title: ROS nodes and topics
description: Self-assess your comprehension on the **ROS basics** including **ROS nodes, publishers, subscribers, message types, and the writing of the code**. Read carefully each question. Once answered, you will receive feedback. Good luck!
questions:

 - type: "fill-single"
   points: 1
   question: "If running the following command to run the teleoperation node `rosrun turtlebot3_teleop turtlebot3_teleop_key`
     and this error appears: `Unable to register with master node [http://localhost:11311]: master may not be running yet. Will keep trying.`
     
     Which command to run to solve the problem? Please type it."
   answer:
    - roscore
   feedback_p: Correct! The ROS master is necessary for running ROS nodes, enabling communication.
   feedback_n: The problem is with the ROS master that is not running.


 - type: "multiple-choice"
   points: 1
   question: "ROS message passing paradigm is based on publish/subscribe."
   items:
    - choice: True.
      correct: true
    - choice: False.
      correct: false

 - type: "matching"
   points: 1
   question: "Match the commands with their function."
   items_a:
    - rostopic pub
    - rostopic echo
    - rostopic list
    - rostopic info
    - rosnode list
    - rosnode info
   items_b:
    - Program to publish messages on a topic. 
    - Program to print on the screen the content of messages exchanged on a topic.
    - Show all current topics registered to the ROS master.
    - Program to show the information about a topic, including message type, publisher, and subscriber.
    - Program to print all nodes running.
    - Program to show the information about a specific node, including subscribed and published topics.

 - type: "multiple-choice"
   points: 1
   question: "More than one node can publish or subscribe to a topic."
   items:
    - choice: True.
      correct: true
    - choice: False.
      correct: false

 - type: "file-upload"
   points: 5
   question: "Modify the ROS node to make the robot follow a predefined shape, for example a square which size is 3m x 3m. You are welcome to try out other shapes. Upload the code and the resulting video of the robot performing the shape."
   feedback: Combine the translation and the rotation motions.