Topic remapping in ROS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output topic name after remapping?

Consider a ROS node that subscribes to the topic /sensor/data. The node is launched with the remapping argument /sensor/data:=/robot/sensor_data. What topic will the node actually subscribe to?

A /robot/sensor_data

B /sensor/data

C /sensor/robot_data

D /robot/data

💡 Hint

Think about how remapping changes the topic names the node uses.

The remapping argument /sensor/data:=/robot/sensor_data tells ROS to replace all uses of /sensor/data with /robot/sensor_data. So the node subscribes to /robot/sensor_data.

📝 Syntax

intermediate

Identify the correct syntax for remapping a topic in a ROS launch file

Which of the following is the correct way to remap the topic /cmd_vel to /robot/cmd_vel in a ROS 1 launch file?

A <remap old="/cmd_vel" new="/robot/cmd_vel"/>

B <remap to="/cmd_vel" from="/robot/cmd_vel"/>

C <remap topic="/cmd_vel" new_topic="/robot/cmd_vel"/>

D <remap from="/cmd_vel" to="/robot/cmd_vel"/>

💡 Hint

Look for the correct attribute names in the remap tag.

The correct syntax uses from and to attributes to specify the original and new topic names respectively.

🔧 Debug

advanced

Why does the node fail to receive messages after remapping?

A ROS node subscribes to /camera/image_raw. The launch file remaps /camera/image_raw:=/front_camera/image. However, the node does not receive any messages. What is the most likely cause?

A The node subscribes to <code>/front_camera/image</code> but the publisher still publishes to <code>/camera/image_raw</code>.

B The remapping syntax is incorrect; it should be <code>/front_camera/image:=camera/image_raw</code>.

C The remapping only affects publishers, not subscribers.

D The node subscribes to <code>/camera/image_raw</code> ignoring remapping.

💡 Hint

Consider which side (publisher or subscriber) remapping affects and what topic the publisher uses.

Remapping changes the topic name the node subscribes to. If the publisher still publishes to the original topic /camera/image_raw, the node will not receive messages on the remapped topic /front_camera/image.

🧠 Conceptual

advanced

What is the effect of remapping a private node topic?

A ROS node has a private topic ~status. If you remap ~status:=/robot/status when launching the node, what happens?

A The remapping has no effect because private topics cannot be remapped.

B The node creates a new topic <code>~robot/status</code> under its namespace.

C The node publishes or subscribes to <code>/robot/status</code> instead of its private <code>~status</code> topic.

D The node publishes to both <code>~status</code> and <code>/robot/status</code> simultaneously.

💡 Hint

Remember that private topics start with ~ and remapping can change their absolute names.

Remapping ~status to /robot/status changes the private topic to the absolute topic /robot/status. The node will use the new topic name.

❓ state_output

expert

What is the final topic name after multiple remappings?

A node subscribes to /odom. The launch file applies two remappings: /odom:=/robot/odom and /robot/odom:=/vehicle/odometry. What topic does the node actually subscribe to?

A /odom

B /robot/odom

C /vehicle/odometry

D /robot/vehicle/odometry

💡 Hint

Remappings are applied only once per topic name at node startup.

ROS applies remapping once per topic name. The node's /odom is remapped to /robot/odom. The second remapping does not apply again to the result.

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