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 mailing lists. Each topic has a name and a data type. Names usually start with a forward slash, such as /bike_state. Data types are defined by ROS (or you). 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.4

Note that there is no single Topic class. There is a Publisher class and a Subscriber class, both of which together do all the stuff you want to do with a topic.

Writing a node

Python (Pi)	Arduino/C++ (Due)	Description
import rospy from std_msgs.msg import Float32	#include <ros.h> #include <std_msgs/Float32.h>	Imports the main ROS "client library" (code needed to interact with ROS) 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 handle. A node handle is an object used to interact with ROS.
pub = rospy.Publisher("hjkl", Float32, queue_size=10)	std_msgs::Float32 hjkl_data; ros::Publisher hjkl_pub("hjkl", &hjkl_data); // In a function: nh.advertise(hjkl_pub);	Sets up a publisher who will publish to the topic hjkl.
pub.publish(42)	hjkl_data.data = 42; hjkl_pub.publish(&hjkl_data);	Publishes the number 42.
def foo_callback(data): print(data.data) rospy.Subscriber("foo", Float32, foo_callback)	void foo_callback(const std_msgs::Float32& data) { Serial.println(data.data); } ros::Subscriber<std_msgs::Float32> foo_sub("foo", &foo_callback); // In a function: nh.subscribe(foo_sub);	Subscribes to the topic foo, which publishes Float32 values.

Working with a Float32MultiArray

Background knowledge. The Float32MultiArray is used to store an array of Float32s in any dimension. So it could be a regular array, or a 2-D array, or a 3-D one, and so on. There's a MultiArrayLayout field that tells you what the layout is. That's a bit wasteful, so really we should be defining custom message types.