Getting XML node matching XPath expression

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.