RE: Boolean true() and false() as strings?

Ah yes, boolean conversion of result tree fragments.  The first thing you
need to know to explain the behavior of your stylesheet is that whenever an
<xsl:variable> element is not empty, it will always return a value of type
Result Tree Fragment.  More to the point, <xsl:value-of/> always copies the
*string-value* of the designated XPath expression, boolean or otherwise.

Try hard-coding the true() value, first as a child of <xsl:variable> and
then as the value of the select attribute.

This returns an RTF:
  <xsl:variable name="units_gt_300">
    <xsl:value-of select="true()"/>
  </xsl:variable>

This returns an XPath boolean:
  <xsl:variable name="units_gt_300" select="true()"/>

Your if test will evaluate differently depending on the variable's type.

In the first case, the comparison is between an RTF and a string.  The
processor first converts the RTF to a string and subsequently performs a
string comparison.  Thus, you are effectively comparing 'true' with 'true'
or 'false' with 'true'.  If you changed the if test to "$units_gt_300" or
"$units_gt_300=true()" (same thing), it would not work correctly, because
'true' and 'false' will always evaluate to true, being non-empty strings.

In the second case, the comparison is between a boolean and a string.  The
string is first converted to a boolean and then the comparison is made.  If
the string you include is not empty, then it will always convert to true,
which effectively gives you the same result as "$units_gt_300" or
"$units_gt_300='anything'".

This behavior is described in the XPath spec as follows:

"If at least one object to be compared is a boolean, then each object to be
compared is converted to a boolean as if by applying the boolean
function." -- http://www.w3.org/TR/xpath#booleans

Hope this helps!  You're keeping me on my toes ;)

Evan Lenz
elenz@xxxxxxxxxxx
http://www.xyzfind.com
XYZFind, the search engine *designed* for XML
Download our free beta software: http://www.xyzfind.com/beta




-----Original Message-----
From: owner-xsl-list@xxxxxxxxxxxxxxxx
[mailto:owner-xsl-list@xxxxxxxxxxxxxxxx]On Behalf Of John E. Simpson
Sent: Thursday, October 05, 2000 1:18 PM
To: xsl-list@xxxxxxxxxxxxxxxx
Subject: Boolean true() and false() as strings?


A curious thing (well, to me!)... consider this template rule:

<xsl:template match="sales">
   <h2>Regions Selling More than 300 Units:</h2>
   <xsl:for-each select="region">
     <xsl:variable name="units_gt_300">
       <xsl:choose>
         <xsl:when test="number(units) &gt; 300">
           <xsl:value-of select="true()"/>
         </xsl:when>
         <xsl:otherwise>
           <xsl:value-of select="false()"/>
         </xsl:otherwise>
       </xsl:choose>
     </xsl:variable>
     <xsl:if test="$units_gt_300='true'">
       <h3><xsl:value-of select="concat(@name, ' (', units, '
units)')"/></h3>
     </xsl:if>
   </xsl:for-each>
</xsl:template>

Note the assignment of Boolean true() or false() to the variable, and the
<xsl:if> test. Why can't the latter be simply:

   <xsl:if test="$units_gt_300">

i.e., why the need to test vs. the string value "true"? Does using the
true() and false() functions in a variable coerce their return values to
string type rather than their "native" Boolean?

Here's a simple XML doc to run it against:

<sales quarter="2001-01">
   <region name="Northeast">
     <units>374</units>
     <amount>12500.26</amount>
   </region>
   <region name="Southeast">
     <units>512</units>
     <amount>17692</amount>
   </region>
   <region name="Southwest">
     <units>161</units>
     <amount>8349.72</amount>
   </region>
   <region name="Northwest">
     <units>465</units>
     <amount>15239.6</amount>
   </region>
</sales>

(Tested with both Saxon and the Sept. release of IE5.5, btw.)
==========================================================
John E. Simpson               | "Curiosity killed the cat,
http://www.flixml.org         | but for a while I was a
XML Q&A: http://www.xml.com   | suspect." (Steven Wright)


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