Action definition (.action files) in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the goal part of a ROS action file.

ROS

int32 [1]
---
# Result
bool success
---
# Feedback
string status

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

Afeedback

Bresult

Cgoal_position

Dtarget_position

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'result' or 'feedback' in the goal section

Leaving out the variable name

The goal section defines the input data for the action. Here, goal_position is a common name for the target value.

2fill in blank

medium

Complete the code to define the result part of a ROS action file.

ROS

int32 target_position
---
bool [1]
---
# Feedback
string status

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

Adone

Bfinished

Ccomplete

Dsuccess

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'done' or 'finished' which are less standard

Omitting the boolean type

The result section defines the output of the action. success is a common boolean to indicate if the action finished correctly.

3fill in blank

hard

Fix the error in the feedback part of this ROS action file.

ROS

int32 target_position
---
bool success
---
string [1]

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

Aresult

Bstatus

Cstate

Dgoal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'result' or 'goal' in feedback

Omitting the string type

The feedback section usually contains a string named status to describe the current progress.

4fill in blank

hard

Fill both blanks to complete a minimal ROS action file with goal and result.

ROS

[1] distance
---
[2] finished
---
# Feedback
string status

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

Aint32

Bbool

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing types between goal and result

Using the wrong order of sections

The goal part defines an integer distance, and the result part defines a boolean finished flag.

5fill in blank

hard

Fill all three blanks to define a complete ROS action file with goal, result, and feedback.

ROS

[1] speed
---
[2] success
---
string [3]

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

Afloat32

Bbool

Cstatus

Dint32

Attempts:

3 left

💡 Hint

Common Mistakes

Using int32 instead of float32 for speed

Wrong names for feedback string

The goal defines a float32 speed, the result a boolean success, and the feedback a string status.

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