Service definition (.srv files) in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a request field for a ROS service.

ROS

int32 [1]

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

Adata

Brequest_data

Cinput

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using reserved keywords as field names

Leaving out the field name

In ROS .srv files, the request fields are defined by specifying the type and the field name. 'data' is a common simple field name.

2fill in blank

medium

Complete the code to separate request and response parts in a .srv file.

ROS

int32 data
---
[1]

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

Aresult int32

Bint32

Cint32 result

Dint32_result

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the field name in response

Using invalid syntax for response fields

The '---' separates request and response. The response fields are defined similarly to request fields, with type and name.

3fill in blank

hard

Fix the error in the service definition by completing the missing separator.

ROS

string command
[1]
bool success

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

A#

B---

C===

D//

Attempts:

3 left

💡 Hint

Common Mistakes

Using comment symbols instead of separator

Omitting the separator line

The '---' line separates the request and response parts in a .srv file.

4fill in blank

hard

Fill both blanks to define a service with a float request and a string response.

ROS

float32 [1]
---
string [2]

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

Atemperature

Bstatus

Cmessage

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up request and response field names

Using invalid field names

The request field is named 'temperature' with type float32, and the response field is named 'message' with type string.

5fill in blank

hard

Fill all three blanks to define a service with int request, bool flag, and string response.

ROS

int32 [1]
bool [2]
---
string [3]

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

Acount

Bis_valid

Cresult

Dflag

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing field names between request and response

Using invalid types or missing separator

The request has 'count' as int32 and 'flag' as bool, and the response has 'result' as string.

Practice

1. What is the main purpose of a .srv file in ROS?

easy

A. To define the structure of a service with request and response parts

B. To list all the topics a node subscribes to

C. To specify the launch configuration for nodes

D. To store parameters for the robot

Solution

1. Step 1: Understand what a .srv file represents

   A .srv file defines a service interface in ROS, which includes a request and a response part.

2. Step 2: Match the purpose to the options

   Only To define the structure of a service with request and response parts correctly states that .srv files define the structure of a service with request and response parts.

3. Final Answer:

   To define the structure of a service with request and response parts -> Option A

4. Quick Check:

   .srv files define service structure = C [OK]

Hint: Remember .srv files split request and response with --- [OK]

Common Mistakes:

• Confusing .srv files with launch files
• Thinking .srv files list topics
• Assuming .srv files store parameters

2. Which of the following is the correct syntax to separate request and response fields in a .srv file?

easy

A. Using a colon ':' between request and response

B. Using a comma ',' between request and response

C. Using a semicolon ';' between request and response

D. Using three dashes '---' between request and response

Solution

1. Step 1: Recall the .srv file format

   The .srv file separates request and response fields with three dashes '---'.

2. Step 2: Identify the correct separator

   Only Using three dashes '---' between request and response correctly uses '---' to separate the two parts.

3. Final Answer:

   Using three dashes '---' between request and response -> Option D

4. Quick Check:

   Request and response separated by --- = A [OK]

Hint: Look for '---' as the separator in .srv files [OK]

Common Mistakes:

• Using commas or semicolons instead of ---
• Confusing with YAML or JSON syntax
• Missing the separator line entirely

3. Given this .srv file content:

int32 a
int32 b
---
int32 sum

What will the response contain when the service is called with request values a=3 and b=4 if the service adds them?

medium

A. sum = 12

B. sum = 34

C. sum = 7

D. sum = 0

Solution

1. Step 1: Understand the request and response fields

   The request has two integers a and b; the response has one integer sum.

2. Step 2: Apply the service logic

   The service adds a and b, so sum = 3 + 4 = 7.

3. Final Answer:

   sum = 7 -> Option C

4. Quick Check:

   3 + 4 = 7 [OK]

Hint: Add request fields to get response sum [OK]

Common Mistakes:

• Concatenating numbers as strings
• Multiplying instead of adding
• Returning zero by mistake

4. Identify the error in this .srv file:

float64 x
float64 y
float64 z
---
float64 result

medium

A. No error, this is a valid .srv file

B. Response field should be on the same line as request fields

C. Missing separator '---' between request and response

D. Request fields must be integers, not float64

Solution

1. Step 1: Check the separator

   The separator '---' is present between request and response fields.

2. Step 2: Validate field types and format

   Using float64 for request and response fields is valid in .srv files.

3. Final Answer:

   No error, this is a valid .srv file -> Option A

4. Quick Check:

   Correct separator and valid types = A [OK]

Hint: Look for '---' and valid types to confirm syntax [OK]

Common Mistakes:

• Thinking separator is missing when it is present
• Believing response must be on same line
• Assuming only integers allowed

5. You want to define a service that takes a string command and an integer timeout as request, and returns a boolean success and a string message as response. Which of these .srv files correctly defines this?

hard

A. string command
int32 timeout
---
bool success, string message

B. string command
int32 timeout
---
bool success
string message

C. string command
int32 timeout
bool success
string message

D. string command
int32 timeout
---
string message
bool success

Solution

1. Step 1: Check for correct separator and field order

   The request fields are before '---' and response fields after. string command int32 timeout --- bool success string message correctly uses '---' and lists request then response fields.

2. Step 2: Validate field types and syntax

   string command int32 timeout --- bool success string message lists request fields (string, int32) and response fields (bool, string) correctly without syntax errors.

3. Final Answer:

   string command int32 timeout --- bool success string message -> Option B

4. Quick Check:

   Request before --- and response after = D [OK]

Hint: Request fields above ---; response fields below --- [OK]

Common Mistakes:

• Placing response fields before separator
• Missing separator line
• Using commas inside .srv files