Random Value Selection using JMeter XPath Post Processor

JMeter has two post processor extractors which are usually used when we need some data extracted from the response. The XPath extractor is useful when the Regex would match other elements in a page (e.g. a link inside an li element). However the XPath extractor is missing a functionality that the Regex Post Processor has. It only returns all the values (equivalent to specifying -1 in a Regex Post Processor). XPath also natively supports returning the "nth" value using the position() function. However it doesn't support returning a value at random.
However using normal JMeter functions we can easily implement this functionality.
Assume that you have an XPath post processor that is attached to a HTTP Sampler and is returning n values into a variable called testXPATH. if we now want to select a value at random what we need to use is

${__V(testXPATH_${__Random(1,${testXPATH_matchNr})})}

But thats a mouthfull and needs an explanation

${testXPATH_matchNr}

returns the count of how many elements matched. We are assuming that some values are going to be selected, otherwise you need an IF controller to check the condition.

${__Random(1,${testXPATH_matchNr})}

Will then select an integer between 1 and the count of the values (TODO check if we need to add 1)

${__V(testXPATH_${__Random(1,${testXPATH_matchNr})})}}

Finally we form a string of the form testXPATH_n e.g. testXPATH_3 which is then passed to the __V function which will get us the value for the variable
http://jakarta.apache.org/jmeter/usermanual/functions.html

Note that running TIDY on HTML , followed by parsing it to create a DOM structure and finally running an XPATH query against it is signifcantly slower than just running a Regex against a String. If you need to generate a large load and you have limited resources available to you, then the XPATH extractor is probably not the best alternative.