[xsl] Nuances of comparisons incorporating node set operands

["G. Ken Holman" <gkholman@xxxxxxxxxxxxxxxxxxxx>]

At 2004-04-02 06:33 +0300, Jarno.Elovirta@xxxxxxxxx wrote:
> > <xsl:for-each select="Reports/GridBody0">
> >
> >      <xsl:if test="Col3 != ''" >
>
> Though correct, it's preferable to use
>
>   not(Col3 = '')

Note that the above two expressions do not always produce the same results, 
though many of my XSL students over the years have assumed that they are 
identical and are surprised to learn the nuances.

If a Col3 element child exists for the current node, they are identical.

If a Col3 element child does not exist for the current node, the first 
expression will produce a false and the second expression will produce a true.

Many forget that a comparison of a node set to another operand is 
initialized to false and if there are no members of the node set the false 
value is returned.  The use of not() in the expression above changes the 
test to true.  An illustration is below.

I hope this is considered helpful, though it is off topic from the original 
post.  I was worried someone might read the above in the context of their 
own transformation problem and assume they were identical.

.................... Ken

T:\ftemp>type jarno.xsl
<?xml version="1.0" encoding="iso-8859-1"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
                xmlns:jarno="test"
                version="1.0">

<xsl:output method="text"/>

<jarno:data1>
  <Col1>abc</Col1>
  <Col3>def</Col3>
</jarno:data1>

<jarno:data2>
  <Col1>abc</Col1>
</jarno:data2>

<xsl:template match="/">
  <xsl:for-each select="document('')/*/jarno:data1">
Data test 1:
    test="Col3 != ''"     - <xsl:value-of select="Col3 != ''"/>
    test="not(Col3 = '')" - <xsl:value-of select="not(Col3 = '')"/>
  </xsl:for-each>
  <xsl:for-each select="document('')/*/jarno:data2">
Data test 2:
    test="Col3 != ''"     - <xsl:value-of select="Col3 != ''"/>
    test="not(Col3 = '')" - <xsl:value-of select="not(Col3 = '')"/>
  </xsl:for-each>
</xsl:template>

</xsl:stylesheet>
T:\ftemp>saxon jarno.xsl jarno.xsl

Data test 1:
    test="Col3 != ''"     - true
    test="not(Col3 = '')" - true
Data test 2:
    test="Col3 != ''"     - false
    test="not(Col3 = '')" - true
T:\ftemp>


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