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Is it possible to use a certain data value of a topic for executing some code?

asked 2016-11-30 08:52:26 -0600

Marcofon gravatar image

updated 2016-12-01 07:29:30 -0600

Hello, i will try to be as clear as possible. I have a ROS topic in which are published only 0 and 1. In another node i have a subscriber that is subscribed to that topic. Then i had an idea to implement a callback function, that as some code to execute inside, that is called by the subscriber. Let's say that every ten data valued 1 a data valued 0 will show up in the topic. So my question is, is it possible to execute the code inside the callback function only when the subscriber reads 0 in the topic? I mean with a sort of if(...) {...} mechanism or something similar. I apologize if it's not clear, i can provide other details or explanations; just let me know. Thank you in advance!

EDIT: here is the correct code of the callback function that i mentioned in the comment:

void orderCallback(const std_msgs::Bool::ConstPtr& msg)

if (msg->data == 0)

system("get_hrv -R my_data_file.txt >doc.txt");
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answered 2016-11-30 09:18:52 -0600

mgruhler gravatar image

Yes. A callback is just a function. Inside this you can do whatever you want. Just check the data and use an if clause.

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Thank you for your reply! Since i don't know how to check the topic values, can you please try to explain it to me? Below there is the code:

void orderCallback()

system("get_hrv -R my_data_file.txt >doc.txt");
Marcofon gravatar imageMarcofon ( 2016-11-30 09:28:21 -0600 )edit

@mig This is the subscriber:

ros::Subscriber sub = n.subscribe("semaphore", 1000, orderCallback);

And, as i said in the question, i have only boolean values.

Marcofon gravatar imageMarcofon ( 2016-11-30 09:30:03 -0600 )edit

check out the tutorial: , your callback is also not correct.

mgruhler gravatar imagemgruhler ( 2016-11-30 10:11:11 -0600 )edit

@mig I have already checked it before posting. Can you help me with the code? If you can, of course. Thank you again.

Marcofon gravatar imageMarcofon ( 2016-11-30 10:18:22 -0600 )edit

What kind of message do you have? this needs to go in the callback function definition. See l.34 of the listener.cpp in the tutorial. Then, depending on the fields of the message, you can access this as shown in l.36 msg->data. But note, that the String.msg has a field called data. You need to

mgruhler gravatar imagemgruhler ( 2016-12-01 01:50:17 -0600 )edit

adapt this accordingly for whatever your message you use. Then, implement your logic in whatever way you like...

mgruhler gravatar imagemgruhler ( 2016-12-01 01:50:53 -0600 )edit

Thank you for your reply! I tried something like this:

void orderCallback(const std_msgs::Bool::ConstPtr& msg)
bool data;
if (data = 0)
system("get_hrv -R my_data_file.txt >doc.txt");
Marcofon gravatar imageMarcofon ( 2016-12-01 04:51:59 -0600 )edit

...i tried to adapt the callback of the tutorial to my case. Anyway, it doesn't seem to work. What i'm missing? P.S. is data == 0 in my real code, it was only a typo in the comment. Thank you again @mig

Marcofon gravatar imageMarcofon ( 2016-12-01 04:55:21 -0600 )edit

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Asked: 2016-11-30 08:52:26 -0600

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Last updated: Dec 01 '16