Setting parameters from launch file in ROS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of setting parameters in a ROS launch file?

Setting parameters in a ROS launch file allows you to configure nodes with specific values before they start. This helps customize node behavior without changing the code.

beginner

How do you set a parameter named robot_speed to 1.5 in a ROS launch file?

You use the <param name="robot_speed" value="1.5" /> tag inside the launch file to set the parameter before nodes start.

intermediate

What is the difference between param and rosparam tags in ROS launch files?

<param> sets simple parameters like numbers or strings. <rosparam> can load complex data like dictionaries or lists from YAML files or inline YAML.

intermediate

Can parameters set in a launch file be changed while the node is running?

Parameters set in a launch file are loaded at node start. To change them while running, you need to use dynamic reconfigure or set parameters manually using command line or code.

beginner

Why is it useful to set parameters in a launch file instead of hardcoding them in the node code?

Setting parameters in a launch file makes your nodes flexible and reusable. You can change behavior without editing code, which is easier and safer for testing and deployment.

Which tag is used to set a simple parameter in a ROS launch file?

A <rosparam>

B <include>

C <node>

D <param>

The tag sets simple parameters like numbers or strings in a launch file.

How do you load a YAML file with parameters in a ROS launch file?

A <rosparam file="params.yaml" />

B <include file="params.yaml" />

C <node params="params.yaml" />

D <param file="params.yaml" />

The tag with the file attribute loads parameters from a YAML file.

If you want to set a parameter named 'max_speed' to 2.0, which is correct?

A <param max_speed="2.0" />

B <param name="max_speed" value="2.0" />

C <rosparam name="max_speed" value="2.0" />

D <param name="max_speed">2.0</param>

The correct syntax uses .

Can parameters set in a launch file be accessed by nodes after they start?

A Yes, nodes can read parameters from the parameter server anytime.

B No, parameters are only available during launch.

C Only if the node is restarted.

D Only if parameters are hardcoded in the node.

Parameters set in launch files are stored on the parameter server and can be accessed anytime by nodes.

What is a benefit of using <rosparam> over <param> in launch files?

A <rosparam> automatically restarts nodes.

B <rosparam> is faster to load parameters.

C <rosparam> can load complex data like lists and dictionaries.

D <rosparam> is deprecated.

supports loading complex YAML data structures, unlike .

Explain how to set a parameter in a ROS launch file and why it is useful.

Think about how launch files help customize nodes without code changes.

Describe the difference between and tags in ROS launch files.

Consider the type of data each tag can handle.

Practice

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

easy

A. To start multiple nodes at the same time

B. To set parameters that nodes can use when they start

C. To define the order in which nodes run

D. To stop nodes after they finish

Solution

1. Step 1: Understand the role of <param> in launch files

   The <param> tag is used to set parameters that nodes read when they start running.

2. Step 2: Differentiate from other launch file tags

   Tags like <node> start nodes, but <param> specifically sets values nodes use.

3. Final Answer:

   To set parameters that nodes can use when they start -> Option B

4. Quick Check:

   Setting parameters = A [OK]

Hint: Remember: <param> sets values, <node> starts nodes [OK]

Common Mistakes:

• Confusing <param> with <node> tag
• Thinking <param> controls node start/stop
• Assuming <param> sets node execution order

2. Which of the following is the correct syntax to set a string parameter named robot_name to "R2D2" in a ROS launch file?

easy

A.

B.

C.

D.

Solution

1. Step 1: Check the correct attribute names

   The <param> tag requires 'name' for the parameter name and 'value' for its value.

2. Step 2: Verify proper quoting of string values

   String values must be inside quotes, so value="R2D2" is correct.

3. Final Answer:

   <param name="robot_name" value="R2D2" /> -> Option C

4. Quick Check:

   Correct attributes and quotes = B [OK]

Hint: Use name="" and value="" with quotes for strings [OK]

Common Mistakes:

• Missing quotes around string values
• Swapping name and value attributes
• Using incorrect attribute names

3. Given this launch file snippet:

<launch>
  <param name="speed" value="2.5" />
  <node pkg="robot_pkg" type="move_node" name="mover" />
</launch>

What will be the value of the speed parameter inside the move_node when it starts?

medium

A. 2.5 (as a float)

B. 0, the default value

C. Undefined, because parameters must be set inside the node tag

D. "2.5" (as a string)

Solution

1. Step 1: Understand parameter value types in launch files

   Values in <param> tags are treated as strings by default unless explicitly converted in code.

2. Step 2: Check the launch file snippet

   The value is "2.5" in quotes, so the parameter is set as a string "2.5".

3. Final Answer:

   "2.5" (as a string) -> Option D

4. Quick Check:

   Parameter values are strings by default = D [OK]

Hint: Parameter values are strings unless converted in code [OK]

Common Mistakes:

• Assuming numeric values are auto-converted to float
• Thinking parameters must be inside <node> tag
• Believing default values apply if parameter is set

4. Identify the error in this launch file snippet:

<launch>
  <param name="use_sim" value=true />
  <node pkg="sim_pkg" type="sim_node" name="simulator" />
</launch>

medium

A. The value true should be in quotes: value="true"

B. The param tag should be inside the node tag

C. The node tag is missing the required args attribute

D. The launch tag is missing a closing slash

Solution

1. Step 1: Check the value attribute format

   Boolean values in <param> tags must be strings, so true should be "true" in quotes.

2. Step 2: Verify tag placement and syntax

   The <param> tag can be outside the <node> tag, and the <launch> tag is correctly closed.

3. Final Answer:

   The value true should be in quotes: value="true" -> Option A

4. Quick Check:

   Boolean values need quotes in <param> = C [OK]

Hint: Always put boolean values in quotes in <param> tags [OK]

Common Mistakes:

• Leaving boolean values unquoted
• Placing <param> tags only inside <node>
• Misunderstanding launch tag syntax

5. You want to set a parameter max_retries to 5 only if it is not already set by another launch file. Which launch file tag should you use to achieve this?

hard

A.

B. 5

C.

D.

Solution

1. Step 1: Understand conditional parameter setting

   The <param> tag supports the 'unless' attribute to set a parameter only if it is not already set.

2. Step 2: Check the options for correct usage

   <param unless="max_retries" name="max_retries" value="5" /> uses 'unless="max_retries"' with name and value, which is the correct syntax.

3. Final Answer:

   <param unless="max_retries" name="max_retries" value="5" /> -> Option A

4. Quick Check:

   Use 'unless' attribute to avoid overwriting = A [OK]

Hint: Use 'unless' attribute to set param only if unset [OK]

Common Mistakes:

• Omitting the 'name' attribute when using 'unless'
• Using <rosparam> without condition
• Misplacing 'unless' attribute