Creating a subscriber node in ROS - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

ROS

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

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

A"subscriber"

B"talker"

C"listener"

D"publisher"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong node name like 'talker' or 'publisher'.

Forgetting the quotes around the node name.

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

2fill in blank

medium

Complete the code to create a NodeHandle object named 'nh'.

ROS

ros::[1] nh;

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

APublisher

BNodeHandle

CSubscriber

DServiceClient

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Publisher' or 'Subscriber' instead of 'NodeHandle'.

Confusing NodeHandle with message types.

The ros::NodeHandle object is used to interact with the ROS system, including creating subscribers.

3fill in blank

hard

Fix the error in the subscriber creation line by filling the blank with the correct message type.

ROS

ros::Subscriber sub = nh.subscribe("chatter", 1000, chatterCallback); // Message type: [1]

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

Astd_msgs::String

Bnav_msgs::Odometry

Csensor_msgs::Image

Dgeometry_msgs::Twist

Attempts:

3 left

💡 Hint

Common Mistakes

Using message types unrelated to the 'chatter' topic.

Confusing message types with topic names.

The subscriber listens to the 'chatter' topic which uses the std_msgs::String message type.

4fill in blank

hard

Fill both blanks to complete the subscriber callback function signature correctly.

ROS

void [1](const [2]::ConstPtr& msg) {
  ROS_INFO("I heard: %s", msg->data.c_str());
}

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

AchatterCallback

Bstd_msgs

Cgeometry_msgs

DlistenerCallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong function names like 'listenerCallback'.

Using incorrect message namespaces like 'geometry_msgs'.

The callback function is named chatterCallback and it receives a constant pointer to a std_msgs message.

5fill in blank

hard

Fill all three blanks to complete the main function that initializes ROS, creates a NodeHandle, subscribes to 'chatter', and spins.

ROS

int main(int argc, char **argv) {
  ros::[1](argc, argv, "listener");
  ros::[2] nh;
  ros::Subscriber sub = nh.subscribe("chatter", 1000, [3]);
  ros::spin();
  return 0;
}

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

Ainit

BNodeHandle

CchatterCallback

DSubscriber

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Subscriber' instead of 'NodeHandle' for the handle.

Forgetting to call ros::spin() to keep the node alive.

Using wrong callback function names.

The main function initializes ROS with init, creates a NodeHandle, subscribes to 'chatter' with the callback chatterCallback, and calls ros::spin() to process callbacks.

Practice

1.

What is the main purpose of a subscriber node in ROS?

easy

A. To send messages to other nodes

B. To listen and react to messages published on a topic

C. To create new topics for communication

D. To store messages permanently

Solution

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

   A subscriber node listens to messages sent on a specific topic in ROS.

2. Step 2: Compare options with the subscriber role

   Only To listen and react to messages published on a topic describes listening and reacting to messages, which matches the subscriber's purpose.

3. Final Answer:

   To listen and react to messages published on a topic -> Option B

4. Quick Check:

   Subscriber node = listens to messages [OK]

Hint: Subscriber nodes always listen to topics, not send [OK]

Common Mistakes:

• Confusing subscriber with publisher
• Thinking subscriber creates topics
• Assuming subscriber stores messages permanently

2.

Which of the following is the correct way to create a subscriber in ROS 2 Python?

node.create_subscription(____, 'chatter', callback, 10)

easy

A. std_msgs.msg.String

B. String

C. std_msgs/String

D. string

Solution

1. Step 1: Identify the correct message type import

   In ROS 2 Python, message types are imported as modules, e.g., std_msgs.msg.String.

2. Step 2: Match the correct syntax for create_subscription

   The first argument must be the full message type class, so std_msgs.msg.String is correct.

3. Final Answer:

   std_msgs.msg.String -> Option A

4. Quick Check:

   Message type = std_msgs.msg.String [OK]

Hint: Use full message type path with .msg in ROS 2 Python [OK]

Common Mistakes:

• Using just 'String' without module path
• Using slash notation like 'std_msgs/String'
• Using lowercase 'string' instead of class name

3.

Given this ROS 2 Python subscriber code snippet, what will be printed when a message with data 'Hello' arrives?

def callback(msg):
    print(f'Received: {msg.data}')

node.create_subscription(std_msgs.msg.String, 'chatter', callback, 10)

medium

A. Received: msg.data

B. Hello

C. Received: Hello

D. msg.data

Solution

1. Step 1: Understand the callback function

   The callback prints 'Received: ' followed by the message's data attribute.

2. Step 2: Substitute the message data value

   When the message data is 'Hello', the print outputs 'Received: Hello'.

3. Final Answer:

   Received: Hello -> Option C

4. Quick Check:

   Prints 'Received: ' + msg.data [OK]

Hint: Callback prints 'Received: ' plus message data [OK]

Common Mistakes:

• Printing just the data without prefix
• Printing the variable name instead of its value
• Confusing string formatting syntax

4.

What is wrong with this ROS 2 Python subscriber code?

def callback(msg):
    print(msg.data)

node.create_subscription(std_msgs.msg.String, 'chatter', callback())

medium

A. Missing message type in create_subscription

B. Queue size argument is missing but required

C. Topic name should be a variable, not a string

D. Callback is called immediately instead of passed as a function

Solution

1. Step 1: Check how callback is passed

   The code uses callback() which calls the function immediately instead of passing the function reference.

2. Step 2: Understand correct callback usage

   We must pass callback without parentheses so it is called on message arrival.

3. Final Answer:

   Callback is called immediately instead of passed as a function -> Option D

4. Quick Check:

   Pass callback function, not call it [OK]

Hint: Pass callback without () to create_subscription [OK]

Common Mistakes:

• Adding parentheses to callback in subscription
• Omitting queue size argument (optional in some versions)
• Using variable instead of string for topic name

5.

You want to create a subscriber node that listens to the topic 'sensor_data' and prints only messages where the data is greater than 10. Which callback function correctly implements this?

def callback(msg):
    # Fill in the condition
    if ____:
        print(f"Data: {msg.data}")

hard

A. msg.data > 10

B. msg.data >= 10

C. msg.data < 10

D. msg.data == 10

Solution

1. Step 1: Understand the requirement

   We want to print messages only if data is greater than 10, so the condition must check msg.data > 10.

2. Step 2: Compare options with condition

   msg.data > 10 matches exactly 'greater than 10'. Others check equal or less, which do not meet the requirement.

3. Final Answer:

   msg.data > 10 -> Option A

4. Quick Check:

   Condition for data > 10 is msg.data > 10 [OK]

Hint: Use > 10 to filter messages with data above 10 [OK]

Common Mistakes:

• Using >= instead of > changes condition
• Using < or == prints wrong messages
• Not adding any condition prints all messages