Counting distinct items in XSLT

Question

I have XML like this:

<assessment>
 <variables>
  <variable>
   <attributes>
    <variable_name value="FRED"/>
   </attributes>
  </variable>
 </variables>
 <variables>
  <variable>
   <attributes>
    <variable_name value="MORTIMER"/>
   </attributes>
  </variable>
 </variables>
 <variables>
  <variable>
   <attributes>
    <variable_name value="FRED"/>
   </attributes>
  </variable>
 </variables>
</assessment>

I know that with this XSLT:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0">
 <xsl:output method="html"/>

 <xsl:template match="assessment">
  <xsl:for-each select=".//variables/variable/attributes/variable_name">
   <xsl:value-of select="@value"/>
   <br/>
  </xsl:for-each>
 </xsl:template>
</xsl:stylesheet>

I can output the following:

FRED 
MORTIMER 
FRED

But what I really want to output is this:

FRED: 2 
MORTIMER: 1 

That is, I want to list the distinct elements and how many times each occurs. Note that I want the elements to appear in the order of their first appearance (which might rule out some solutions that use sorting).

How do I do this?

Answer 1

This transformation:

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

 <xsl:key name="kValueByVal" match="variable_name/@value"
  use="."/>

 <xsl:template match="/">
  <xsl:for-each select="
   /*/*/variable/attributes/variable_name/@value
             [generate-id()
             =
              generate-id(key('kValueByVal', .)[1])
             ]
   ">
     <xsl:value-of select=
     "concat(., ' ', count(key('kValueByVal', .)), '&#xA;')"/>
  </xsl:for-each>
 </xsl:template>
</xsl:stylesheet>

when applied on the provided XML document, produces the wanted, correct result:

FRED 2
MORTIMER 1