Getting XML node matching XPath expression - xpath

Question

I'm hoping you can help with this. I tried a number of XPath testers online and wasn't able to find a solution.
I'm trying to get the value of the first node with the '~tid' attribute from this XML:
<Response status="ok" version="1.1">
<Action>
<Page>
<Params>
<Param name="~tid" value="1345811517165"/>
<Param name="~action-type" value="DATA"/>
<Param name="~from-page" value="login"/>
<Param name="~from-act" value="submitData"/>
<Param name="login_USERNAME" value="" type="0"/>
<Param name="login_PASSWORD" value="" type="0"/>
</Params>
</Page>
<Data name="clientData" method="POST">
<Params>
<Param name="timezoneoffset" value=""/>
<Param name="daylightsavings" value=""/>
</Params>
</Data>
</Action>
<Action>
<Page>
<Params>
<Param name="~tid" value="1345811517165"/>
<Param name="~action-type" value="NAV"/>
<Param name="~from-page" value="login"/>
<Param name="~from-act" value="resetPassword"/>
<Param name="~to-page" value="login_resetPassword"/>
</Params>
</Page>
</Action>
<Action>
<Page>
<Params>
<Param name="~tid" value="1345811517165"/>
<Param name="~action-type" value="NAV"/>
<Param name="~from-page" value="login"/>
<Param name="~from-act" value="newUser"/>
<Param name="~to-page" value="login_newUser"/>
</Params>
</Page>
</Action>
</Response>
The XPath I'm using is:
//Params[1]/Param[#name='~tid']/#value
It gets all three values. How can I get just the first one?

Answer 1

Try
/descendant::Params[1]/Param[#name='~tid']/#value
From the W3C XPath specification:
NOTE: The location path //para[1] does not mean the same as the location path /descendant::para[1]. The latter selects the first descendant para element; the former selects all descendant para elements that are the first para children of their parents.

Answer 2

This is the second most FAQ in XPath.
Use:
(//Params/Param[#name='~tid'])[1]/#value
Or even:
(//Param[#name='~tid'])[1]/#value
Explanation:
The XPath pseudo-operator // has lower precedence (priority) than the [] operator.
The usual solution in any laguage when default priority needs to be overriden is to use brackets that explicitly specify the new, wanted priorities.