Publish-subscribe communication model in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a ROS publisher that publishes messages on the 'chatter' topic.

ROS

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

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

Asubscribe

Badvertise

Cinit

Dspin

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'subscribe' instead of 'advertise' to create a publisher.

Using 'spin' which is for processing callbacks.

The advertise function creates a publisher for a topic in ROS.

2fill in blank

medium

Complete the code to subscribe to the 'chatter' topic with a callback function.

ROS

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

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

Asubscribe

Bpublish

Cadvertise

Dinit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'advertise' instead of 'subscribe' to receive messages.

Using 'publish' which is for sending messages.

The subscribe function creates a subscriber to listen to messages on a topic.

3fill in blank

hard

Fix the error in the publisher loop to publish messages correctly.

ROS

std_msgs::String msg;
msg.data = "hello world";
[1].publish(msg);

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

Anode

Bpublisher

Cpub

Dsubscriber

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'subscriber' to publish messages.

Using 'node' which is not the publisher object.

The publisher object is usually named pub and is used to send messages.

4fill in blank

hard

Fill both blanks to create a subscriber with a callback and queue size.

ROS

ros::Subscriber sub = nh.[1]("sensor_data", [2], sensorCallback);

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

Asubscribe

B1000

C10

Dadvertise

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'advertise' instead of 'subscribe' for receiving messages.

Setting queue size too large or using a string instead of a number.

Use subscribe to listen to a topic and set the queue size to 10 for buffering messages.

5fill in blank

hard

Fill all three blanks to create a publisher, publish a message, and spin the node.

ROS

ros::Publisher [1] = nh.[2]<std_msgs::String>("status", 1000);
std_msgs::String msg;
msg.data = "running";
[3].publish(msg);
ros::spin();

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

Apub

Badvertise

Dsub

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'sub' instead of 'pub' for publishing.

Using 'subscribe' instead of 'advertise' to create the publisher.

We create a publisher named pub using advertise, then publish the message with pub.publish(msg).

Practice

1. What is the main role of a publisher in the ROS publish-subscribe model?

easy

A. To create new topics dynamically

B. To receive messages from a topic

C. To store messages in a database

D. To send messages to a topic

Solution

1. Step 1: Understand the publish-subscribe roles

   In ROS, publishers send messages to topics, while subscribers receive messages from topics.

2. Step 2: Identify the publisher's function

   The publisher's job is to send data to a topic so subscribers can receive it.

3. Final Answer:

   To send messages to a topic -> Option D

4. Quick Check:

   Publisher = sends messages [OK]

Hint: Publisher sends, subscriber receives messages [OK]

Common Mistakes:

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

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

easy

A. pub = rospy.Publisher('topic_name', callback, queue_size=10)

B. pub = rospy.Subscriber('topic_name', String, callback)

C. pub = rospy.Publisher('topic_name', String, queue_size=10)

D. pub = rospy.Subscriber('topic_name', String)

Solution

1. Step 1: Recall publisher syntax in ROS Python

   The correct syntax uses rospy.Publisher with topic name, message type, and queue size.

2. Step 2: Check each option

   pub = rospy.Publisher('topic_name', String, queue_size=10) matches the correct syntax. Using Subscriber instead of Publisher is incorrect. pub = rospy.Publisher('topic_name', callback, queue_size=10) incorrectly uses callback instead of message type.

3. Final Answer:

   pub = rospy.Publisher('topic_name', String, queue_size=10) -> Option C

4. Quick Check:

   Publisher syntax = rospy.Publisher(topic, type, queue_size) [OK]

Hint: Publisher uses rospy.Publisher(topic, type, queue_size) [OK]

Common Mistakes:

• Using Subscriber instead of Publisher
• Passing callback instead of message type
• Omitting queue_size parameter

3. Given the following ROS Python code snippet, what will be printed when a message with data 'Hello' is published?

import rospy
from std_msgs.msg import String

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

rospy.init_node('listener')
sub = rospy.Subscriber('chatter', String, callback)
rospy.spin()

medium

A. Received: Hello

B. Hello

C. Received: msg

D. No output

Solution

1. Step 1: Understand the callback function

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

2. Step 2: Analyze the message content

   The message data is 'Hello', so the print statement outputs 'Received: Hello'.

3. Final Answer:

   Received: Hello -> Option A

4. Quick Check:

   Callback prints 'Received: ' + msg.data [OK]

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

Common Mistakes:

• Printing message object instead of data
• Expecting just the message data without prefix
• Thinking no output occurs without explicit print

4. Identify the error in this ROS Python subscriber code:

import rospy
from std_msgs.msg import String

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

rospy.init_node('listener')
sub = rospy.Subscriber('chatter', String)
rospy.spin()

medium

A. Incorrect message type used

B. Missing callback function in Subscriber

C. Node not initialized

D. No import for rospy

Solution

1. Step 1: Review Subscriber syntax

   Subscriber requires three arguments: topic name, message type, and callback function.

2. Step 2: Check the given code

   The Subscriber call misses the callback argument, causing an error.

3. Final Answer:

   Missing callback function in Subscriber -> Option B

4. Quick Check:

   Subscriber needs callback argument [OK]

Hint: Subscriber must include callback function [OK]

Common Mistakes:

• Omitting callback in Subscriber
• Confusing Publisher and Subscriber arguments
• Forgetting to initialize node

5. You want to create a ROS system where multiple nodes publish sensor data to the same topic, and one node subscribes to process all data. Which feature of the publish-subscribe model makes this possible?

hard

A. Topics allow multiple publishers and subscribers to communicate independently

B. Each topic can have only one publisher and one subscriber

C. Publishers must know all subscribers to send messages

D. Subscribers can only listen to one topic at a time

Solution

1. Step 1: Understand topic communication in ROS

   Topics support multiple publishers and subscribers simultaneously, enabling flexible communication.

2. Step 2: Analyze the scenario

   Multiple nodes publishing to the same topic and one subscriber processing all data is possible because topics allow many publishers and subscribers.

3. Final Answer:

   Topics allow multiple publishers and subscribers to communicate independently -> Option A

4. Quick Check:

   Topics support many-to-many communication [OK]

Hint: Topics support many publishers and subscribers [OK]

Common Mistakes:

• Thinking topics allow only one publisher or subscriber
• Believing publishers must know subscribers
• Assuming subscribers can listen to only one topic