How to use XSLT to create distinct values

I have XML like this:

<items>
  <item>
    <products>
      <product>laptop</product>
      <product>charger</product>
    </products>
  </item>
  <item>
    <products>
      <product>laptop</product>
      <product>headphones</product>  
    </products>  
  </item>
</items>

I want it to output like

laptop
charger
headphones

I was trying to use distinct-values() but I guess i m doing something wrong. Can anyone tell me how to achieve this using distinct-values()? Thanks.

<xsl:template match="/">			
  <xsl:for-each select="//products/product/text()">
    <li>
      <xsl:value-of select="distinct-values(.)"/>
    </li>				
  </xsl:for-each>
</xsl:template>

but its giving me output like this:

<li>laptop</li>
<li>charger</li>
<li>laptop></li>
<li>headphones</li>

An XSLT 1.0 solution that uses key and the generate-id() function to get distinct values:

<?xml version="1.0" encoding="UTF-8"?>
  <xsl:stylesheet
   version="1.0"
   xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="xml" encoding="UTF-8" indent="yes"/>

<xsl:key name="product" match="/items/item/products/product/text()" use="." />

<xsl:template match="/">

  <xsl:for-each select="/items/item/products/product/text()[generate-id()
                                       = generate-id(key('product',.)[1])]">
    <li>
      <xsl:value-of select="."/>
    </li>
  </xsl:for-each>

</xsl:template>

</xsl:stylesheet>

Here's an XSLT 1.0 solution that I've used in the past, I think it's more succinct (and readable) than using the generate-id() function.

  <xsl:template match="/">           
    <ul> 
      <xsl:for-each select="//products/product[not(.=preceding::*)]">
        <li>
          <xsl:value-of select="."/>
        </li>   
      </xsl:for-each>            
    </ul>
  </xsl:template>

Returns:

<ul xmlns="http://www.w3.org/1999/xhtml">
  <li>laptop</li>
  <li>charger</li>
  <li>headphones</li>
</ul>

You don't want "output (distinct-values)", but rather "for-each (distinct-values)":

<xsl:template match="/">              
  <xsl:for-each select="distinct-values(/items/item/products/product/text())">
    <li>
      <xsl:value-of select="."/>
    </li>
  </xsl:for-each>
</xsl:template>

I came to this problem while working with a Sitecore XSL rendering. Both the approach that used key() and the approach that used the preceding axis performed very slowly. I ended up using a method similar to key() but that did not require using key(). It performs very quickly.

<xsl:variable name="prods" select="items/item/products/product" />
<xsl:for-each select="$prods">
  <xsl:if test="generate-id() = generate-id($prods[. = current()][1])">
    <xsl:value-of select="." />
    <br />
  </xsl:if>
</xsl:for-each>

distinct-values(//product/text())

I found that you can do what you want with XSLT 1.0 without generate-id() and key() functions.

Here is Microsoft-specific solution (.NET's XslCompiledTransform class, or MSXSLT.exe or Microsoft platfocm COM-objects).

It is based on this answer. You can copy sorted node set to variable ($sorted-products in the stylesheet below), then convert it to node-set using ms:node-set function. Then you able for-each second time upon sorted node-set:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0" xmlns:ms="urn:schemas-microsoft-com:xslt" exclude-result-prefixes="ms">

  <xsl:output method="html" indent="yes" />

  <xsl:template match="/">
    <xsl:variable name="sorted-products">
        <xsl:for-each select="//products/product">
            <xsl:sort select="text()" />
            
            <xsl:copy-of select=".|@*" />
        </xsl:for-each>
    </xsl:variable>
    
    <xsl:variable name="products" select="ms:node-set($sorted-products)/product" />

    <xsl:for-each select="$products">
      <xsl:variable name='previous-position' select="position()-1" />
      
      <xsl:if test="normalize-space($products[$previous-position]) != normalize-space(./text())">
        <li>
          <xsl:value-of select="./text()" />
        </li>
      </xsl:if>
    </xsl:for-each>
  </xsl:template>
  
</xsl:stylesheet>

output:

<li>charger</li>
<li>headphones</li>
<li>laptop</li>

You can try it out in online playground.