XPath find if node exists

Question

Using a XPath query how do you find if a node (tag) exists at all?

For example if I needed to make sure a website page has the correct basic structure like /html/body and /html/head/title.

Answer 1

<xsl:if test="xpath-expression">...</xsl:if>

so for example

<xsl:if test="/html/body">body node exists</xsl:if>
<xsl:if test="not(/html/body)">body node missing</xsl:if>

Answer 2

Try the following expression: boolean(path-to-node)

Answer 3

Patrick is correct, both in the use of the xsl:if, and in the syntax for checking for the existence of a node. However, as Patrick's response implies, there is no xsl equivalent to if-then-else, so if you are looking for something more like an if-then-else, you're normally better off using xsl:choose and xsl:otherwise. So, Patrick's example syntax will work, but this is an alternative:

<xsl:choose>
 <xsl:when test="/html/body">body node exists</xsl:when>
 <xsl:otherwise>body node missing</xsl:otherwise>
</xsl:choose>

Answer 4

Might be better to use a choice, don't have to type (or possibly mistype) your expressions more than once, and allows you to follow additional different behaviors.

I very often use count(/html/body) = 0, as the specific number of nodes is more interesting than the set. For example... when there is unexpectedly more than 1 node that matches your expression.

<xsl:choose>
    <xsl:when test="/html/body">
         <!-- Found the node(s) -->
    </xsl:when>
    <!-- more xsl:when here, if needed -->
    <xsl:otherwise>
         <!-- No node exists -->
    </xsl:otherwise>
</xsl:choose>

Answer 5

I work in Ruby and using Nokogiri I fetch the element and look to see if the result is nil.

require 'nokogiri'

url = "http://somthing.com/resource"

resp = Nokogiri::XML(open(url))

first_name = resp.xpath("/movies/actors/actor[1]/first-name")

puts "first-name not found" if first_name.nil?

Answer 6

A variation when using xpath in Java using count():

int numberofbodies = Integer.parseInt((String) xPath.evaluate("count(/html/body)", doc));
if( numberofbodies==0) {
    // body node missing
}