XPath and negation searches - ruby

Question

I have the following code sample in an xmlns root:
<ol class="stan">
<li>Item one.</li>
<li>
<p>Paragraph one.</p>
<p>Paragraph two.</p>
</li>
<li>
<pre>Preformated one.</pre>
<p>Paragraph one.</p>
</li>
</ol>
I would like to perform a different operation on the first item in <li> depending on the type of tag it resides in, or no tag, i.e. the first <li> in the sample.
EDIT:
My logic in pursuing the task turns out to be incorrect.
How do I query a <li> that has no descendants as in the first list item?
I tried negation:
#doc.xpath("//xmlns:ol[#class='stan']//xmlns:li/xmlns:*[1][not(p|pre)]")
That gives me the exact opposite for what I think I am asking for.
I think I am making the expression more complicated since I can't find the right solution.
UPDATE:
Navin Rawat has answered this one in the comments. The correct code would be:
#doc.xpath("//xmlns:ol[#class='stan']/xmlns:li[not(xmlns:*)]")
CORRECTION:
The correct question involves both an XPath search and a Nokogiri method.
Given the above xhtml code, how do I search for first descendant using xpath? And how do I use xpath in a conditional statement, e.g.:
#doc.xpath("//xmlns:ol[#class='stan']/xmlns:li").each do |e|
if e.xpath("e has no descendants")
perform task
elsif e.xpath("e first descendant is <p>")
perform second task
elsif e.xpath("e first descendant is <pre>")
perform third task
end
end
I am not asking for complete code. Just the part in parenthesis in the above Nokogiri code.

Answer 1

Pure XPath answer...
If you have the following XML :
<ol class="stan">
<li>Item one.</li>
<li>
<p>Paragraph one.</p>
<p>Paragraph two.</p>
</li>
<li>
<pre>Preformated one.</pre>
<p>Paragraph one.</p>
</li>
</ol>
And want to select <li> that has no child element as in the first list item, use :
//ol/li[count(*)=0]
If you have namespaces problem, please give to whole XML (with the root element and namespaces declaration) so that we can help you dealing with it.
EDIT after our discussion, here is your final tested code :):
#doc.xpath("//xmlns:ol[#class='footnotes']/xmlns:li").each do |e|
if e.xpath("count(*)=0")
puts "No children"
elsif e.xpath("count(*[1]/self::xmlns:p)=1")
puts "First child is <p>"
elsif e.xpath("count(*[1]/self::xmlns:pre)=1")
puts "First child is <pre>"
end
end