Action definition (.action files) in ROS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the purpose of the Goal section in a ROS .action file?

In a ROS action definition file, the Goal section specifies what?

Choose the best description.

A It defines the error messages sent if the action fails.

B It defines the feedback data sent from the server to the client during task execution.

C It defines the data sent from the client to the action server to request a task.

D It defines the final result data sent from the server to the client after task completion.

💡 Hint

Think about what the client sends to start an action.

The Goal section defines the input data the client sends to the action server to request a specific task. Feedback and Result are separate sections.

📝 Syntax

intermediate

Which of these is a valid ROS .action file syntax for defining feedback?

Select the correct syntax snippet for the Feedback section in a ROS .action file.

A

float32 progress
string status_message

B float32 progress: string status_message

C float32 progress, string status_message

D float32 progress; string status_message;

💡 Hint

ROS .action files use simple field declarations without semicolons or commas.

ROS .action files list fields line by line with type and name separated by space, no semicolons or commas.

❓ state_output

advanced

What will be the output of this .action file's Result section after completion?

Given this Result section in a ROS .action file:

bool success
string message
int32 final_count

What data types will the client receive in the result?

A A boolean indicating success, a string message, and an integer count.

B Only a boolean success flag.

C A string message and an integer count, but no boolean.

D An integer count and a float progress value.

💡 Hint

Look carefully at the declared fields and their types.

The Result section defines all fields sent back after action completion, including success (bool), message (string), and final_count (int32).

🔧 Debug

advanced

Why does this .action file cause a syntax error?

Consider this snippet from a ROS .action file:

Goal:
int32 target
---
Feedback:
float32 progress
string status_message
---
Result:
bool success
string message,

What causes the syntax error?

ROS

Goal:
int32 target
---
Feedback:
float32 progress
string status_message
---
Result:
bool success
string message,

A The 'int32 target' field is missing a semicolon.

B The '---' separators are missing between sections.

C The 'Goal:' and 'Feedback:' labels are not allowed in .action files.

D The comma after 'string message' in the Result section is invalid syntax.

💡 Hint

Check punctuation at the end of lines.

ROS .action files do not use commas or semicolons at the end of field lines. The comma after 'string message' causes a syntax error.

🧠 Conceptual

expert

How does the Feedback section in a ROS .action file improve client-server interaction?

Choose the best explanation of the role of the Feedback section in ROS actions.

A It defines the final output data sent once the action completes successfully.

B It allows the action server to send ongoing status updates to the client while the action is running.

C It specifies the initial parameters the client sends to start the action.

D It handles error messages sent if the action fails.

💡 Hint

Think about what happens during the action execution, not just at the start or end.

The Feedback section is used by the action server to send periodic updates to the client about progress or status while the action is still running.

Practice

1. What is the main purpose of an .action file in ROS?

easy

A. To define goals, results, and feedback for long-running tasks

B. To list all the nodes in a ROS system

C. To configure robot sensors

D. To specify message types for topics

Solution

1. Step 1: Understand the role of .action files

   .action files are designed to define the structure of actions, which include goals, feedback, and results for tasks that take time.

2. Step 2: Compare with other ROS files

   Unlike message or service files, .action files specifically handle long-running tasks with progress updates.

3. Final Answer:

   To define goals, results, and feedback for long-running tasks -> Option A

4. Quick Check:

   Action files = goals + feedback + results [OK]

Hint: Remember: .action files split into goal, result, feedback [OK]

Common Mistakes:

• Confusing .action files with .msg or .srv files
• Thinking .action files configure hardware
• Assuming .action files list nodes

2. Which of the following is the correct structure of a ROS .action file?

easy

A. goal fields --- feedback fields --- result fields

B. goal fields --- result fields --- feedback fields

C. result fields --- feedback fields --- goal fields

D. feedback fields --- goal fields --- result fields

Solution

1. Step 1: Recall the .action file format

   The .action file has three parts separated by lines with three dashes (---): first goal, then result, then feedback.

2. Step 2: Match the order

   The correct order is goal fields first, result fields second, and feedback fields last.

3. Final Answer:

   goal fields --- result fields --- feedback fields -> Option B

4. Quick Check:

   Order = goal, result, feedback [OK]

Hint: Remember order: Goal first, Result second, Feedback last [OK]

Common Mistakes:

• Mixing up the order of feedback and result
• Using only two sections instead of three
• Forgetting the --- separators

3. Given this .action file snippet:

int32 goal_value
---
int32 result_value
---
float32 progress

What is the type of the feedback field?

medium

A. int32

B. bool

C. string

D. float32

Solution

1. Step 1: Identify the feedback section

   The .action file is split by --- lines. The third section after the second --- is the feedback part.

2. Step 2: Check the feedback field type

   The third section contains float32 progress, so the feedback field type is float32.

3. Final Answer:

   float32 -> Option D

4. Quick Check:

   Feedback type = float32 [OK]

Hint: Feedback is always after second --- line [OK]

Common Mistakes:

• Confusing result and feedback sections
• Choosing goal field type instead
• Ignoring the --- separators

4. What is wrong with this .action file?

int32 goal_value
---
float32 feedback_value
---
int32 result_value

medium

A. Result section must come before feedback

B. The order of sections is correct; no error

C. Feedback section cannot have fields

D. Feedback section is in the wrong place

Solution

1. Step 1: Check the order of sections

   The file has goal fields first, then feedback fields, then result fields, separated by --- lines.

2. Step 2: Verify field types in each section

   Goal has int32, feedback has float32, result has int32. However, the correct order is goal, result, feedback.

3. Final Answer:

   Result section must come before feedback -> Option A

4. Quick Check:

   Correct order is goal, result, feedback [OK]

Hint: Check section order and field types carefully [OK]

Common Mistakes:

• Thinking feedback must come last
• Assuming feedback cannot have fields
• Mixing up result and feedback order

5. You want to define an action for a robot to move to a position with feedback on distance left and a result indicating success. Which .action file snippet correctly defines this?

hard

A. float64 target_x
float64 target_y
---
bool success
---
float64 distance_left

B. bool success
---
float64 target_x
float64 target_y
---
float64 distance_left

C. float64 target_x
float64 target_y
---
float64 distance_left
---
bool success

D. float64 distance_left
---
float64 target_x
float64 target_y
---
bool success

Solution

1. Step 1: Identify goal, feedback, and result roles

   The goal should specify the target position (target_x, target_y). Feedback should provide ongoing info (distance_left). Result should indicate success (bool success).

2. Step 2: Match the correct order in the .action file

   The order is goal fields first, then feedback fields, then result fields. float64 target_x float64 target_y --- float64 distance_left --- bool success matches this order correctly.

3. Final Answer:

   float64 target_x float64 target_y --- float64 distance_left --- bool success -> Option C

4. Quick Check:

   Goal=target, Feedback=distance, Result=success [OK]

Hint: Goal first, feedback second, result last with correct fields [OK]

Common Mistakes:

• Swapping feedback and result sections
• Putting result fields in goal section
• Mislabeling feedback as result