XSL Conditionals

This tutorial is designed as a supplement to the XSL Conditionals tutorial, which covers two conditional branching structures in detail, but does not cover XPath if-then-else. XPath if-then-else is explained in this tutorial on slide 4. An example of a possible solution to an XSLT task using XPath if-then-else is given on slide 5.

XSL Conditionals: ifs, chooses, whens and otherwises Almost all programming languages have conditional branching structures. XSL has two: xsl:if and xsl:choose. XPath also has the if-then-else structure.

This tutorial is designed as a supplement to the XSL Conditionals tutorial, which covers xsl:if and xsl:choose in detail, but does not cover XPath if-then-else. XPath if-then-else is explained in this tutorial on slide 4. An example of a possible solution to an XSLT task using XPath if-then-else is given on slide 5.

Using xsl:if

A simple example: using xsl:if to pluralize author if there are multiple authors.

]]> Let's look at this in detail: xsl:if test=count($docAuthors/author) gt 1sxsl:if ]]> The test attribute contains an XPath expression which evaluates to true() or false(). What is inside the xsl:if tag is only implemented if it evaluates to true().

The example above uses xsl:if to pluralize the word "author" if there is more than one listed. The test attribute on xsl:if asks the XSLT processor to count the number of author elements within docAuthor, and then to compare that number to one. If the number is greater than one, the text s is added after after "Author"

Every test attribute contains an XPath expression. The expression either evaluates as true or as false. If it is true, then the content of xsl:if is applied. If it is false, nothing is done.

In this example, the content of xsl:if is a simple text node, but we can imagine more complex examples like checking if there is an ref on persName, and if there is, then proceeding with a whole additional snippet of code.

Using xsl:choose Sometimes you need to handle two or more conditions. This is done with xsl:choose: DHSI Dress Code <]]>xsl:choose <]]>xsl:when test=surname='Bauman' tie, no footwear xsl:when <]]>xsl:when test=surname='Holmes' footwear, no tie xsl:when <]]>xsl:otherwise unpredictable xsl:otherwise xsl:choose ]]> The processor looks at each xsl:when in turn; when it finds one whose test evaluates to true(), it processed that one, and then exits the xsl:choose. If none are true, it processes xsl:otherwise (assuming there is one).

The xsl:if element works well if you only want to apply constructors (i.e., do things) when a simple test proves to be true (or false, as you can always reverse the logic of a test by wrapping it in a not() function), and don't want to do anything if said simple test proves to be false (or true). But there may be instances in which you want to do something when the test evaluates as true, and something else when it evaluates as false. Indeed, you may want to apply rules to things that aren't even simple true/false statements.

In these cases, you will want to use xsl:choose, which allows you to specify rules for several different scenarios. The children of xsl:choose are one or more xsl:when statements followed by zero or one xsl:otherwise statement.

The processor evaluates the test attribute of each xsl:when in order. If the XPath expression is false, the processor goes to the next xsl:when. When the processor finds that the test of an xsl:when is true, it performs the content of that xsl:when, and stops looking at the rest of the xsl:whens and the xsl:otherwise (if there even is one).

If none of the tests for xsl:when are true, then the processor will execute the content of the xsl:otherwise (if there is one).

Thus at most one child of an xsl:choose is executed. If there is no xsl:otherwise child of the xsl:choose, it is possible that no child of the xsl:choose will be executed.

Using XPath if-then-else If your condition is very simple, and the processing you want to do as a result of it does not involve creating elements or attributes, then you can just use an if-then-else structure in XPath: xsl:value-of select=count($docAuthors/author) gt 1'Authors: ''Author: ']]>

As we saw in the slide explaining how to use xsl:if, this slide shows a simple example of using the XPath if-then-else structure to pluralize author is there is more than one author element within docAuthor.

In this case the xsl:value-of element has a select attribute that contains an XPath if-then-else construct The first condition, if, contains the XPath expression:

count(
                $docAuthors/author ) gt 1

. This XPath expression starts by calling the function count(), which counts the number of items in its argument sequence. In this case, the argument to count() is the sequence of child author elements found in the variable $docAuthors (presumably the list of authors of the current document :-) The XPath expression then asks the question is this (the value of the count() function) greater than one?. That's what the gt means: greater than. The answer to that question (is this (the value of the count() function) greater than one?) is either true() or false(). If the XPath expression after if is true, then the content following then is returned. Otherwise, the content following else is returned. In this example, if there is more than one author, then the condition will return the text: Authors: ; if not, then it will return the text: Author:

XPath if-then-else is useful for when your condition is very simple, and the processing you want to do as a result of it does not involve creating XML constructs like elements and attributes. Also, unlike xsl:if, XPath if-then-else allows you to process conditions for both the true() or false() evaluation of an XPath expression.

XSL Conditionals: Task

Here's a simple task you can try:

Open Places XML (demos/xslt_intro/places.xml). Open XSLT (demos/xslt_intro/mdh_xsl_conditionals.xsl). Switch to the XSLT debugger in oXygen. Follow the instructions in the XSLT file. You'll need to write some conditional XSLT code to complete the task.

Download the two linked files and open them in the XSLT Debugger in Oxygen. If you haven’t used the XSLT Debugger view before, please refer to the the tutorial here: XSLT Debugger.

On the next slide are possible solutions to the task proposed in the XSLT document using xsl:choose and the XPath if-then-else structure.

XSL Conditionals: Task

This is one possible solution:

]]> <xsl:choose> <xsl:when test="count( location/geo ) eq 1">(point)</xsl:when> <xsl:when test="location[ @type eq 'path' ]">(path)</xsl:when> <xsl:otherwise>(polygon)</xsl:otherwise> </xsl:choose>

]]>

As always, there is more than one way to go about this. The solution here, using xsl:choose, is one way to produce the desired output, conditionally based on the input location. We might also use the XPath if-then-else structure to accomplish the same thing:

]]><xsl:value-of select=" if ( count( location/geo ) eq 1 ) then '(point)' else if ( location[@type eq 'path'] ) then '(path)' else '(polygon)' "/>

]]> This tutorial is complete, please see links below to continue: Proceed to next tutorial in Transformation and Publication Primer Return to Transformation and Publication Primer Return to main tutorial page