ROS parameter server - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this ROS Python node when reading a parameter?

Consider this ROS Python node snippet that reads a parameter named robot_speed from the parameter server. What will it print if the parameter is not set?

ROS

import rospy

rospy.init_node('speed_reader')
speed = rospy.get_param('robot_speed', 1.0)
print(f"Speed is {speed}")

A Speed is 0

B Speed is None

C KeyError exception

D Speed is 1.0

💡 Hint

Check the default value provided to get_param.

The get_param function returns the default value if the parameter is not found. Here, 1.0 is the default, so it prints 'Speed is 1.0'.

❓ state_output

intermediate

What is the value of the parameter after this ROS C++ code runs?

Given this ROS C++ snippet that sets a parameter, what will be the value of robot_mode on the parameter server after execution?

ROS

#include <ros/ros.h>

int main(int argc, char **argv) {
  ros::init(argc, argv, "param_setter");
  ros::NodeHandle nh;
  nh.setParam("robot_mode", "AUTO");
  nh.setParam("robot_mode", "MANUAL");
  return 0;
}

A "MANUAL"

B Compilation error

C Parameter does not exist

D "AUTO"

💡 Hint

The last setParam call overwrites the previous value.

The parameter 'robot_mode' is first set to 'AUTO', then immediately overwritten to 'MANUAL'. The final value is 'MANUAL'.

📝 Syntax

advanced

Which option correctly retrieves a nested parameter from the ROS parameter server in Python?

You want to get the nested parameter /robot/config/max_speed using rospy. Which code snippet correctly retrieves it?

A max_speed = rospy.get_param('/robot/config/max_speed')

B max_speed = rospy.get_param('robot.config.max_speed')

C max_speed = rospy.get_param('~robot/config/max_speed')

D max_speed = rospy.get_param('robot/config/max_speed/')

💡 Hint

Use the full absolute parameter name starting with a slash.

The parameter server uses slash-separated paths. The correct absolute path is '/robot/config/max_speed'.

🔧 Debug

advanced

What error will this ROS Python code raise?

This code tries to get a parameter without a default value. What error occurs if the parameter robot_name is missing?

ROS

import rospy

rospy.init_node('name_reader')
name = rospy.get_param('robot_name')
print(name)

A ROSException

B KeyError

C TypeError

D No error, prints None

💡 Hint

Check rospy.get_param behavior when no default is given and parameter is missing.

If the parameter is missing and no default is provided, rospy.get_param raises a KeyError.

🧠 Conceptual

expert

How many parameters are on the server after this sequence of ROS commands?

Assume the parameter server is empty initially. After running these commands in order, how many parameters exist on the server? 1. rosparam set /robot/speed 5 2. rosparam set /robot/mode AUTO 3. rosparam delete /robot/speed 4. rosparam set /robot/speed 10

A 1

B 3

C 2

D 0

💡 Hint

Deleting a parameter removes it, but setting it again adds it back.

After step 1 and 2, two parameters exist. Step 3 deletes one, leaving one. Step 4 adds it back, so total is two.

Practice

1. What is the main purpose of the ROS parameter server?

easy

A. To compile ROS packages

B. To store and share configuration settings between ROS nodes

C. To execute robot movement commands

D. To visualize sensor data in real-time

Solution

1. Step 1: Understand the role of the parameter server

   The ROS parameter server is designed to hold parameters that nodes can access and share, such as configuration values.

2. Step 2: Compare options with this role

   Options A, B, and D describe other ROS functionalities unrelated to parameter storage.

3. Final Answer:

   To store and share configuration settings between ROS nodes -> Option B

4. Quick Check:

   Parameter server = shared settings [OK]

Hint: Parameter server holds shared settings, not commands or data [OK]

Common Mistakes:

• Confusing parameter server with command execution
• Thinking it compiles code
• Mixing it up with visualization tools

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

easy

A. rosparam set robot_speed 1.5

B. rosparam get robot_speed 1.5

C. rosparam delete robot_speed 1.5

D. rosparam list robot_speed 1.5

Solution

1. Step 1: Identify the command to set a parameter

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

2. Step 2: Verify the syntax

   The 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 A

4. Quick Check:

   Set parameter = rosparam set [OK]

Hint: Use 'rosparam set' to assign values to parameters [OK]

Common Mistakes:

• Using 'get' instead of 'set' to assign values
• Trying to delete or list with value arguments
• Incorrect command syntax

3. Given the following Python ROS node code snippet, what will be printed if the parameter max_speed is not set on the parameter server?

import rospy
rospy.init_node('speed_node')
max_speed = rospy.get_param('max_speed', 2.0)
print(max_speed)

medium

A. It will print 0

B. It will print None

C. It will print 2.0

D. It will raise a KeyError

Solution

1. Step 1: Understand the use of rospy.get_param with default

   The function rospy.get_param('max_speed', 2.0) tries to get the parameter 'max_speed'. If it does not exist, it returns the default value 2.0.

2. Step 2: Apply this to the scenario

   Since 'max_speed' is not set, the default 2.0 will be assigned and printed.

3. Final Answer:

   It will print 2.0 -> Option C

4. Quick Check:

   Missing param returns default = 2.0 [OK]

Hint: get_param returns default if param missing [OK]

Common Mistakes:

• Expecting an error if parameter missing
• Assuming None is returned by default
• Confusing default value usage

4. You wrote this code to get a parameter but it crashes with a KeyError. What is the problem?

import rospy
rospy.init_node('node')
value = rospy.get_param('unknown_param')
print(value)

medium

A. The parameter 'unknown_param' does not exist and no default was provided

B. The node was not initialized before getting the parameter

C. The print statement is incorrect syntax

D. The parameter server is not running

Solution

1. Step 1: Analyze the get_param call without default

   Calling rospy.get_param('unknown_param') without a default raises a KeyError if the parameter does not exist.

2. Step 2: Confirm the cause of the crash

   Since 'unknown_param' is missing and no default is given, the code crashes with KeyError.

3. Final Answer:

   The parameter 'unknown_param' does not exist and no default was provided -> Option A

4. Quick Check:

   Missing param + no default = KeyError [OK]

Hint: Always provide default or check param exists to avoid KeyError [OK]

Common Mistakes:

• Assuming node init causes KeyError
• Thinking print syntax is wrong
• Ignoring parameter server status

5. You want to set multiple parameters at once for a robot configuration using a YAML file named config.yaml. Which command correctly loads all parameters from this file onto the ROS parameter server under the namespace /robot?

hard

A. rosparam set config.yaml /robot

B. rosparam delete config.yaml /robot

C. rosparam get config.yaml /robot

D. rosparam load config.yaml /robot

Solution

1. Step 1: Identify the command to load parameters from a file

   The rosparam load command loads parameters from a YAML file onto the parameter server.

2. Step 2: Confirm the correct syntax for namespace

   Using rosparam load config.yaml /robot loads all parameters under the /robot namespace.

3. Final Answer:

   rosparam load config.yaml /robot -> Option D

4. Quick Check:

   Load YAML file with rosparam load [OK]

Hint: Use 'rosparam load' to import YAML parameters [OK]

Common Mistakes:

• Using 'set' instead of 'load' for files
• Trying to 'get' or 'delete' files
• Omitting namespace or using wrong syntax