Writing ROS code

This page covers writing ROS nodes, which are programs that connect (often with TCP) to a central server program, called the ROS core. After connecting with the core, ROS nodes can communicate with each other. Three ways ROS nodes communicate are topics, services, and parameters. We mostly use topics. This page has Python and C++ (Arduino) examples for communicating between two nodes using topics.

Topics are like Slack channels. Each topic has a name and a data type. Names usually start with a forward slash (which means they're in the "global namespace", which is like Java's default package), such as /bike_state. Data types are ROS-defined. Here's a list of the standard ones. We mostly use Float32 and Float32MultiArray. Nodes publish to and subscribe from topics. Multiple nodes can publish to and subscribe from the same topic. There are also command-line and graphical tools for publishing to a topic and showing messages published to a topic.

Writing a node

Python	Arduino	Description
import rospy from std_msgs.msg import Float32	#include <ros.h> #include <std_msgs/Float32.h>	Imports the main ROS client library and the Float32 message type.
rospy.init_node("asdf") The node name is "asdf". Also takes an optional argument anonymous=True, which lets you simultaneously run multiple copies of the same node	typedef ros::NodeHandle_<ArduinoHardware, 1, 3, 500, 500> RosNodeHandle; RosNodeHandle nh; // in a function: nh.initNode(); Note: ros::init may also be required, but we don't call it on the Arduino Note 2: The Arduino node is given a name on the Pi, so we don't pass in a name here.	Initializes a node
pub = rospy.Publisher("hjkl", Float32, queue_size=10)	std_msgs::Float32 hjkl_data; ros::Publisher hjkl_pub("hjkl", &hjkl_data);	Sets up a publisher.