XML in a Nutshell, 3rd Edition

Booleans

A Boolean is a value that has exactly two states, true or false. Every Boolean must have one of these binary values. XPath does not provide any Boolean literals. If you use <xsl:value-of select="true"/> in an XSLT stylesheet, then the XSLT processor looks for a child element of the context node named true. However, the XPath functions true() and false( ) can substitute for the missing literals quite easily.

Most of the time, however, Booleans are created by comparisons between other objects, most commonly numbers. XPath provides all the usual relational operators including =, !=, <, >, >=, and <=. In addition, the and and or operators can combine Boolean expressions according to the usual rules of logic.

Booleans are most commonly used in predicates of location paths. For example, in the location step person[profession="physicist"], profession="physicist" is a Boolean. It is either true or false; there is no other possibility. Booleans are also commonly used in the test attribute of xsl:if and xsl:when elements. For example, this XSLT template rule includes the profession element in the output only if its contents are “physicist” or “computer scientist”:

 <xsl:template match="profession">
  <xsl:if test=".='computer scientist' or .='physicist'">
    <xsl:value-of select="."/>
  </xsl:if>
</xsl:template>

This XSLT template rule italicizes the profession element if and only if its content is the string “computer scientist”:

<xsl:template match="profession"> <xsl:choose> <xsl:when ...