Creating a publisher node in ROS - Quick Revision & Summary

Recall & Review

beginner

What is a publisher node in ROS?

A publisher node in ROS is a program that sends messages to a topic. Other nodes can subscribe to this topic to receive the messages.

beginner

Which ROS function is used to create a publisher?

The function rospy.Publisher() in Python or node_handle.advertise() in C++ is used to create a publisher that sends messages on a specific topic.

beginner

What are the main steps to create a publisher node in ROS?

1. Initialize the ROS node.
2. Create a publisher object for a topic.
3. Publish messages in a loop or callback.
4. Keep the node running to send messages continuously.

beginner

Why do we use rospy.Rate() in a publisher node?

rospy.Rate() controls how fast the loop runs, so messages are published at a steady speed, like sending updates every second.

beginner

What message type is commonly used for simple text messages in ROS publishers?

The std_msgs/String message type is used to send simple text messages in ROS.

What does a ROS publisher node do?

A Stores messages in a database

B Receives messages from a topic

C Sends messages to a topic

D Visualizes messages on screen

A publisher node sends messages to a topic that other nodes can subscribe to.

Which function initializes a ROS node in Python?

A rospy.init_node()

B rospy.Publisher()

C rospy.spin()

D rospy.Rate()

rospy.init_node() starts the ROS node with a given name.

What is the purpose of rospy.spin() in a publisher node?

A Keeps the node running to process callbacks

B Publishes messages once

C Stops the node immediately

D Creates a new topic

rospy.spin() keeps the node alive to handle callbacks and continue running.

Which message type is used to send text in ROS?

A sensor_msgs/Image

B std_msgs/String

C geometry_msgs/Twist

D nav_msgs/Odometry

std_msgs/String is the standard message type for text.

How do you control the publishing frequency in a ROS publisher node?

A Using rospy.Subscriber()

B Using rospy.Publisher()

C Using rospy.init_node()

D Using rospy.Rate()

rospy.Rate() sets how often messages are published in a loop.

Describe the process of creating a simple ROS publisher node that sends text messages.

Think about the steps from starting the node to sending messages repeatedly.

Explain why controlling the publishing rate is important in a ROS publisher node.

Consider what happens if messages are sent too fast or too slow.

Practice

1. What is the main purpose of a publisher node in ROS?

easy

A. To store data in a database

B. To receive messages from other nodes

C. To compile the ROS package

D. To send messages on a topic to share information

Solution

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

   A publisher node sends messages to other nodes by publishing on a topic.

2. Step 2: Compare options with the role

   Only To send messages on a topic to share information describes sending messages on a topic, which matches the publisher's role.

3. Final Answer:

   To send messages on a topic to share information -> Option D

4. Quick Check:

   Publisher node = sends messages [OK]

Hint: Publisher nodes send messages, subscribers receive them [OK]

Common Mistakes:

• Confusing publisher with subscriber
• Thinking publisher stores data
• Assuming publisher compiles code

2. Which of the following is the correct way to create a publisher in a ROS2 Python node?

easy

A. self.createPublisher(String, 'topic_name', 10)

B. self.create_publisher(String, 'topic_name', 10)

C. create_publisher(String, 'topic_name', 10)

D. self.create_publisher('topic_name', String, 10)

Solution

1. Step 1: Recall ROS2 Python publisher syntax

   The method is called on the node instance as self.create_publisher, with message type first, then topic name, then queue size.

2. Step 2: Check each option's syntax

   self.create_publisher(String, 'topic_name', 10) matches the correct method name and argument order. self.createPublisher(String, 'topic_name', 10) has wrong method name casing. create_publisher(String, 'topic_name', 10) misses self. self.create_publisher('topic_name', String, 10) swaps argument order.

3. Final Answer:

   self.create_publisher(String, 'topic_name', 10) -> Option B

4. Quick Check:

   Correct method and argument order = self.create_publisher(String, 'topic_name', 10) [OK]

Hint: Use self.create_publisher with message type first [OK]

Common Mistakes:

• Wrong method name casing
• Missing self when calling method
• Swapping topic name and message type

3. Given this ROS2 Python publisher code snippet, what will be printed when the timer callback runs?

def timer_callback(self):
    msg = String()
    msg.data = 'Hello ROS'
    self.publisher.publish(msg)
    print('Published:', msg.data)

medium

A. Published: Hello ROS

B. Hello ROS

C. Published: msg.data

D. Error: msg.data undefined

Solution

1. Step 1: Analyze the print statement

   The print outputs the string 'Published:' followed by the value of msg.data, which is 'Hello ROS'.

2. Step 2: Confirm msg.data value

   msg.data is assigned 'Hello ROS' before publishing, so the print shows 'Published: Hello ROS'.

3. Final Answer:

   Published: Hello ROS -> Option A

4. Quick Check:

   Print shows 'Published:' plus msg.data [OK]

Hint: Print uses string plus msg.data value [OK]

Common Mistakes:

• Printing variable name instead of value
• Assuming print shows only msg.data
• Confusing msg.data with undefined

4. Identify the error in this ROS2 Python publisher node snippet:

def __init__(self):
    self.publisher = self.create_publisher(String, 'chatter', 10)

def timer_callback(self):
    msg = String()
    msg.data = 'Hello'
    self.publisher.publish(msg)

self.timer = self.create_timer(1.0, timer_callback)

medium

A. timer_callback is missing 'self' parameter

B. create_publisher is called with wrong arguments

C. Timer is created outside the class constructor

D. msg.data is assigned incorrectly

Solution

1. Step 1: Check code structure and indentation

   The self.timer = self.create_timer(1.0, timer_callback) line lacks indentation and appears outside __init__ at class level, where 'self' is undefined, causing NameError.

2. Step 2: Verify other options

   A: timer_callback includes 'self'. B: create_publisher args are correct (type, topic, qos). D: msg.data = 'Hello' is valid.

3. Final Answer:

   Timer is created outside the class constructor -> Option C

4. Quick Check:

   self assignments must be inside instance methods like __init__ [OK]

Hint: Create timers inside __init__ with self.create_timer(1.0, self.timer_callback) [OK]

Common Mistakes:

• Omitting 'self' in method parameters
• Misordering create_publisher arguments
• Creating timer outside class context

5. You want to create a ROS2 publisher node that publishes integers incrementing every second starting from 0. Which code snippet correctly implements this behavior?

hard

A. class IntPublisher(Node):
def __init__(self):
super().__init__('int_publisher')
self.publisher = self.create_publisher(Int32, 'numbers', 10)
self.i = 0
self.timer = self.create_timer(1.0, self.timer_callback)
def timer_callback(self):
msg = Int32()
msg.data = self.i
self.publisher.publish(msg)
self.i += 1

B. class IntPublisher(Node):
def __init__(self):
super().__init__('int_publisher')
self.publisher = self.create_publisher(Int32, 'numbers', 10)
self.i = 0
self.timer = self.create_timer(1.0, self.timer_callback)
def timer_callback():
msg = Int32()
msg.data = self.i
self.publisher.publish(msg)
self.i += 1

C. class IntPublisher(Node):
def __init__(self):
super().__init__('int_publisher')
self.publisher = self.create_publisher(Int32, 'numbers', 10)
self.i = 0
self.timer = self.create_timer(1.0, self.timer_callback)
def timer_callback(self):
msg = Int32()
msg.data = 0
self.publisher.publish(msg)
self.i += 1

D. class IntPublisher(Node):
def __init__(self):
super().__init__('int_publisher')
self.publisher = self.create_publisher(Int32, 'numbers', 10)
self.i = 1
self.timer = self.create_timer(1.0, self.timer_callback)
def timer_callback(self):
msg = Int32()
msg.data = self.i
self.publisher.publish(msg)
self.i = 0

Solution

1. Step 1: Check timer_callback method signatures

   Options A and C define timer_callback(self); B omits 'self', causing AttributeError on self.i and self.publisher.

2. Step 2: Verify increment logic

   class IntPublisher(Node): def __init__(self): super().__init__('int_publisher') self.publisher = self.create_publisher(Int32, 'numbers', 10) self.i = 0 self.timer = self.create_timer(1.0, self.timer_callback) def timer_callback(self): msg = Int32() msg.data = self.i self.publisher.publish(msg) self.i += 1 initializes self.i=0 and does self.i += 1 after publishing, yielding 0,1,2,... class IntPublisher(Node): def __init__(self): super().__init__('int_publisher') self.publisher = self.create_publisher(Int32, 'numbers', 10) self.i = 0 self.timer = self.create_timer(1.0, self.timer_callback) def timer_callback(self): msg = Int32() msg.data = 0 self.publisher.publish(msg) self.i += 1 always publishes 0. class IntPublisher(Node): def __init__(self): super().__init__('int_publisher') self.publisher = self.create_publisher(Int32, 'numbers', 10) self.i = 1 self.timer = self.create_timer(1.0, self.timer_callback) def timer_callback(self): msg = Int32() msg.data = self.i self.publisher.publish(msg) self.i = 0 publishes 1 then sets self.i=0 repeatedly.

3. Final Answer:

   self.i = 0; msg.data = self.i; self.i += 1 -> Option A

4. Quick Check:

   self.i starts 0, increment after publish [OK]

Hint: Use self.i and increment after publishing in timer callback [OK]

Common Mistakes:

• Missing 'self' in timer_callback
• Not incrementing the counter
• Resetting counter inside callback