ROS2: Retrieve the total number of laser rays using the python interpreter (adjust topic name if neccesary)

print_number_of_laser_rays.py

import rclpy
from sensor_msgs.msg import LaserScan
from rclpy.qos import ReliabilityPolicy, QoSProfile

def scan_callback(msg):
    global g_node
    g_node.get_logger().info('Number of rays: "%d"' % len(msg.ranges))
    g_node.get_logger().info('Angle of first ray: "%.2f"' % msg.angle_min)
    g_node.get_logger().info('Angle of last ray: "%.2f"' % msg.angle_max)
    g_node.get_logger().info('Total angular area of coverage: "%.2f"' % (msg.angle_max-msg.angle_min))



g_node = None
global g_node
rclpy.init()
qos_policy = rclpy.qos.QoSProfile(reliability=rclpy.qos.ReliabilityPolicy.BEST_EFFORT,history=rclpy.qos.HistoryPolicy.KEEP_LAST,depth=1)
g_node = rclpy.create_node('minimal_subscriber')
subscription = g_node.create_subscription(LaserScan, '/scan', scan_callback, qos_profile=qos_policy)
if rclpy.ok():
    rclpy.spin_once(g_node)
#g_node.destroy_node()
#rclpy.shutdown()

How to use it:

1. In your terminal start the python interpreter by typing:
   python3

2. Copy the lines of code shown above and paste them in the python interpreter prompt. Hit enter two times.
   You should see the total number of laser rays printed to the screen.

3. Exit the python interpreter by typing: exit()

Alternatively you can run from the command line:

ros2 topic echo --full-length --once
or:

ros2 topic echo -f --once

Which will output all elements for the ranges arrays. By default the length of arrays gets truncated to 128 elements. (--once option for ros2 humble and above).

example: ros2 topic echo -f --once /scan > scan.txt

Alternatively you can consult the data sheet or the gazebo plugin.

ROS1 version:

import rospy
from sensor_msgs.msg import LaserScan

def scan_callback(msg):
  rospy.loginfo('Number of rays: "%d"' % len(msg.ranges))
  rospy.loginfo('Angle of first ray: "%.2f"' % msg.angle_min)
  rospy.loginfo('Angle of last ray: "%.2f"' % msg.angle_max)
  rospy.loginfo('Total angular area of coverage: "%.2f"' % (msg.angle_max - msg.angle_min))

def main():
  rospy.init_node('minimal_subscriber')
  subscription = rospy.Subscriber('/kobuki/laser/scan', LaserScan, scan_callback)
  
  try:
    rospy.spin()
  except KeyboardInterrupt:
    rospy.loginfo("Ctrl+C detected. Stopping the script.")

if __name__ == '__main__':
  main()