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Conditionals

XSLT has two conditional instructions: xsl:if for a single yes/no test, and xsl:choose for picking one branch out of several.

xsl:if

xsl:if outputs its body only when the test expression is true. There is no else — for that you need xsl:choose.

<xsl:for-each select="catalog/cd">
  <p>
    <xsl:value-of select="title"/>
    <xsl:if test="price &gt; 10">
      <span style="color:red"> (premium)</span>
    </xsl:if>
  </p>
</xsl:for-each>

Escaping < and > in tests

A stylesheet is XML, so a literal > is fine but < must be written &lt; and, by convention, > is often written &gt;. So "price less than 10" is test="price &lt; 10".

xsl:choose / when / otherwise

xsl:choose is XSLT's switch/if-else-if. The processor takes the first xsl:when whose test is true; if none match, it takes xsl:otherwise (which is optional).

cdcatalog-choose.xsl
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<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/">
<html>
<body>
  <h2>My CD Collection</h2>
  <table border="1">
    <tr bgcolor="#9acd32">
      <th>Title</th>
      <th>Artist</th>
    </tr>
    <xsl:for-each select="catalog/cd">
    <tr>
      <td><xsl:value-of select="title"/></td>
      <xsl:choose>
        <xsl:when test="price &gt; 10">                <!-- (1)! -->
          <td bgcolor="#ff00ff"><xsl:value-of select="artist"/></td>
        </xsl:when>
        <xsl:otherwise>                                <!-- (2)! -->
          <td><xsl:value-of select="artist"/></td>
        </xsl:otherwise>
      </xsl:choose>
    </tr>
    </xsl:for-each>
  </table>
</body>
</html>
</xsl:template>
</xsl:stylesheet>
  1. CDs priced over 10 get a highlighted cell…
  2. …everything else falls through to the default cell.

Against the catalog, only Empire Burlesque (10.90) is over 10, so its artist cell is highlighted:

Title Artist
Empire Burlesque Bob Dylan
Hide your heart Bonnie Tyler
Greatest Hits Dolly Parton

Writing good tests

The test is an XPath expression. XPath coerces the result to a boolean:

Expression True when
price &gt; 10 numeric comparison holds
artist = 'Bob Dylan' string equality holds
year a <year> child exists (non-empty node-set)
not(@discontinued) the discontinued attribute is absent
position() mod 2 = 0 current node is at an even position

That last node exists idiom is the usual way to guard optional content: <xsl:if test="notes">…</xsl:if>.

Next

Tests let you change what you output. XPath predicates push that filtering into the node selection itself.