Why ROS is the robotics standard - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

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

ROS

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

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

A"robot_controller"

Brobot_controller

C"controller_node"

Dcontroller_node

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put the node name in quotes causes a compile error.

The ros::init function requires the node name as a string literal, so it must be in quotes.

2fill in blank

medium

Complete the code to create a ROS publisher for topic 'chatter' with message type std_msgs::String.

ROS

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

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

A"chatter"

Bstd_msgs/String

Cchatter

D"std_msgs/String"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the message type or unquoted topic name instead of a quoted string.

The topic name must be a string literal, so it needs quotes around it.

3fill in blank

hard

Fix the error in the ROS subscriber callback signature to accept a shared pointer to the message.

ROS

void chatterCallback(const std_msgs::String::ConstPtr& [1]) {

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

Acallback

Bmessage

Cdata

Dmsg

Attempts:

3 left

💡 Hint

Common Mistakes

Using variable names that are not descriptive or conventional.

The conventional variable name for the message pointer in ROS callbacks is msg.

4fill in blank

hard

Fill both blanks to create a ROS service server named 'add_two_ints' with callback function 'add'.

ROS

ros::ServiceServer service = nh.advertiseService([1], [2]);

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

A"add_two_ints"

Badd

C"addInts"

Dadd_two_ints

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted service names or quoted callback names.

The service name must be a string literal, and the callback is the function name without quotes.

5fill in blank

hard

Fill all three blanks to create a ROS node that publishes 'Hello ROS' every second.

ROS

ros::NodeHandle nh;
ros::Publisher pub = nh.advertise<std_msgs::String>([1], 10);
ros::Rate loop_rate([2]);

while (ros::ok()) {
  std_msgs::String msg;
  msg.data = [3];
  pub.publish(msg);
  ros::spinOnce();
  loop_rate.sleep();
}

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

A"chatter"

B1

C"Hello ROS"

D"hello"

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong topic names, incorrect loop rate, or missing quotes around strings.

The topic name is 'chatter' as a string, the loop rate is 1 Hz, and the message data is the string 'Hello ROS'.

Practice

1. Why is ROS considered the standard framework in robotics?

easy

A. Because it requires no programming knowledge

B. Because it is the only software that can control robots

C. Because it is developed by a single company

D. Because it allows easy connection of robot parts and code sharing

Solution

1. Step 1: Understand ROS's main feature

   ROS connects different robot parts easily and allows sharing of code among developers.

2. Step 2: Compare options

   Only Because it allows easy connection of robot parts and code sharing correctly describes this key feature; others are incorrect or misleading.

3. Final Answer:

   Because it allows easy connection of robot parts and code sharing -> Option D

4. Quick Check:

   ROS connects parts and shares code = A [OK]

Hint: Remember ROS connects parts and shares code easily [OK]

Common Mistakes:

• Thinking ROS is the only robot software
• Believing ROS is made by one company
• Assuming no programming is needed

2. Which of the following is the correct way to start a ROS node in a launch file?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall ROS launch file syntax

   The correct tag to start a node is <node> with attributes pkg, type, and name.

2. Step 2: Identify correct option

   <node pkg="my_robot" type="controller" name="robot_controller" /> uses the correct <node> tag and attributes; others use invalid tags.

3. Final Answer:

   <node pkg="my_robot" type="controller" name="robot_controller" /> -> Option A

4. Quick Check:

   Use <node> tag to start nodes = B [OK]

Hint: Remember launch files use <node> tag to start nodes [OK]

Common Mistakes:

• Using incorrect tags like <start> or <run>
• Confusing launch file syntax with other XML formats
• Omitting required attributes

3. Given the following ROS Python code snippet, what will be printed when the node runs?

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

medium

A. Node started

B. test_node

C. rospy.init_node called

D. No output

Solution

1. Step 1: Understand rospy.loginfo behavior

   rospy.loginfo prints the message to the console when the node runs.

2. Step 2: Identify printed message

   The message 'Node started' is passed to loginfo, so it will be printed.

3. Final Answer:

   Node started -> Option A

4. Quick Check:

   rospy.loginfo prints message = A [OK]

Hint: rospy.loginfo prints the given message to console [OK]

Common Mistakes:

• Thinking init_node prints output
• Confusing node name with printed message
• Expecting no output without explicit print

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

import rospy
rospy.init_node('my_node')
rospy.spin()
print('Node running')

medium

A. print statement runs before spin

B. rospy.spin() blocks, so print never runs

C. init_node is missing parameters

D. Missing import for print function

Solution

1. Step 1: Understand rospy.spin()

   rospy.spin() blocks the program to keep the node alive and waits for callbacks.

2. Step 2: Analyze print statement position

   Since spin blocks, the print statement after it will never execute.

3. Final Answer:

   rospy.spin() blocks, so print never runs -> Option B

4. Quick Check:

   spin blocks execution after it = C [OK]

Hint: Remember spin() blocks; code after it won't run [OK]

Common Mistakes:

• Assuming print runs before spin
• Thinking init_node needs extra parameters
• Confusing Python 2/3 print syntax

5. A robotics team wants to reuse existing ROS packages to build a new robot quickly. Which feature of ROS makes this possible?

hard

A. ROS does not allow sharing code between teams

B. ROS requires all code to be written from scratch

C. ROS's modular design with reusable nodes and packages

D. ROS only supports one robot model at a time

Solution

1. Step 1: Understand ROS modularity

   ROS is designed with modular nodes and packages that can be reused across projects.

2. Step 2: Evaluate options

   ROS's modular design with reusable nodes and packages correctly states this feature; others contradict ROS's collaborative nature.

3. Final Answer:

   ROS's modular design with reusable nodes and packages -> Option C

4. Quick Check:

   Modular reusable packages = D [OK]

Hint: ROS modularity enables fast reuse of code and tools [OK]

Common Mistakes:

• Believing ROS forces writing all new code
• Thinking ROS supports only one robot model
• Assuming ROS prevents code sharing