Setting parameters from launch file in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a parameter named 'robot_speed' to 1.0 in the launch file.

ROS

<launch>
  <param name="robot_speed" value="[1]" />
</launch>

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

A10

Btrue

C"fast"

D1.0

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around numeric values when not needed.

Setting the value to a boolean instead of a number.

The <param> tag sets a parameter with the given name and value. Here, 'robot_speed' is set to 1.0.

2fill in blank

medium

Complete the code to set a parameter 'use_sim_time' to true in the launch file.

ROS

<launch>
  <param name="use_sim_time" value="[1]" />
</launch>

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

Atrue

B"true"

C1

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Putting quotes around true, which makes it a string.

Using 1 instead of true for boolean.

The use_sim_time parameter is a boolean and should be set to true without quotes.

3fill in blank

hard

Fix the error in the launch file to correctly set the parameter 'max_velocity' to 2.5.

ROS

<launch>
  <param name="max_velocity" value="[1]" />
</launch>

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

A"2.5"

B2.5

Ctrue

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the value unquoted causing XML parsing errors.

Using a boolean value instead of a number.

When the value attribute is not quoted, it causes an error. The numeric value should be inside quotes as a string in XML.

4fill in blank

hard

Fill both blanks to set parameters 'robot_name' to 'alpha' and 'robot_id' to 42 in the launch file.

ROS

<launch>
  <param name="robot_name" value="[1]" />
  <param name="robot_id" value="[2]" />
</launch>

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

A"alpha"

B42

C"beta"

D24

Attempts:

3 left

💡 Hint

Common Mistakes

Not quoting string values.

Mixing up the numeric values.

String values must be quoted, so 'alpha' is "alpha". Numeric values like 42 do not need quotes but can be quoted as strings.

5fill in blank

hard

Fill all three blanks to set parameters 'enable_logging' to true, 'log_level' to 'debug', and 'max_retries' to 5.

ROS

<launch>
  <param name="enable_logging" value="[1]" />
  <param name="log_level" value="[2]" />
  <param name="max_retries" value="[3]" />
</launch>

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

Afalse

B"debug"

C5

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Quoting boolean values.

Not quoting string values.

Using wrong numeric values.

Boolean true is unquoted, string 'debug' must be quoted, and number 5 can be unquoted.

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