Added rostopic FAQ details

pps_wiki/faq.md

@@ -249,11 +249,39 @@ Now the ``.otherCrazyflies`` property of the ``request`` variable that is passed
 
 
 
-### I added some advertise, publish, subscribe topics but not getting the desired behaviour, what debugging tools are avilable?
+### I added some advertise, publish, subscribe topics but I am not getting the desired behaviour, what debugging tools are avilable?
 
 <details>
 <summary>Click here to expand</summary>
 
-If you code changes compile successfully and your node runs without crashing, then the command line tool ``rostopic`` is the most useful tool for debugging errors.
+If your code changes compile successfully and your node runs without crashing, then the command line tool ``rostopic`` is the most useful tool for debugging errors. Open a new terminal and type `rostopic` to read the desription and help files.
+
+After launching your node, open a separate command window and type the command
+```
+rostopic list
+```
+this will list all the topics that are currently active, including both advertised and subscribe to topics.
+
+Common pitfalls to watch out for are:
+- If there are two topics with a very similar name, then double check the spelling of the topic is identical in both the ``.advertise`` and ``.subscribe`` lines of code.
+- Even if the topic name is exactly matching, the name space may be different.
+- If you advertise or subscribe to a topic from with a block of code enclosed by curly braces, for example:
+```
+if (true)
+{
+		ros::NodeHandle nodeHandle("~");
+		ros::Subscriber my_subscriber = nodeHandle.subscribe("/fortytwo",1,fortytwoCallback);
+}
+```
+then the subscriber only exists while the code between the ``{}`` is being exectute and removed after the ``if`` statement has completed it execution. To make the subscriber persist, then you need to declare the subscriber varaible outside of the ``if`` statement's context, i.e.,
+```
+ros::Subscriber my_subscriber;
+if (true)
+{
+		ros::NodeHandle nodeHandle("~");
+		my_subscriber = nodeHandle.subscribe("/fortytwo",1,fortytwoCallback);
+}
+```
+This would work if it is in the ``main()`` function of a node, but if it is in other functions then you should declare the subscriber as a class variable.
 
 </details>