ROS node concept - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize a ROS node named 'talker'.

ROS

ros::init(argc, argv, [1]);

Drag options to blanks, or click blank then click option'

A"listener"

B"talker"

C"node"

D"publisher"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different node name than intended.

Forgetting to put the node name in quotes.

The ros::init function initializes the ROS node with the given name. Here, the node is named 'talker'.

2fill in blank

medium

Complete the code to create a ROS node handle object named 'nh'.

ROS

ros::[1] nh;

Drag options to blanks, or click blank then click option'

ANode

BSubscriber

CPublisher

DNodeHandle

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Node' instead of 'NodeHandle'.

Confusing Publisher or Subscriber with NodeHandle.

The ros::NodeHandle object is used to interact with the ROS system, such as creating publishers and subscribers.

3fill in blank

hard

Fix the error in the code to create a publisher that publishes std_msgs::String messages on the 'chatter' topic.

ROS

ros::Publisher pub = nh.advertise<std_msgs::String>([1], 1000);

Drag options to blanks, or click blank then click option'

A"chatter"

B1000

Cstd_msgs::String

Dnh

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the queue size instead of the topic name.

Passing the NodeHandle object instead of a string.

The advertise function requires the topic name as a string. Here, the topic is 'chatter'.

4fill in blank

hard

Fill both blanks to create a subscriber that listens to 'chatter' topic with a callback function named 'chatterCallback'.

ROS

ros::Subscriber sub = nh.[1]<std_msgs::String>([2], 1000, chatterCallback);

Drag options to blanks, or click blank then click option'

Asubscribe

B"chatter"

C"listener"

Dadvertise

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'advertise' instead of 'subscribe'.

Passing wrong topic name string.

The subscribe method creates a subscriber. The first argument is the topic name as a string, here 'chatter'.

5fill in blank

hard

Fill all three blanks to create a ROS node that initializes, creates a NodeHandle, and spins to process callbacks.

ROS

int main(int argc, char **argv) {
  ros::init(argc, argv, [1]);
  ros::[2] nh;
  ros::[3]();
  return 0;
}

Drag options to blanks, or click blank then click option'

A"listener"

BNodeHandle

Cspin

D"talker"

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong node name string.

Forgetting to call ros::spin().

Using wrong class name for NodeHandle.

The node is initialized with the name 'talker'. Then a NodeHandle is created. Finally, ros::spin() keeps the node alive to process callbacks.

Practice

1. What is a ROS node in simple terms?

easy

A. A type of sensor used in robots

B. A hardware part of the robot

C. A small program that runs in the robot system

D. A communication cable between robots

Solution

1. Step 1: Understand the role of a ROS node

   A ROS node is a software program that runs to perform tasks in a robot system.

2. Step 2: Differentiate from hardware and other parts

   It is not hardware or cables but a small program that communicates with others.

3. Final Answer:

   A small program that runs in the robot system -> Option C

4. Quick Check:

   ROS node = small program [OK]

Hint: Remember: node = small program in ROS [OK]

Common Mistakes:

• Thinking a node is hardware
• Confusing nodes with sensors
• Believing nodes are cables

2. Which of the following is the correct way to initialize a ROS node in Python?

easy

A. rospy.init_node('node_name')

B. ros.init('node_name')

C. node.start('node_name')

D. init.node('node_name')

Solution

1. Step 1: Recall ROS Python syntax for node initialization

   The correct function to start a node is rospy.init_node with the node name as argument.

2. Step 2: Check other options for correctness

   Other options use incorrect function names or syntax not used in ROS Python.

3. Final Answer:

   rospy.init_node('node_name') -> Option A

4. Quick Check:

   Initialize node with rospy.init_node() [OK]

Hint: Use rospy.init_node('name') to start a node [OK]

Common Mistakes:

• Using ros.init instead of rospy.init_node
• Calling node.start which doesn't exist
• Wrong function order or names

3. Consider this Python ROS node snippet:

import rospy
rospy.init_node('talker')
rospy.loginfo('Node started')

What will this code do when run?

medium

A. Cause a syntax error due to missing parentheses

B. Start a node named 'talker' and print 'Node started' info message

C. Start a node but no message will be printed

D. Start a node named 'listener' and print 'Node started'

Solution

1. Step 1: Analyze node initialization

   The code calls rospy.init_node with 'talker', so it starts a node named 'talker'.

2. Step 2: Check logging output

   rospy.loginfo prints the message 'Node started' to the console or log.

3. Final Answer:

   Start a node named 'talker' and print 'Node started' info message -> Option B

4. Quick Check:

   Node 'talker' starts and logs info [OK]

Hint: rospy.loginfo prints messages after node starts [OK]

Common Mistakes:

• Confusing node name with 'listener'
• Thinking no message prints
• Assuming syntax error without checking code

4. This ROS Python node code has an error:

import rospy
rospy.init_node('listener')
rospy.spin

What is the error?

medium

A. rospy.init_node should be called after rospy.spin

B. Wrong node name 'listener'

C. Missing import of std_msgs

D. Missing parentheses after rospy.spin

Solution

1. Step 1: Check usage of rospy.spin

   rospy.spin is a function and must be called with parentheses to keep the node alive.

2. Step 2: Verify other parts

   Node name 'listener' is valid, std_msgs import is not required here, and init_node must be called before spin.

3. Final Answer:

   Missing parentheses after rospy.spin -> Option D

4. Quick Check:

   Call rospy.spin() with parentheses [OK]

Hint: Always call rospy.spin() with parentheses [OK]

Common Mistakes:

• Forgetting parentheses on rospy.spin
• Thinking node name causes error
• Misordering init_node and spin

5. You want to create two ROS nodes that communicate by sending messages. Which is the best way to organize your code?

hard

A. Write two separate node programs, each with its own init_node call

B. Write one program with two init_node calls for both nodes

C. Use one node and switch its name dynamically

D. Create one node and run it twice without changes

Solution

1. Step 1: Understand ROS node design

   Each node is a separate program with its own init_node call to run independently.

2. Step 2: Evaluate other options

   One program cannot have two init_node calls; switching names dynamically or running one node twice without changes won't create two communicating nodes properly.

3. Final Answer:

   Write two separate node programs, each with its own init_node call -> Option A

4. Quick Check:

   Separate programs = separate nodes [OK]

Hint: One node per program with init_node call [OK]

Common Mistakes:

• Trying to call init_node twice in one program
• Reusing one node for two roles
• Not separating node logic into programs