Launch file XML syntax in ROS - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the error in this ROS launch XML snippet

What error will this ROS launch file snippet cause when run?

ROS

<launch>
  <node pkg="turtlesim" type="turtlesim_node" name="sim"/>
  <param name="background_r" value="255"/>
  <param name="background_g" value="255"/>
  <param name="background_b" value="255"/>
</launch>

A SyntaxError due to missing closing angle bracket in </launch>

B Runtime error because 'param' tags are not allowed inside 'launch'

C No error, the file runs correctly and sets background color

D Error because 'node' tag must have a closing tag, not self-closing

💡 Hint

Check the closing tags carefully in XML.

The closing launch tag is missing the closing angle bracket '>'. This causes XML parsing to fail.

❓ component_behavior

intermediate

What happens when you use in a ROS launch file?

Given this snippet, what is the effect of the tag in a ROS launch file?

ROS

<launch>
  <include file="$(find turtlesim)/launch/turtlesim.launch"/>
</launch>

A It ignores the included file and only runs nodes in the current file

B It runs the included launch file as part of this launch, merging nodes and params

C It causes a syntax error because <include> is not a valid tag

D It runs the included file in a separate ROS master instance

💡 Hint

Think about how you can reuse launch files.

The tag imports another launch file, running its nodes and parameters as part of the current launch.

❓ state_output

advanced

What is the value of the 'robot_name' parameter after launch?

Consider this launch file snippet. What will be the value of the 'robot_name' parameter in the ROS parameter server after launch?

ROS

<launch>
  <param name="robot_name" value="alpha"/>
  <param name="robot_name" value="beta"/>
</launch>

A "beta" because the second param overrides the first

B "alpha" because the first param sets it and second is ignored

C An error occurs due to duplicate param names

D The parameter is unset because of conflicting definitions

💡 Hint

Later parameters override earlier ones.

In ROS launch files, if the same parameter is set multiple times, the last value overrides previous ones.

🔧 Debug

advanced

Why does this launch file fail to start the node?

This launch file fails to start the node. What is the cause?

ROS

<launch>
  <node pkg="my_package" type="my_node" name="my_node">
</launch>

A The 'name' attribute cannot be the same as the 'type'

B The 'pkg' attribute must be the full path, not just package name

C The 'type' attribute must be 'executable', not 'my_node'

D Missing closing </node> tag causes XML parsing error

💡 Hint

Check if all tags are properly closed.

The tag is not closed properly. It needs a closing tag or self-closing slash.

🧠 Conceptual

expert

How does the 'arg' tag affect launch file behavior?

What is the main purpose of the tag in a ROS launch file?

A To define environment variables for nodes to use at runtime

B To include external launch files dynamically based on conditions

C To declare a variable that can be set when launching to customize parameters or node names

D To specify the order in which nodes start during launch

💡 Hint

Think about how you can pass values into a launch file.

The tag declares arguments that can be set when running the launch file to customize behavior.

Practice

1. What is the main purpose of a <launch> file in ROS?

easy

A. To compile the robot's source code

B. To write the robot's main program logic

C. To store sensor data during robot operation

D. To start and configure multiple robot nodes easily

Solution

1. Step 1: Understand the role of launch files

   Launch files in ROS are XML files used to start and configure multiple nodes and parameters together.

2. Step 2: Compare options with launch file purpose

   Only To start and configure multiple robot nodes easily correctly describes this purpose. Other options describe unrelated tasks.

3. Final Answer:

   To start and configure multiple robot nodes easily -> Option D

4. Quick Check:

   Launch files start nodes = A [OK]

Hint: Launch files group node startups in XML [OK]

Common Mistakes:

• Confusing launch files with program code
• Thinking launch files store data
• Assuming launch files compile code

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

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall correct node tag attributes

   The correct attributes are pkg, type, and name inside a <node> tag.

2. Step 2: Match options with correct syntax

   Only <node pkg="my_pkg" type="my_node" name="node1" /> uses the correct tag and attribute names. Others use wrong attribute names or tags.

3. Final Answer:

   <node pkg="my_pkg" type="my_node" name="node1" /> -> Option B

4. Quick Check:

   Correct node syntax = A [OK]

Hint: Use pkg, type, name in <node> tag [OK]

Common Mistakes:

• Using wrong attribute names like package or exec
• Using incorrect tag names like launch_node
• Confusing name with id or label

3. Given this launch file snippet, what will happen when it runs?

<launch>
  <node pkg="turtlesim" type="turtlesim_node" name="sim" />
  <param name="background_r" value="255" />
</launch>

medium

A. Starts the turtlesim node with a red background color

B. Fails because <param> is outside <node>

C. Starts the turtlesim node with default background color

D. Starts a node named 'background_r' with value 255

Solution

1. Step 1: Understand node and param placement

   The <param> tag outside <node> sets a global parameter, not specific to the node.

2. Step 2: Check if turtlesim uses this param

   The parameter background_r is not recognized globally by turtlesim, so it uses default background color.

3. Final Answer:

   Starts the turtlesim node with default background color -> Option C

4. Quick Check:

   Param outside node is global but unused = C [OK]

Hint: Params outside nodes are global, may not affect node [OK]

Common Mistakes:

• Assuming param inside launch affects node color
• Thinking param outside node causes error
• Confusing param tag as node

4. Identify the error in this launch file snippet:

<launch>
  <node pkg="demo_pkg" type="demo_node" name="demo">
</launch>

medium

A. Missing closing tag for <node>

B. Incorrect attribute name 'type'

C. Missing 'pkg' attribute in <node>

D. Extra closing tag for <launch>

Solution

1. Step 1: Check XML tag completeness

   The <node> tag is opened but not closed properly (no closing tag or self-closing slash).

2. Step 2: Verify other attributes and tags

   Attributes 'pkg' and 'type' are correct. The <launch> tag is properly closed.

3. Final Answer:

   Missing closing tag for <node> -> Option A

4. Quick Check:

   Unclosed node tag = D [OK]

Hint: Every opened tag must be closed or self-closed [OK]

Common Mistakes:

• Forgetting to close <node> tag
• Confusing attribute names
• Miscounting launch tag closures

5. How can you include another launch file named camera.launch inside your main launch file to start its nodes as well?

hard

A.

B.

C.

D.

Solution

1. Step 1: Recall correct include syntax in ROS launch files

   The <include> tag uses the attribute file to specify the file path.

2. Step 2: Understand file path usage with $(find package)

   To locate a launch file inside a package, use $(find package_name)/launch/filename.launch with file attribute.

3. Step 3: Compare options

   Only <include file="$(find package_name)/launch/camera.launch" /> uses the correct attribute file and the proper path syntax with $(find ...).

4. Final Answer:

   <include file="$(find package_name)/launch/camera.launch" /> -> Option A

5. Quick Check:

   Include uses file and $(find) = C [OK]

Hint: Use [OK]

Common Mistakes:

• Using 'launchfile' instead of 'file' attribute
• Not using $(find package_name) for path
• Omitting include tag or wrong syntax