For 'def callback(data)', the argument 'data' is just a standard Python function parameter. You can name it whatever you want. It just so happens that std_msgs/String has a 'data' field as well. - Ken On Sun, Apr 18, 2010 at 1:08 PM, hitesh dhiman wrote: > Hi Ken, > Thanks for the help. > I just have a small question regarding this example node. The callback > function takes in a 'data' argument. Does this mean that the message being > published has a 'data' attribute? Is it true for all messages? > > On Mon, Apr 19, 2010 at 3:55 AM, Ken Conley wrote: >> >> Hi Hitesh, >> >> Please look at the "Writing the Subscriber Node" section here: >> >> >> http://www.ros.org/wiki/rospy_tutorials/Tutorials/WritingPublisherSubscriber#Writing_the_Subscriber_Node >> >> Your subscriber has no callback. The callback is what receives the >> message instance. >> >> Hope this helps, >> Ken >> >> On Sun, Apr 18, 2010 at 12:51 PM, hitesh dhiman >> wrote: >> > Hi Ken, >> > Actually i started out by following the tutorials. >> > Here's the code I'm using: >> > >> > velocity = rospy.Subscriber("cmd_vel", Twist) >> > >> > I checked using rxgraph, my node is subscribing to cmd_vel. I also used >> > rostopic echo /cmd_vel, i can see the velocities being published. Now >> > the >> > problem is, how to extract the information from the message. >> > >> > If i try to type : >> > >> > print velocity.linear.x >> > >> > I get an error: velocity has no attribute x >> > >> > Even if i just do velocity.linear[0], or simply velocity.linear, i get >> > the >> > error: >> > >> > velocity has no attribute linear >> > >> > The cmd_vel topic has a type Twist message, that means two cpp vectors, >> > each >> > for linear and angular velocities. I'm not able to get the correct >> > format to >> > access these vectors. >> > >> > On Mon, Apr 19, 2010 at 3:00 AM, Ken Conley >> > wrote: >> >> >> >> Hi Hitesh, >> >> >> >> If you follow the rospy tutorials for publishing and subscribing [1] >> >> you will be able to subscribe to any type of ROS topic, including >> >> cmd_vel, and extract the fields that you want. >> >> >> >> [1] http://www.ros.org/wiki/rospy/Tutorials >> >> >> >>  - Ken >> >> >> >> On Sun, Apr 18, 2010 at 11:55 AM, hitesh dhiman >> >> wrote: >> >> > Hi ken, >> >> > Well I think you're correct. If I just do a print vel, it says >> >> > subscriber >> >> > object at 'address'...Basically what I'm doing is getting the cmd_vel >> >> > published from navigation stack. >> >> > On this note, is it possible to find out the correct way to get the >> >> > linear >> >> > and angular velocities from cmd_vel ? >> >> > >> >> > On Apr 19, 2010 2:34 AM, "Ken Conley" wrote: >> >> > >> >> > Hi hitesh, >> >> > >> >> > It would help if you cut and paste the exact error message and >> >> > include a >> >> > sample program. I suspect that the program is using the subscriber >> >> > object >> >> > instead of the message object, but this is hard to tell without more >> >> > information. >> >> > >> >> > Ken >> >> > >> >> >> > On Apr 18, 2010 9:43 AM, "hitesh dhiman" >> >> >> > >> >> >> > wrote: > > Hi all, > I'... >> >> > >> >> > >> >> > >> >> > ------------------------------------------------------------------------------ >> >> > Download Intel® Parallel Studio Eval >> >> > Try the new software tools for yourself. Speed compiling, find bugs >> >> > proactively, and fine-tune applications for parallel performance. >> >> > See why Intel Parallel Studio got high marks during beta. >> >> > http://p.sf.net/sfu/intel-sw-dev >> >> > _______________________________________________ >> >> > ros-users mailing list >> >> > ros-users@lists.sourceforge.net >> >> > https://lists.sourceforge.net/lists/listinfo/ros-users >> >> > >> >> > _______________________________________________ >> >> > ros-users mailing list >> >> > ros-users@code.ros.org >> >> > https://code.ros.org/mailman/listinfo/ros-users >> >> > >> >> > >> >> > _______________________________________________ >> >> > ros-users mailing list >> >> > ros-users@code.ros.org >> >> > https://code.ros.org/mailman/listinfo/ros-users >> >> > >> >> > >> >> > _______________________________________________ >> >> > ros-users mailing list >> >> > ros-users@code.ros.org >> >> > https://code.ros.org/mailman/listinfo/ros-users >> >> > >> >> > >> >> _______________________________________________ >> >> ros-users mailing list >> >> ros-users@code.ros.org >> >> https://code.ros.org/mailman/listinfo/ros-users >> > >> > >> > >> > -- >> > Regards, >> > Hitesh Dhiman >> > Electrical Engineering >> > National University of Singapore >> > >> > _______________________________________________ >> > ros-users mailing list >> > ros-users@code.ros.org >> > https://code.ros.org/mailman/listinfo/ros-users >> > >> > >> _______________________________________________ >> ros-users mailing list >> ros-users@code.ros.org >> https://code.ros.org/mailman/listinfo/ros-users > > > > -- > Regards, > Hitesh Dhiman > Electrical Engineering > National University of Singapore > > _______________________________________________ > ros-users mailing list > ros-users@code.ros.org > https://code.ros.org/mailman/listinfo/ros-users > >