rosparam command-line tools - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding rosparam get command output

What will be the output of the command rosparam get /robot_speed if the parameter /robot_speed is set to 1.5?

A /robot_speed: 1.5

B 1.5

C Parameter /robot_speed not set

D Error: rosparam get requires a file argument

💡 Hint

Think about what rosparam get returns when the parameter exists.

The rosparam get command outputs the value of the parameter directly. Since /robot_speed is set to 1.5, the output is just 1.5.

❓ component_behavior

intermediate

Behavior of rosparam load with nested YAML

Given the YAML file config.yaml containing:

robot:
  speed: 2.0
  mode: auto

What will be the result of running rosparam load config.yaml /robot_config and then rosparam get /robot_config/robot/speed?

A 2.0

B auto

C Parameter /robot_config/robot/speed not set

D Error: rosparam load does not accept -f option

💡 Hint

Consider how rosparam load handles nested YAML structures.

The rosparam load command loads the YAML file and sets the parameter tree under /robot_config. So /robot_config/robot/speed will be 2.0.

📝 Syntax

advanced

Identifying syntax error in rosparam command

Which option contains a syntax error when trying to delete a parameter named /sensor_threshold?

A rosparam del /sensor_threshold

B rosparam delete /sensor_threshold

C rosparam delete sensor_threshold

D rosparam delete /sensor_threshold/

💡 Hint

Check the exact command name for deleting parameters.

The correct command to delete a parameter is rosparam delete. Option A uses del which is not a valid rosparam command and causes a syntax error.

❓ state_output

advanced

Effect of rosparam dump on parameter server

After running rosparam dump params.yaml /robot, what will be the content of params.yaml if the parameter server has:

/robot/speed: 3.5
/robot/mode: manual
/robot/sensors/camera: enabled

A

/robot/speed: 3.5
/robot/mode: manual
/robot/sensors/camera: enabled

B

/robot:
  speed: 3.5
  mode: manual
  sensors:
    camera: enabled

C

speed: 3.5
mode: manual
camera: enabled

D

speed: 3.5
mode: manual
sensors:
  camera: enabled

💡 Hint

rosparam dump saves parameters under the specified namespace without the namespace prefix.

The rosparam dump command saves parameters under the given namespace without including the namespace key itself. So the YAML file contains keys speed, mode, and nested sensors under root.

🔧 Debug

expert

Diagnosing rosparam load failure

You run rosparam load config.yaml /robot but parameters are not set on the server. The config.yaml contains:

robot:
  speed: 4.0
  mode: auto

What is the most likely reason the parameters are missing after the command?

A The command should be <code>rosparam set /robot -f config.yaml</code> instead

B The parameter server is down, so rosparam load silently fails

C The YAML file has an extra root key <code>robot</code> causing parameters to be nested under /robot/robot

D rosparam load requires the -f flag to load YAML files

💡 Hint

Think about how rosparam load merges YAML keys with the namespace argument.

When loading a YAML file with a root key robot under the namespace /robot, parameters are set under /robot/robot. So /robot/speed is not set, causing confusion.

Practice

1. What is the primary purpose of the rosparam command-line tool in ROS?

easy

A. To compile ROS packages

B. To visualize robot models

C. To launch ROS nodes

D. To manage parameters on the ROS parameter server

Solution

1. Step 1: Understand the role of rosparam

   rosparam is designed to interact with the ROS parameter server, allowing users to set, get, and delete parameters.

2. Step 2: Differentiate from other ROS tools

   Compiling packages is done by catkin_make, launching nodes by roslaunch, and visualization by tools like rviz. These are unrelated to parameter management.

3. Final Answer:

   To manage parameters on the ROS parameter server -> Option D

4. Quick Check:

   rosparam manages parameters = B [OK]

Hint: rosparam always deals with parameters, not nodes or builds [OK]

Common Mistakes:

• Confusing rosparam with roslaunch
• Thinking rosparam compiles code
• Assuming rosparam visualizes data

2. Which of the following is the correct syntax to set a parameter named robot_speed to 1.5 using rosparam?

easy

A. rosparam get robot_speed 1.5

B. rosparam set robot_speed 1.5

C. rosparam load robot_speed 1.5

D. rosparam delete robot_speed 1.5

Solution

1. Step 1: Identify the command to assign a value

   The set command is used to assign a value to a parameter on the ROS parameter server.

2. Step 2: Check the syntax format

   The correct syntax is rosparam set <param_name> <value>. So rosparam set robot_speed 1.5 is correct.

3. Final Answer:

   rosparam set robot_speed 1.5 -> Option B

4. Quick Check:

   Set command assigns value = A [OK]

Hint: Use 'set' to assign values, not 'get' or 'load' [OK]

Common Mistakes:

• Using 'get' to set a value
• Confusing 'load' with 'set'
• Adding extra arguments after parameter name

3. Given the following commands run in order:

rosparam set /max_speed 2.0
rosparam get /max_speed
rosparam delete /max_speed
rosparam get /max_speed

What will be the output of the second rosparam get /max_speed command?

medium

A. Parameter /max_speed not set

B. 0

C. 2.0

D. Error: invalid command

Solution

1. Step 1: Understand the sequence of commands

   First, the parameter /max_speed is set to 2.0. Then it is retrieved, so the first get returns 2.0. Next, the parameter is deleted.

2. Step 2: Check the second get command after deletion

   After deletion, the parameter no longer exists on the server. So the second get command will report that the parameter is not set.

3. Final Answer:

   Parameter /max_speed not set -> Option A

4. Quick Check:

   Deleted parameter returns 'not set' = C [OK]

Hint: Deleted parameters return 'not set' on get [OK]

Common Mistakes:

• Assuming deleted parameters return default 0
• Expecting an error instead of a message
• Confusing set and delete effects

4. You run the command rosparam load config.yaml but get an error saying the file does not exist. What is the most likely cause?

medium

A. The YAML file path is incorrect or file is missing

B. rosparam cannot load YAML files

C. You need to use rosparam set instead of load

D. The parameter server is not running

Solution

1. Step 1: Understand rosparam load requirements

   The rosparam load command requires a valid YAML file path to load parameters from.

2. Step 2: Identify common causes of file errors

   If the file does not exist or the path is wrong, rosparam will report an error. The parameter server must be running, but that usually causes a different error.

3. Final Answer:

   The YAML file path is incorrect or file is missing -> Option A

4. Quick Check:

   File missing causes load error = A [OK]

Hint: Check file path carefully before loading YAML [OK]

Common Mistakes:

• Thinking rosparam can't load YAML files
• Using set instead of load for files
• Ignoring parameter server status

5. You want to save all current parameters under the namespace /robot to a file named robot_params.yaml. Which command correctly does this?

hard

A. rosparam get /robot > robot_params.yaml

B. rosparam load robot_params.yaml /robot

C. rosparam dump robot_params.yaml /robot

D. rosparam set /robot robot_params.yaml

Solution

1. Step 1: Identify the command to save parameters

   rosparam dump saves parameters to a YAML file. It takes the filename and optionally a namespace.

2. Step 2: Match the syntax to the requirement

   To save parameters under /robot to robot_params.yaml, the command is rosparam dump robot_params.yaml /robot.

3. Final Answer:

   rosparam dump robot_params.yaml /robot -> Option C

4. Quick Check:

   Dump saves parameters to file = D [OK]

Hint: Use 'dump' to save parameters, 'load' to restore [OK]

Common Mistakes:

• Using load instead of dump to save
• Trying to redirect get output to file
• Confusing set with dump