing a preceding axis, but 
> this one has got me stumped:
>
> <page>
>         <front_back>F</front_back>
>         <page_no>1</page_no>
>         <colours>
>                 <colour>Red</colour>
>                 <rgb>00FFFF</rgb>
>                 <colour>Green</colour>
>                 <rgb>00FF00</rgb>
>                 <colour>Blue</colour>
>                 <rgb>FFFF00</rgb>
>         </colours>
> </page>
> <page>
>         <front_back>F</front_back>
>         <page_no>2</page_no>
>         <colours>
>                 <colour>Green</colour>
>                 <rgb>FFFFFF</rgb>
>         </colours>
> </page>
> <page>
>         <front_back>F</front_back>
>         <page_no>3</page_no>
>         <colours>
>                 <colour>Green</colour>
>                 <rgb>00FF00</rgb>
>         </colours>
> </page>
>
> I need to return a nodeset with a list of DISTINCT colour nodes, 
> that I can then process in a for-each element.
>
> The other conditions for selection are:
>
>     colour is not Red or Blue
>     rgb value is not FFFFFF
>
>
> I somehow need to combine the following predicates (I think)
>
>     colours/colour[. != 'Red' and . != 'Blue']
>     colours/rgb[. != 'FFFFFF']
>     colours/colour[not(. = preceding-sibling::colour)]
>
> I'm stuck with XSL 1.0 and would like to avoid using keys if possible
>
> Any suggestions greatly appreciated
>
> Regards
>
> Mark


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Wendell Piez                            mailto:wapiez@xxxxxxxxxxxxxxxx
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