Topic remapping in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remap the topic '/chatter' to '/talk' in a ROS node.

ROS

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

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

AAnonymousName

BNoRemap

CRemap

DNoSigintHandler

Attempts:

3 left

💡 Hint

Common Mistakes

Using ros::init_options::NoRemap which disables remapping.

Confusing Remap with AnonymousName option.

The ros::init_options::AnonymousName option allows remapping by creating a unique node name, enabling topic remapping.

2fill in blank

medium

Complete the launch file snippet to remap the topic '/input' to '/output'.

ROS

<node pkg="my_pkg" type="talker" name="talker">
  <remap from="[1]" to="/output"/>
</node>

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

A/chatter

B/output

C/talk

D/input

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the 'from' and 'to' values.

Using the new topic name in the 'from' attribute.

The from attribute specifies the original topic name to remap, which is '/input' here.

3fill in blank

hard

Fix the error in the remapping argument to remap '/sensor_data' to '/data'.

ROS

ros::NodeHandle nh("~");
ros::Subscriber sub = nh.subscribe("[1]", 1000, callback);

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

Asensor_data

B/sensor_data

Cdata

D/data

Attempts:

3 left

💡 Hint

Common Mistakes

Using absolute topic names with private NodeHandle.

Confusing original and remapped topic names.

Using a private NodeHandle ("~") means topic names are relative, so 'data' (without slash) is correct for remapping.

4fill in blank

hard

Fill both blanks to remap '/camera/image_raw' to '/image'.

ROS

<node pkg="image_proc" type="image_proc" name="image_proc">
  <remap from="[1]" to="[2]"/>
</node>

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

A/camera/image_raw

B/image_raw

C/image

D/camera/image

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping 'from' and 'to' values.

Using incomplete topic names.

The from attribute is the original topic '/camera/image_raw', and to is the new topic '/image'.

5fill in blank

hard

Fill all three blanks to remap topics in a ROS2 node using rclcpp.

ROS

auto node = rclcpp::Node::make_shared("[2]");
auto sub = node->create_subscription<std_msgs::msg::String>("[1]", 10, callback);

rclcpp::init(argc, argv);
rclcpp::spin(node);
rclcpp::shutdown();

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

A/chatter

B/talk

C/listener

D/talker

Attempts:

3 left

💡 Hint

Common Mistakes

Using original topic name instead of remapped topic in subscription.

Confusing node name with topic names.

The subscription topic is remapped from '/chatter' to '/talk'. The node name is 'talker', and the original topic is '/chatter'.

Practice

1. What is the main purpose of topic remapping in ROS?

easy

A. To create new topics dynamically during runtime

B. To change the message type of a topic

C. To stop a node from publishing messages

D. To change topic names without modifying the node's source code

Solution

1. Step 1: Understand topic remapping concept

   Topic remapping allows changing the topic names nodes subscribe or publish to without changing the node's code.

2. Step 2: Identify the correct purpose

   Changing message types or stopping nodes are unrelated to remapping. Creating new topics dynamically is not the main purpose.

3. Final Answer:

   To change topic names without modifying the node's source code -> Option D

4. Quick Check:

   Topic remapping = rename topics without code change [OK]

Hint: Remapping means renaming topics without touching code [OK]

Common Mistakes:

• Thinking remapping changes message types
• Confusing remapping with stopping nodes
• Assuming remapping creates new topics automatically

2. Which of the following is the correct syntax to remap a topic when launching a ROS node from the command line?

easy

A. rosrun package node _old_topic:=new_topic

B. rosrun package node __old_topic:=new_topic

C. rosrun package node old_topic:=new_topic

D. rosrun package node _old_topic=new_topic

Solution

1. Step 1: Recall ROS remapping syntax

   ROS uses the syntax old_topic:=new_topic to remap topics on the command line.

2. Step 2: Identify correct option

   rosrun package node old_topic:=new_topic matches the correct syntax without extra underscores or wrong symbols.

3. Final Answer:

   rosrun package node old_topic:=new_topic -> Option C

4. Quick Check:

   Remap syntax = old_topic:=new_topic [OK]

Hint: Use := between old and new topic names for remapping [OK]

Common Mistakes:

• Adding underscores before topic names
• Using = instead of :=
• Confusing remapping syntax with parameters

3. Given the launch command:
rosrun turtlesim turtlesim_node turtle1/cmd_vel:=robot1/cmd_vel
What will happen when the node publishes velocity commands to turtle1/cmd_vel?

medium

A. The commands will be sent to robot1/cmd_vel topic instead

B. The commands will be ignored because the topic name changed

C. The node will publish to both turtle1/cmd_vel and robot1/cmd_vel

D. The node will fail to start due to invalid remapping

Solution

1. Step 1: Understand remapping effect on publishing

   Remapping turtle1/cmd_vel to robot1/cmd_vel means the node publishes to robot1/cmd_vel instead of the original topic.

2. Step 2: Analyze options

   Commands are not ignored or duplicated; remapping redirects the topic. The node starts normally with valid remapping.

3. Final Answer:

   The commands will be sent to robot1/cmd_vel topic instead -> Option A

4. Quick Check:

   Remapping redirects publishing topic [OK]

Hint: Remapping changes topic destination for publishing [OK]

Common Mistakes:

• Thinking remapping duplicates topics
• Assuming remapping disables publishing
• Believing remapping causes node failure

4. You tried to remap a topic using the command:
rosrun my_package my_node /input:=/sensor_data
but the node still subscribes to /input instead of /sensor_data. What is the likely error?

medium

A. Using a leading slash in the remapping name causes remapping to fail

B. The remapping syntax is incorrect; it should be input:=sensor_data without slashes

C. The node does not support topic remapping

D. The ROS master is not running

Solution

1. Step 1: Check remapping syntax rules

   Remapping uses relative topic names without leading slashes; slashes can cause remapping to fail.

2. Step 2: Identify correct syntax

   Correct syntax is input:=sensor_data without slashes. Leading slashes make remapping ineffective.

3. Final Answer:

   The remapping syntax is incorrect; it should be input:=sensor_data without slashes -> Option B

4. Quick Check:

   Use relative names without slashes for remapping [OK]

Hint: Avoid leading slashes in remapping topic names [OK]

Common Mistakes:

• Using absolute topic names with slashes in remapping
• Assuming node does not support remapping without checking
• Ignoring ROS master status

5. You have two nodes: nodeA publishes to /camera/image_raw and nodeB subscribes to /image. You want nodeB to receive images from nodeA without changing their code. How can you achieve this using topic remapping in a launch file?

hard

A. inside nodeB's launch tag

B. inside nodeA's launch tag

C. inside nodeB's launch tag

D. inside nodeA's launch tag

Solution

1. Step 1: Understand remapping direction

   NodeB subscribes to /image, but nodeA publishes /camera/image_raw. To connect them, remap nodeB's /image to /camera/image_raw.

2. Step 2: Apply remapping in nodeB's launch tag

   Use <remap from="/image" to="/camera/image_raw"/> inside nodeB's launch tag to redirect subscriptions.

3. Final Answer:

   <remap from="/image" to="/camera/image_raw"/> inside nodeB's launch tag -> Option A

4. Quick Check:

   Remap subscriber topic to publisher's topic [OK]

Hint: Remap subscriber's topic to publisher's topic in launch file [OK]

Common Mistakes:

• Remapping publisher's topic instead of subscriber's
• Swapping from and to attributes
• Changing code instead of using remapping