comparing nodes in xslt

[inquirydog]

Hi-

Does anyone know a way to compare whether two nodes contain
the same information in xslt (the name, attributes, and all content
recursivly should be the same. I am interested in the case where node
ordering matters, and also the case where it doesn't, but perhaps that
is an advanced topic). Ideally the method should be available to
xpath expressions, so I think that creating new templates which
compare nodes will not work (well, it will if you store the results of
the apply-template in a variable and then use that in an expression,
but this seems way too complex at times, especially since variables
are fixed after they are evaluated).

thanks
-I

 

[Joris Gillis]

Hi,

I'm not quite sure this reply will be of any use to you.
This solution only works with nodes with the same ordering.
As far as I know, there are not (yet) Xpath expressions to deal with
node comparison (or operations).

to compare e.g. node[1] and node[2], use :

<xsl:variable name="identical">
<xsl:call-template name="check_identical">
<xsl:with-param name="comp1"><xsl:copy-of
select="//node[1]"/></xsl:with-param>
<xsl:with-param name="comp2"><xsl:copy-of
select="//node[2]"/></xsl:with-param>
</xsl:call-template>
</xsl:variable>
<xsl:if test="$identical='true'">nodes completely are
identical</xsl:if>
<xsl:if test="$identical='false'">nodes differ in at least one
aspect</xsl:if>

These 2 templates should be included/inserted in the stylesheet:

<xsl:template name="check_identical">
<xslaram name="comp1"/>
<xslaram name="comp2"/>
<xsl:variable name="string1">
<xsl:call-template name="stringify">
<xsl:with-param name="node"><xsl:copy-of
select="$comp1"/></xsl:with-param>
</xsl:call-template>
</xsl:variable>
<xsl:variable name="string2">
<xsl:call-template name="stringify">
<xsl:with-param name="node"><xsl:copy-of
select="$comp2"/></xsl:with-param>
</xsl:call-template>
</xsl:variable>
<xsl:value-of select="$string1=$string2"/>
</xsl:template>

The following template 'stringifies' a given node i.e. convert it to
the ascii equivalent like one would see when opening an xml file with
a text editor.

<xsl:template name="stringify">
<xslaram name="node"/>
<xsl:for-each select="$node/*/*|$node/*/text()">
<xsl:choose>
<xsl:when test="boolean(local-name())">
&lt;<xsl:value-of select="local-name()"/>
<xsl:variable name="pos" select="position()"/>
<xsl:for-each select="@*">
<xsl:text> </xsl:text><xsl:value-of
select="local-name()"/>="<xsl:value-of select="."/>"
</xsl:for-each>
<xsl:call-template name="stringify">
<xsl:with-param name="node"><xsl:copy-of
select="."/></xsl:with-param>
</xsl:call-template>
&lt;/<xsl:value-of select="local-name()"/>&gt;
</xsl:when>
<xsltherwise><xsl:value-of
select="normalize-space(.)"/></xsltherwise>
</xsl:choose>
</xsl:for-each>
</xsl:template>

Joris Gillis

 

[inquirydog]

Hi-

Thanks for the reply. I looked into it a bit and found that
in XSLT 2 you can define xpath functions using xslt elements. This
certainly makes a solution like this more reasonable. It does still
leave certain problems like counting repeat nodes in a list.... For
instance, if you wanted to eliminate doubles in a list like this

<list>
<element name="1" />
<element name="2" />
<element name="1" />
<element name="1" />
<element name="1" />
<element name="2" />
</list>

converting to

<list>

<element name"1" count="4" />
<element name="2" count="2" />

</list>

you need to group by element type and then count the number of
elements. Except for very constrained cases this seems to me to be
impossible to do in xslt (yes I know about the muenchian method- it
only works in a constrained case). To do this grouping you need to
first be able to check for node equality (which I think you can do in
xslt 2) and then do the grouping based on the equality (which is
better supported in xslt 2 but still can't do what I describe in this
email).

thanks
-I

Hi,

I'm not quite sure this reply will be of any use to you.
This solution only works with nodes with the same ordering.
As far as I know, there are not (yet) Xpath expressions to deal with
node comparison (or operations).

to compare e.g. node[1] and node[2], use :

<xsl:variable name="identical">
<xsl:call-template name="check_identical">
<xsl:with-param name="comp1"><xsl:copy-of
select="//node[1]"/></xsl:with-param>
<xsl:with-param name="comp2"><xsl:copy-of
select="//node[2]"/></xsl:with-param>
</xsl:call-template>
</xsl:variable>
<xsl:if test="$identical='true'">nodes completely are
identical</xsl:if>
<xsl:if test="$identical='false'">nodes differ in at least one
aspect</xsl:if>

These 2 templates should be included/inserted in the stylesheet:

<xsl:template name="check_identical">
<xslaram name="comp1"/>
<xslaram name="comp2"/>
<xsl:variable name="string1">
<xsl:call-template name="stringify">
<xsl:with-param name="node"><xsl:copy-of
select="$comp1"/></xsl:with-param>
</xsl:call-template>
</xsl:variable>
<xsl:variable name="string2">
<xsl:call-template name="stringify">
<xsl:with-param name="node"><xsl:copy-of
select="$comp2"/></xsl:with-param>
</xsl:call-template>
</xsl:variable>
<xsl:value-of select="$string1=$string2"/>
</xsl:template>

The following template 'stringifies' a given node i.e. convert it to
the ascii equivalent like one would see when opening an xml file with
a text editor.

<xsl:template name="stringify">
<xslaram name="node"/>
<xsl:for-each select="$node/*/*|$node/*/text()">
<xsl:choose>
<xsl:when test="boolean(local-name())">
&lt;<xsl:value-of select="local-name()"/>
<xsl:variable name="pos" select="position()"/>
<xsl:for-each select="@*">
<xsl:text> </xsl:text><xsl:value-of
select="local-name()"/>="<xsl:value-of select="."/>"
</xsl:for-each>
<xsl:call-template name="stringify">
<xsl:with-param name="node"><xsl:copy-of
select="."/></xsl:with-param>
</xsl:call-template>
&lt;/<xsl:value-of select="local-name()"/>&gt;
</xsl:when>
<xsltherwise><xsl:value-of
select="normalize-space(.)"/></xsltherwise>
</xsl:choose>
</xsl:for-each>
</xsl:template>

Joris Gillis

 

[David Carlisle]

Hi-

Thanks for the reply. I looked into it a bit and found that
in XSLT 2 you can define xpath functions using xslt elements. This
certainly makes a solution like this more reasonable. It does still
leave certain problems like counting repeat nodes in a list.... For
instance, if you wanted to eliminate doubles in a list like this

<list>
<element name="1" />
<element name="2" />
<element name="1" />
<element name="1" />
<element name="1" />
<element name="2" />
</list>

converting to

<list>

<element name"1" count="4" />
<element name="2" count="2" />

</list>

you need to group by element type and then count the number of
elements. Except for very constrained cases this seems to me to be
impossible to do in xslt (yes I know about the muenchian method- it
only works in a constrained case). To do this grouping you need to
first be able to check for node equality (which I think you can do in
xslt 2) and then do the grouping based on the equality (which is
better supported in xslt 2 but still can't do what I describe in this
email).

What do you mean by

but still can't do what I describe in this email.

?

Your grouping example seems quite simple to do in either XSLT1 or XSLT2,
it might loook a bit simpler in 2 using the explict xsl:for-each-group
mechanism, but it doesn't take much code in either case.

For example in 1.0 it would be:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

<xsl:key name="x" match="element" use="@name"/>

<xsl:template match="list">
<list>
<xsl:for-each select="element[generate-id()=generate-id(key('x',@name))]">
<element name="{@name}" count="{count(key('x',@name))}"/>
</xsl:for-each>
</list>
</xsl:template>

</xsl:stylesheet>



David

 

[inquirydog]

Hi-

The method you describe is the Muenchian Method that I
mentioned in my posting.... And it only works for very simple cases,
where 1). you know ahead of time where the list to be counted is, and
2). where the list items are trivial content (all of the same type).
For instance just change one of the elements by adding content to it

<element name="1"> => <element name="1"><content /></element>

the example counts this as the same type of element as the ones
without content.

I repeat what I stated before- XSLT 1.0 and 2.0 are (I am pretty sure)
not capable of counting repeat nodes in a list in the general case,
where the list can appear anywhere in the node tree and where the
elements in the list are not of some predetermined type (they can
contain arbitrary content). This is a limitation of xslt that I
believe should be fixed in future revisions.

thanks
-I

 

[David Carlisle]

You say

<element name="1"> => <element name="1"><content /></element>

the example counts this as the same type of element as the ones
without content.

well yes of course because you (and so I) are only grouping on the name
attribute. If you want to consider something else as being a
distinguishing factor then just include it in the test eg
use
concat(@name,':',count(*))
would distinguish nodes based on their name attribute and the number of
children. I suspect that you want a test that is more like deep-equal
(which is a predefined function in xpath2) so perhaps your request

This is a limitation of xslt that I
believe should be fixed in future revisions.

has been granted in Xpath2. If that is the case you probably can't express that as
a single Xpath in Xpath 1 (without using an extension function,) but you
can certainly express it in xslt1 so you can do the grouping, you
probably just can't use the optimisation of using a key as the key index
needs to be expressed as a single xpath. But keys are only an
optimisation feature anything expressable using keys is expressable
without them (just may take longer to run)

of course any xpath extension that allows xpath functions to be defined
using xslt would allow this kind of key to be used even in an xslt1
context (saxon and the exslt project have such function definition
possibilitiesfor xslt1, and they are a standard part of xslt2 draft)

David