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Re: [xsl] preceding with the same ancestor as self


Subject: Re: [xsl] preceding with the same ancestor as self
From: "Michael Kay mike@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Fri, 27 Feb 2015 21:14:56 -0000

> I think I have it:
>
> preceding::fig[.!="" and ancestor::grp = current()/ancestor::grp][1]

It's a bad mistake to use "=" to test whether two nodes are the same node (so
to speak). You're actually comparing the string values of the nodes, which is
a very expensive operation for nodes that have large subtrees, and if you're
unlucky it could give you a false positive. In 2.0, use the "is" operator. In
1.0, compare node identity using generate-id(), or count($X|$Y)=1.

The other problem with this expression is that if there isn't a preceding
figure within the subtree, it could take a long search to find out (back to
the start of the document).

It might be worth noting that preceding::x gives the same nodes as
ancestor-or-self::*/preceding-sibling::*/descendant-or-self::x. Using this
equivalence, you can stop the search earlier by qualifying the first step:

ancestor-or-self::*[ancestor::grp]/preceding-sibling::*/descendant-or-self::x

Michael Kay
Saxonica

>
> -----Original Message-----
> From: Rick Quatro rick@xxxxxxxxxxxxxx
> [mailto:xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx]
> Sent: Friday, February 27, 2015 3:44 PM
> To: xsl-list@xxxxxxxxxxxxxxxxxxxxxx
> Subject: [xsl] preceding with the same ancestor as self
>
> Hi,
>
> When I am at a <fig> with no text, I want to find the previous <fig> that
> has text, but only within the same ancestor group.
>
> I am using this:
>
> preceding::fig[.!=""][1]
>
> This works for all of the empty <fig> elements in the first <grp>, but in
> the second <grp>, it incorrectly picks up the <fig> with the 4 value from
> the previous <grp>. How can I restrict the preceding axis so a found node
> will have the same ancestor as the context node? Thanks.
>
> Rick
>
>
> <?xml version="1.0" encoding="UTF-8"?>
> <doc>
>    <grp>
>        <fig/>
>        <fig>1</fig>
>        <fig>2</fig>
>        <fig/>
>        <sub>
>            <fig>3</fig>
>        </sub>
>        <fig/>
>        <fig>4</fig>
>    </grp>
>    <grp>
>        <fig/>
>        <fig>1</fig>
>        <fig>2</fig>
>        <fig>3</fig>
>        <sub>
>            <fig/>
>        </sub>
>        <fig/>
>    </grp>
> </doc>


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