Getting Transformer exception while running Jmeter script - xpath

Question

While running my JMeter script with 3 threads, ramp-up 5 sec I am getting transformer exception. Actually in my scenario I want to click on the multiple links to the page randomly so I have used the random variable in JMeter But after running, I am getting below error, however, my XPath expression in XPath extractor looks fine.
2016/09/23 23:44:03 INFO - jmeter.engine.StandardJMeterEngine: Running the test!
2016/09/23 23:44:03 INFO - jmeter.samplers.SampleEvent: List of sample_variables: []
2016/09/23 23:44:03 INFO - jmeter.gui.util.JMeterMenuBar: setRunning(true,*local*)
2016/09/23 23:44:03 INFO - jmeter.engine.StandardJMeterEngine: Starting ThreadGroup: 1 : Thread Group
2016/09/23 23:44:03 INFO - jmeter.engine.StandardJMeterEngine: Starting 3 threads for group Thread Group.
2016/09/23 23:44:03 INFO - jmeter.engine.StandardJMeterEngine: Thread will continue on error
2016/09/23 23:44:03 INFO - jmeter.threads.ThreadGroup: Starting thread group number 1 threads 3 ramp-up 5 perThread 1666.6666 delayedStart=false
2016/09/23 23:44:03 INFO - jmeter.threads.JMeterThread: Thread started: Thread Group 1-1
2016/09/23 23:44:03 INFO - jmeter.threads.ThreadGroup: Started thread group number 1
2016/09/23 23:44:03 INFO - jmeter.engine.StandardJMeterEngine: All thread groups have been started
2016/09/23 23:44:05 INFO - jmeter.threads.JMeterThread: Thread started: Thread Group 1-2
2016/09/23 23:44:07 INFO - jmeter.threads.JMeterThread: Thread started: Thread Group 1-3
2016/09/23 23:44:09 WARN - jmeter.extractor.XPathExtractor: TransformerException while processing ((//span[#class="title"]/a/#href)[$(RandomNumber)]) Expected ], but found: RandomNumber
2016/09/23 23:44:10 INFO - jmeter.threads.JMeterThread: Thread is done: Thread Group 1-1
2016/09/23 23:44:10 INFO - jmeter.threads.JMeterThread: Thread finished: Thread Group 1-1
2016/09/23 23:44:10 WARN - jmeter.extractor.XPathExtractor: TransformerException while processing ((//span[#class="title"]/a/#href)[$(RandomNumber)]) Expected ], but found: RandomNumber
2016/09/23 23:44:11 INFO - jmeter.threads.JMeterThread: Thread is done: Thread Group 1-3
2016/09/23 23:44:11 INFO - jmeter.threads.JMeterThread: Thread finished: Thread Group 1-3
2016/09/23 23:44:11 WARN - jmeter.extractor.XPathExtractor: TransformerException while processing ((//span[#class="title"]/a/#href)[$(RandomNumber)]) Expected ], but found: RandomNumber
2016/09/23 23:44:12 INFO - jmeter.threads.JMeterThread: Thread is done: Thread Group 1-2
2016/09/23 23:44:12 INFO - jmeter.threads.JMeterThread: Thread finished: Thread Group 1-2
2016/09/23 23:44:12 INFO - jmeter.engine.StandardJMeterEngine: Notifying test listeners of end of test
2016/09/23 23:44:12 INFO - jmeter.gui.util.JMeterMenuBar: setRunning(false,*local*)

Answer 1

JMeter Variables are referenced as ${RandomNumber} - you need to use braces instead of parentheses. Alternatively you can use __V() function like
${__V(RandomNumber)}
So you need to change your XPath query to one of the following:
(//span[#class="title"]/a/#href)[${RandomNumber}]
(//span[#class="title"]/a/#href)[${__V(RandomNumber)}]
Your approach seems to be fragile. I don't know how many links are at the page and how do you generate your ${RandomNumber} variable, however I would rather go the following way:
Store all links at the page into JMeter variables like:
link_1=http://example.com
link_2=http://jmeter.apache.org
...
link_matchNr=50
Use random JMeter variable from previously extracted ones where required via aforementioned __V() and __Random() functions combination like:
${__V(link_${__Random(1,${link_matchNr},)})}
Demo:
See Here’s What to Do to Combine Multiple JMeter Variables article for more information on how you can combine different JMeter Functions and Variables into a single expression.