how to split the code so that it runs on two computers

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Thanks a lot, you saved my day.

Happy to help :)

I understood that I need to use the same topic and it is a Publisher on master and Subscriber in slave, it is easy with from std_msgs.msg import String. how to do it if I'd like to send numbers as float?

Slave is only physically apart from master, otherwise have access to all the topics/nodes/services and master has same for slave. Just think of it as you are using the same machine, nothing is different except your access to larger messages like image and pointcloud might be slower. Can you explain last part of your question, I didn't quite get what you are trying to do.

I am changing in these codes in order to understand it , (http://wiki.ros.org/ROS/Tutorials/Wri...) it works with me because of the topic works as a Publisher and as a Subscriber at the same time but if I need to use the topic of velocity. how to implement it? because it is always working as a Publisher, it is not working with me. another thing, this ex. hello world works with String as type (string data). if I need to use type (float data) which one should I use instead of String?

I got a float(http://wiki.ros.org/msg) but the topic of velocity still not clear with me how to implement it?

I would suggest you go over ROS beginner tutorials and get a grasp of how ROS ecosystem works. From this you will get at least basic understanding of what you can do and how to do it and you will be better able to navigate your way instead of asking every small detail. To answer your question ROS distro comes with a lot of messages and you can also make your own messages if needed. Float is part of std_msgs/Float32 and so is string. You can also use command line to publish a message like rostopic pub /topic_name message_type message. Where you can tab autocomplete message_type and message.

For velocity you can use Twist or any variant of it like TwistStamped. You can use rosmsg show geometry_msgs/Twist to see what message contains. Like everything ROS related tab autocomplete works here as well and you can see all variants of Twist or all the messages avaiable to you.