4.8 Doing the Math with Expressions

Expressions allow you to perform simple arithmetic and Boolean logic when processing nodes. Here's an example of some simple addition and multiplication. The document math.xml contains a group of operand elements, each containing an integer:

<math>  <operand>12</operand>  <operand>23</operand>  <operand>45</operand>  <operand>56</operand>  <operand>75</operand> </math>

You can use an expression to add and multiply 25 with these operands, as shown in Example 4-10, the stylesheet math.xsl.

Example 4-10. A stylesheet that does simple math

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:output method="text"/>     <xsl:template match="math">  <xsl:apply-templates select="operand"/> </xsl:template>     <xsl:template match="operand">  <xsl:value-of select="."/>  <xsl:text> + 25 = </xsl:text>  <xsl:value-of select=". + 25"/>  <xsl:text>&#10;</xsl:text>  <xsl:value-of select="."/>  <xsl:text> * 25 = </xsl:text>  <xsl:value-of select=". * 25"/>  <xsl:text>&#10;</xsl:text> </xsl:template>     </xsl:stylesheet>

The expression is the value of several select attributes of value-of that add and multiply the content of each operand element with 25. The value-of element returns a string value, but the presence of + or * automatically converts the content of operand to a number, if possible. If the content of operand were a nonnumerical string, however, the number conversion wouldn't take place. This won't cause an error, but you will get NaN (Not a Number) in response.

When you process math.xsl against math.xml using:

xalan math.xml math.xsl

you get this result:

12 + 25 = 37 12 * 25 = 300 23 + 25 = 48 23 * 25 = 575 45 + 25 = 70 45 * 25 = 1125 56 + 25 = 81 56 * 25 = 1400 75 + 25 = 100 75 * 25 = 1875

The stylesheet shown in Example 4-11, boolean.xsl, combines addition and multiplication with some Boolean logic. It uses expressions in predicates to test whether the content of operand nodes are both greater-than and less-than a value.

Example 4-11. A stylesheet demonstrating more mathematical capability

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:output method="text"/>     <xsl:template match="/">  <xsl:apply-templates select="math"/> </xsl:template>     <xsl:template match="math">  <xsl:apply-templates select="operand[(. &lt; 50) and (. &gt; 30)]"/> </xsl:template>     <xsl:template match="operand[(. &lt; 50) and (. &gt; 30)]">  <xsl:value-of select="."/>  <xsl:text> + 25 = </xsl:text>  <xsl:value-of select=". + 25"/>  <xsl:text>&#10;</xsl:text>  <xsl:value-of select="."/>  <xsl:text> * 25 = </xsl:text>  <xsl:value-of select=". * 25"/>  <xsl:text>&#10;</xsl:text> </xsl:template>     </xsl:stylesheet>

In ordinary English, the expression:

(. < 50) and (. > 30)

tests whether the operand is less than 50 and greater than 30. The entity references &lt; and &gt; are used in the predicates instead of < and > because < is forbidden in attribute values in XML (see Section 3.1 of the XML specification). To balance this limitation, XML uses entity references for both symbols, even though > is legal in attribute values. The parentheses distinguish the greater-than and less-than tests, which are compared with the and operator. For a complete list of Boolean and math operators in XPath, see Table 4-5.

Table 4-5. XPath operators

Operator	Type	Description
and	Boolean	Boolean AND
or	Boolean	Boolean OR
=	Boolean	Equals
!=	Boolean	Not equal
&lt; (<)	Boolean	Less than
&lt;= (<=)	Boolean	Less than or equal to
&gt; (>)	Boolean	Greater than
&gt;= (>=)	Boolean	Greater than or equal to
+	Number	Addition
-	Number	Subtraction
*	Number	Multiplication
div	Number	Division
mod	Number	Modulo (remainder of division)

This concludes your mini math lesson in XPath and XSLT. To learn more about math in XPath, see Sections 3.4 and 3.5 in the XPath specification.