How to read the laserscan,store and display it simultaneously (Topics_quiz)

bayodesegun · October 10, 2019, 7:12pm

Looking at your code, I can see the following:

You are trying to put your LaserScan message into lsvalue, but it seems it’s not being done rightly. But…you don’t even need to do that! In your laser_callback, just print the messages you are printing in main() by accessing msg directly.
Your are missing ros::spin() in main(), so your subscriber is not working properly (spinOnce() is not enough for this case, as you need to keep subscribing to the laser scan. See the difference between spin() and spinOnce() in the post below:
What is rospy.spin(), ros::spin(), ros::spinOnce(), and what are they for?
Don’t stop the robot in main(), like you’re doing here:

vel.linear.x=0;
pub.publish(vel);

You don’t need the Rate object here:
Proper use of rospy.Rate or ros::Rate
You don’t need to do any publishing in the main function - do them in the laser_callback, based on the sensor reading. If you must do any publication in main(), let it be the initial one and then do the rest in the callback. Your main should be something like:

int main(int argc,char** argv){
   ros::init(argc, argv, “topics_quiz_node”);
   ros::NodeHandle nh;
   pub=nh.advertise<geometry_msgs::Twist>("/cmd_vel", 1000);
   sub=nh.subscribe("/kobuki/laser/scan",1000, laser_callback);
   // Do just the initial publication here; do the rest in the callback
   // Not really necessary though
   for(int i=0; i<5;i++){
     vel.linear.x=0.2;
     pub.publish(vel);
   }
   // This is necessary for the LaserScan message to keep arriving 
   // and for the program to keep running
   ros::spin();
}

Your laser_callback() could be something like:

void laser_callback(const sensor_msgs::LaserScan::ConstPtr& msg){
  // print your ranges array for debugging.
  // the middle values represent the front-center of the robot
  // the first and last represent the extreme sides
  if (/*robot is not too close for comfort*/) {
    vel.linear.x = 0.2; // move forward
    vel.anguar.z = 0; // no turning for now
  }
  else {
    vel.angular.z = -0.2 // or 0.2, turn
    vel.angular.x = 0; // no forward movement for now
  }
  pub.publish(vel);
}