Exists function in XSL When Statement - xslt

Question

I am using an XSL when clause to transform one XML file to another XML file. I need to use something like an "exists" function in my when test.
Here's an example source XML:
<People>
<Person personid="1" location="US" fullname="John Doe"/>
<Person personid="2" location="US" fullname="Jane Doe"/>
</People>
<Nicknames>
<Nickname personid="1" nname="Johnny D"/>
</Nicknames>
Here's my example XSL:
<xsl:element name="HASNICKNAME">
<xsl:choose>
<!-- If nickname exists in source XML, return true -->
<xsl:when test="boolean exists function"
<xsl:text>TRUE</xsl:text>
</xsl:when>
<xsl:otherwise>
<xsl:text>FALSE</xsl:text>
</xsl:otherwise>
</xsl:choose>
</xsl:element>
Can someone help with the exists part?

Answer 1

Suppose the variable $personid contains the #personid of the Person you are checking, then this only checks existence:
<xsl:when test="boolean(//Nickname[#personid=$personid]/#nname)">
For similar issues I normally prefer to check for a non-empty/non-whitespace value:
<xsl:when test="normalize-space(//Nickname[#personid=$personid]/#nname)!=''">
If you use a key like <xsl:key name="nn" match="//Nickname" use="#personid"/>, the following is also possible:
<xsl:when test="normalize-space(key('nn',$personid)/#nname)!=''">
The latter does not need the $personid variable but can directly work with the #personid of the Person you are checking…