If you want, you can define a default value for a parameter. There are two ways to define a default value; the simplest is to use a select attribute on the <xsl:param> element:

In this example, the default values of the parameters bgColor and width are 'blue' and 150, respectively. If we invoke this template without specifying values for these parameters, the default values are used. Also notice that we generated the values of the width and bgcolor attributes of the HTML <td> tag with attribute value templates, the values in curly braces. For more information, see Section 3.3, "Attribute Value Templates" in Chapter 3, "XPath: A Syntax for Describing Needles and Haystacks".

WARNING: Notice that in the previous sample, we put single quotes around the value blue, but we didn't do it around the value 150. Without the single quotes around blue, the XSLT processor assumes we want to select all the <blue> elements in the current context, which is probably not what we want. The XSLT processor is clever enough to realize that the value 150 can't be an XML element name (the XML 1.0 Specification says element names can't begin with numbers), so we don't need the single quotes around a numeric value.

Try to keep this in mind when you're using parameters. You'll probably forget it at some point, and you'll probably go nuts trying to figure out the strange behavior you're getting from the XSLT processor.

The second way to define a default value for a parameter is to include content inside the <xsl:param> element:

<template name="addTableCell">
  <xsl:param name="bgColor">
    <xsl:text>blue</xsl:text>
  </xsl:param>
  <xsl:param name="width">
    <xsl:value-of select="7+8"/><xsl:text>0</xsl:text>
  </xsl:param>
  <xsl:param name="content"/>
  <td width="{$width}" bgcolor="{$bgColor}">
    <xsl:apply-templates select="$content"/>
  </td>
</template>

In this example, we used <xsl:text> and <xsl:value-of> elements to define the default values of the parameters. Out of sheer perverseness, we defined the value of width as the concatenation of the numeric expression 7+8, followed by the string "0". This example produces the same results as the previous one.

4.4.2. Passing Parameters

If we invoke a template by name, which is similar to calling a subroutine, we'll need to pass parameters to those templates. We do this with the <xsl:with-param> element. For example, let's say we want to call a template named draw-box, and pass the parameters startX, startY, endX, and endY to it. Here's what we'd do:

<xsl:call-template name="draw-box">
  <xsl:with-param name="startX" select="50"/>
  <xsl:with-param name="startY" select="50"/>
  <xsl:with-param name="endX" select="97"/>
  <xsl:with-param name="endY" select="144"/>
</xsl:call-template>

In this sample, we've called the template named draw-box with the four parameters we mentioned earlier. Notice that up until now, <xsl:call-template> has always been an empty tag; here, though, the parameters are the content of the <xsl:call-template> element. (If you want, you can do the same thing with <xsl:apply-templates>.) We used the <xsl:with-param> element with the <xsl:call-template> element here, but you can also use it with <xsl:apply-templates>.

If we're going to pass parameters to a template, we have to set up the template so that it expects the parameters we're passing. To do this, we'll use the <xsl:param> element inside the template. Here are some examples:

<xsl:template name="draw-box">
<xsl:param name="startX"/>
<xsl:param name="startY" select="'0'"/>
<xsl:param name="endX"> 
10
</xsl:param>
<xsl:param name="endY">
10
</xsl:param>
...
</xsl:template>

A couple of notes about the <xsl:param> element:

• If you define any <xsl:param> elements in a template, they must be the first thing in the template.

• The <xsl:param> element allows you to define a default value for the parameter. If the calling template doesn't supply a value, the default is used instead. The last three <xsl:param> elements in our previous example define default values.

• The <xsl:param> element has the same content model as <xsl:variable>. With no content and no select attribute, the default value of the parameter is an empty string (""). With a select attribute, the default value of the parameter is the value of the select attribute. If the <xsl:param> element contains content, the default value of the parameter is the content of the <xsl:param> element.

XSLT allows you to define parameters whose scope is the entire stylesheet. You can define default values for these parameters and you can pass values to those parameters externally to the stylesheet. Before we talk about how to pass in values for global parameters, we'll show you how to create them. Any parameters that are top-level elements (any <xsl:param> elements whose parent is <xsl:stylesheet>) are global parameters. Here's an example: