Conditional Templates

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Conditional templates are output only if certain conditions exist within the source document. Conditional templates are defined with the <xsl:if> and <xsl:choose> elements.

In the following portfolio data, the "stock" element has an attribute named "exchange". You might want to generate some output only when this attribute has a certain value.

<?xml version="1.0"?>
<portfolio xmlns:dt="urn:schemas-microsoft-com:datatypes">
  <stock exchange="nyse">
    <name>zacx corp</name>
    <symbol>ZCXM</symbol>
    <price dt:dt="number">28.875</price>
  </stock>
  <stock exchange="nasdaq">
    <name>zaffymat inc</name>
    <symbol>ZFFX</symbol>
    <price dt:dt="number">92.250</price>
  </stock>
  <stock exchange="nasdaq">
    <name>zysmergy inc</name>
    <symbol>ZYSZ</symbol>
    <price dt:dt="number">20.313</price>
  </stock>
</portfolio>

You can easily create another row in the table in which to place the attribute value. However, if you want to indicate stocks from a particular exchange, not by a completely new table row, but by noting them with an asterisk (*). The <xsl:if> element provides a mechanism for conditionally inserting structure into the output tree.

In the following example, the <xsl:if> element inserts an asterisk after the symbol for those stocks listed on the NASDAQ stock exchange. The <xsl:if> contents can be simple text, as in this example, or any structure allowed by XSLT, such as elements or attributes.

<?xml version='1.0'?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
  <xsl:template match="/">
    <HTML>
      <BODY>
        <TABLE BORDER="2">
          <TR>
            <TD>Symbol</TD>
            <TD>Name</TD>
            <TD>Price</TD>
          </TR>
          <xsl:for-each select="portfolio/stock">
            <TR>
            <TD>
                <xsl:value-of select="symbol"/>
                <xsl:if test="@exchange[.='nasdaq']">*</xsl:if>
              </TD>
              <TD><xsl:value-of select="name"/></TD>
              <TD><xsl:value-of select="price"/></TD>
            </TR>
          </xsl:for-each>
        </TABLE>
        <P>* Listed on Nasdaq stock exchange</P>
      </BODY>
    </HTML>
  </xsl:template>
</xsl:stylesheet>

The test attribute takes a pattern. If the query described by the pattern selects one or more nodes, the <xsl:if> template is inserted. If the selection is empty, the conditional is skipped. In this case, the query checks to see if the <stock> element has an exchange attribute, and then verifies that the value of the exchange attribute is equal to "nasdaq". For more information about qualifying a pattern using brackets ([]) and constructing patterns that compare values, see Introduction to the Syntax of XPath.

Choosing Alternatives

The <xsl:choose> element provides a mechanism for "either/or" processing. <xsl:choose> contains a series of <xsl:when> elements that are tested in order from top to bottom until a match is found. An <xsl:otherwise> element can be used to insert a template if no match is found.

The following code can be added to the preceding example to color-code the rows by price. 0-25 are displayed in green; 25-50 are displayed in blue; and 50 or higher are displayed in red. The color is changed by conditionally generating a portion of the value of the STYLE attribute on the table row.

<TR>
  <xsl:attribute name="STYLE">color:
    <xsl:choose>
      <xsl:when test="price[. &lt; 25]">green</xsl:when>
      <xsl:when test="price[. &lt; 50]">blue</xsl:when>
      <xsl:otherwise>red</xsl:otherwise>
    </xsl:choose>
  </xsl:attribute>
  <TD>
    ...

 Last updated on Saturday, April 10, 2004

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