Homework Exercise: Sending and Receiving our First ROS Messages Sample Solutions

These are sample solutions for the Python scripts from the sending and receiving our first ROS nodes homework exercise.

point_location_sender.py

#!/usr/bin/env python3
""" This script publishes ROS messages containing the 3D coordinates of a single point """
import rospy
from geometry_msgs.msg import Point, PointStamped
from std_msgs.msg import Header

rospy.init_node('send_point_location')

my_header = Header(stamp=rospy.Time.now(), frame_id="odom")
my_point = Point(1.0, 2.0, 0.0)
my_point_stamped = PointStamped(header=my_header, point=my_point)

publisher = rospy.Publisher('/my_point', PointStamped, queue_size=10)

r = rospy.Rate(2)
while not rospy.is_shutdown():
    my_point_stamped.header.stamp = rospy.Time.now()
    publisher.publish(my_point_stamped)
    r.sleep()
      

point_location_receiver.py

#!/usr/bin/env python3
""" This script receives ROS messages containing the 3D coordinates of a single point and prints them out """
import rospy
from geometry_msgs.msg import PointStamped

rospy.init_node('receive_point_location')

def process_point(data):
    print("[{:.2f}, {:.2f}, {:.2f}]".format(data.point.x, data.point.y, data.point.z))

rospy.Subscriber("/my_point", PointStamped, process_point)

rospy.spin()
      

point_location_sender.py (object-oriented)

#!/usr/bin/env python3

import rospy
from geometry_msgs.msg import Point, PointStamped
from std_msgs.msg import Header

class SendPointLocation(object):
    """ This node publishes ROS messages containing the 3D coordinates of a single point """

    def __init__(self):
        rospy.init_node('send_point_location')
        self.point_location_pub = rospy.Publisher('/my_point', PointStamped, queue_size=10)

    def run(self):
        my_header = Header(stamp=rospy.Time.now(), frame_id="odom")
        my_point = Point(1.0, 2.0, 0.0)
        my_point_stamped = PointStamped(header=my_header, point=my_point)

        r = rospy.Rate(2)
        while not rospy.is_shutdown():
            my_point_stamped.header.stamp = rospy.Time.now()
            self.point_location_pub.publish(my_point_stamped)
            r.sleep()

if __name__ == '__main__':
    node = SendPointLocation()
    node.run()
      

point_location_receiver.py (object-oriented)

#!/usr/bin/env python3
import rospy
from geometry_msgs.msg import PointStamped

class ReceivePointLocation(object):
    """ This node receives ROS messages containing the 3D coordinates of a single point and prints them out """

    def __init__(self):
        rospy.init_node('receive_point_location')
        rospy.Subscriber("/my_point", PointStamped, self.process_point)

    def process_point(self, data):
        print("[{:.2f}, {:.2f}, {:.2f}]".format(data.point.x, data.point.y, data.point.z))

    def run(self):
        rospy.spin()

if __name__ == '__main__':
    node = ReceivePointLocation()
    node.run()