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Attempting to publish and subscribe to individual portions of an advertised topic

asked 2011-11-05 20:42:59 -0500

AlgorithmSeeker gravatar image

updated 2011-11-05 23:40:18 -0500

Hey guys. I've just started using ROS a couple of days back. I'm trying to write a publisher(talker) that will take certain parts of a topic and publish them, and a subscriber that can take whatever this talker is publishing. I know this question sounds a bit vague, but can you help me? I've read the tutorials but I still can't use that to my purpose.

Let's say we have some topic '/XXXX/DATA'. I want to extract, say, '/XXX/DATA.acc_x' using the publisher and send it to the subscriber. Is this even possible?

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answered 2011-11-05 21:41:48 -0500

makokal gravatar image

Yes it is possible, The data that is available on the topic is specified by messages. In you case I assume you define some message that contains some element like

float64 acc_x

for instance. Then you subscribe to this topic and just access the data element via the callback say

void dataCallback(const SomePkg::SomeMsgTypeConstPtr msg) { float accx = msg->acc_x; }

And you are set. You can go ahead and do anything with it afterwards.

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You can also publish in the callback too.
tfoote gravatar image tfoote  ( 2011-11-16 09:12:22 -0500 )edit

answered 2011-11-06 00:30:18 -0500

AlgorithmSeeker gravatar image

updated 2011-11-06 00:33:49 -0500

Let me be even more specific. See this link 'Writing a simple Publisher and Subscriber in C++':

Now this is part of the code written for the tutorial on how to write a Publisher.

while (ros::ok())

{ /** * This is a message object. You stuff it with data, and then publish it. */ std_msgs::String msg;

std::stringstream ss;
ss << "hello world " << count; = ss.str();


Notice how a string object 'ss' is created, and "Hello World" is assigned to 'ss'. Now what I want is, instead of assigning 'Hello World' to this string object, I want to assign information that is being published by the topic /XXX/DATA

I hope my question is clear.

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the easiest way is to just subscribe to the topic and buffer the information on some variable and then use in the other publisher.
makokal gravatar image makokal  ( 2011-11-06 05:04:19 -0500 )edit

answered 2011-11-05 22:10:16 -0500

AlgorithmSeeker gravatar image

Thanks for that, makokal. However, I am still encountering problems in accessing the data. Let me explain my problem more clearly.

I have a certain robot which is producing data under several topics, one of which includes this /XXX/Data. Now /XXX/Data has several members in its definition, for example acc_x, acc_y, acc_z, angle_roll, angle_pitch, angle_yaw etc. What I'm trying to do, is to write a listener that will extract specifically acc_x, which is of int16 type. To access this acc_x, I have tried including the header file, e.g. "information.h" that specifically contains the structure that is publishing /XXX/Data, and this structure contains acc_x, acc_y etc. Then, in my listener, I tried to create an object of the type of the structure I mentioned above. But it's not working.

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Asked: 2011-11-05 20:42:59 -0500

Seen: 314 times

Last updated: Nov 06 '11