Flow Control
- How to implement flow control in XSLT.
- How to loop through XPath result-sets.
- How to sort results.
- How to write if statements in XSLT.
- How to use xsl:choose for more complicated conditionals.
Looping in XSLT
The tag for looping in XSLT is <xsl:for-each>. It takes the select attribute, which uses XPath to point to a node-set and it outputs the contents of the xsl:for-each one time for each node in the set.
Code Sample: FlowControl/Demos/BeatlesForEach.xsl
<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="html"/>
<xsl:template match="/">
<html>
<head>
<title>Beatles</title>
</head>
<body>
<table border="1">
<xsl:for-each select="beatles/beatle">
<tr>
<td><a href="{@link}"><xsl:value-of select="name/lastname"/></a></td>
<td><a href="{@link}"><xsl:value-of select="name/firstname"/></a></td>
</tr>
</xsl:for-each>
</table>
</body>
</html>
</xsl:template>
</xsl:stylesheet>
In the code above, you will also notice that the Beatles' first and last names are made into links. The value of the href attribute of the <a> tag is {@link}. This is the equivalent of <xsl:value-of select="@link"/>. However, it would not be well-formed XML to place the <xsl:value-of /> tag (or any tag, for that matter) inside the angle brackets of another open tag, so the curly-bracket syntax is used instead. The output in a browser would look like this.
Sorting with XSLT
The tag for sorting in XSLT is <xsl:sort>, which takes the select attribute with a value of an XPath to identify the node to sort by. The xsl:sort element can also take the order attribute, which has two possible values: ascending (the default) and descending.
Code Sample: FlowControl/Demos/BeatlesSort.xsl
<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="html"/>
<xsl:template match="/">
<html>
<head>
<title>Beatles</title>
</head>
<body>
<table border="1">
<xsl:for-each select="beatles/beatle">
<xsl:sort select="name/lastname" order="descending"/>
<tr>
<td><a href="{@link}"><xsl:value-of select="name/lastname"/></a></td>
<td><a href="{@link}"><xsl:value-of select="name/firstname"/></a></td>
</tr>
</xsl:for-each>
</table>
</body>
</html>
</xsl:template>
</xsl:stylesheet>
Notice that the xsl:sort is nested within an xsl:for-each element. This is very common as xsl:sort elements need to be nested within repeating structures.
When FlowControl/Demos/BeatlesSort.xml is transformed against FlowControl/Demos/BeatlesSort.xsl, the output is the same as it would be when transformed against FlowControl/Demos/BeatlesForEach.xsl , except the results are sorted by last name in descending order.
Conditions with XSLT
xsl:if
The <xsl:if> tag is used to create a simple if condition. Its test attribute holds the condition, which is written in the form of an XPath, often using the XPath operators in the "XPath Operators" table.
Code Sample: FlowControl/Demos/BeatlesIf.xsl
<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="html"/>
<xsl:template match="/">
<html>
<head>
<title>Beatles</title>
</head>
<body>
<table border="1">
<tr>
<th>First Name</th>
<th>Last Name</th>
</tr>
<xsl:for-each select="beatles/beatle">
<xsl:sort select="name/lastname" />
<xsl:if test="not(@real='no')">
<tr>
<td><xsl:value-of select="name/firstname"/></td>
<td><xsl:value-of select="name/lastname"/></td>
</tr>
</xsl:if>
</xsl:for-each>
</table>
</body>
</html>
</xsl:template>
</xsl:stylesheet>
The xsl:for-each loop causes XSLT to look at each beatle element. The nested xsl:if element uses XPath to test to see if the value of real attribute of the beatle element is no. It then negates the result with the not() function. This way beatle elements that have no real attribute and beatle elements that do have a real attribute with the value of ANYTHING BUT no will both be included in the result set.
When FlowControl/Demos/BeatlesIf.xml is transformed against FlowControl/Demos/BeatlesIf.xsl, the output looks like this in a browser.
Note that there are no <xsl:else> or <xsl:elseif> elements.
xsl:choose
For multi-level conditions, <xsl:choose> is used. It takes as children one or more <xsl:when> tags with an optional <xsl:otherwise> tag at the end.
Code Sample: FlowControl/Demos/BeatlesChoose.xsl
<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="html"/>
<xsl:template match="/">
<html>
<head>
<title>Beatles</title>
</head>
<body>
<table border="1">
<tr>
<th>First Name</th>
<th>Last Name</th>
</tr>
<xsl:for-each select="beatles/beatle">
<xsl:sort select="name/lastname" />
<xsl:choose>
<xsl:when test="not(@real='no')">
<tr bgcolor="red">
<td><xsl:value-of select="name/firstname"/></td>
<td><xsl:value-of select="name/lastname"/></td>
</tr>
</xsl:when>
<xsl:otherwise>
<tr bgcolor="orange">
<td><s><xsl:value-of select="name/firstname"/></s></td>
<td><s><xsl:value-of select="name/lastname"/></s></td>
</tr>
</xsl:otherwise>
</xsl:choose>
</xsl:for-each>
</table>
</body>
</html>
</xsl:template>
</xsl:stylesheet>
This code ouputs a table row with a red background for every real Beatle and a table row with an orange background for every fake Beatle. The fake Beatles are also crossed out with the <s> tag.
When FlowControl/Demos/BeatlesChoose.xml is transformed against FlowControl/Demos/BeatlesChoose.xsl, the output looks like this in a browser.
Flow Control Conclusion
In this lesson of the XSLT tutorial, you learned how loop through nodesets and how to adjust the output of an XSLT based on conditions.