Clearly define problem constraints

Oct 13, 2025 by wbrum.org 75 views

Problem Overview

The problem "User Activity for the Past 30 Days I" on LeetCode is designed to retrieve the number of active users for each day within a specified date range. However, a critical issue arises when the problem constraints are not properly enforced, leading to incorrect test case results. In this article, we will delve into the problem description, identify the missing test case, and provide a corrected solution.

Problem Description

The problem statement requires us to find the number of active users for each day within a 30-day period. The date range is specified as '2019-06-28' to '2019-07-27'. However, the problem constraints are not properly enforced, allowing dates beyond '2019-07-27' to be included in the results.

Code Analysis

The provided MySQL code snippet is used to retrieve the active users for each day within the specified date range:

select activity_date as day, count(distinct user_id) as active_users from Activity
where activity_date between '2019-06-28' and '2019-07-27' 
group by activity_date

However, this code does not enforce the problem constraints, allowing dates beyond '2019-07-27' to be included in the results.

Missing Test Case

The missing test case arises when the problem constraints are not properly enforced. In this case, the test case should include a date beyond '2019-07-27' to ensure that the code correctly excludes such dates.

Corrected Solution

To correct the solution, we need to modify the code to properly enforce the problem constraints. We can achieve this by adding a check to ensure that the activity_date is within the specified date range:

select activity_date as day, count(distinct user_id) as active_users from Activity
where activity_date between '2019-06-28' and '2019-07-27' 
and activity_date <= '2019-07-27' 
group by activity_date

By adding the additional check activity_date <= '2019-07-27', we ensure that the code correctly excludes dates beyond '2019-07-27'.

Conclusion

In conclusion, the missing test case for the problem "User Activity for the Past 30 Days I" arises when the problem constraints are not properly enforced. By modifying the code to include an additional check, we can ensure that the code correctly excludes dates beyond '2019-07-27'. This corrected solution provides a more accurate and reliable result.

Additional Considerations

When working with date ranges, it is essential to properly enforce the problem constraints to ensure accurate results. This includes checking for dates beyond the specified range and excluding them from the results.

Best Practices

To avoid missing test cases like this one, follow these best practices:

Clearly define problem constraints: Ensure that the problem constraints are clearly defined and understood by all team members.
Properly enforce constraints: Ensure that the code properly enforces the problem constraints to prevent incorrect results.
Test for edge cases: Test the code for edge cases, including dates beyond the specified range, to ensure that it correctly handles such scenarios.

Q: What is the problem with the original code?

A: The original code does not properly enforce the problem constraints, allowing dates beyond '2019-07-27' to be included in the results.

Q: Why is this a problem?

A: This is a problem because it can lead to incorrect results. If the code includes dates beyond '2019-07-27', it may incorrectly count active users for those dates.

Q: How can we fix this issue?

A: We can fix this issue by adding a check to ensure that the activity_date is within the specified date range. This can be done by adding the additional check activity_date <= '2019-07-27' to the code.

Q: What is the corrected code?

A: The corrected code is:

select activity_date as day, count(distinct user_id) as active_users from Activity
where activity_date between '2019-06-28' and '2019-07-27' 
and activity_date <= '2019-07-27' 
group by activity_date

Q: Why is this check necessary?

A: This check is necessary because it ensures that the code correctly excludes dates beyond '2019-07-27'. Without this check, the code may incorrectly include such dates in the results.

Q: What are some best practices for working with date ranges?

A: Some best practices for working with date ranges include:

Clearly define problem constraints: Ensure that the problem constraints are clearly defined and understood by all team members.
Properly enforce constraints: Ensure that the code properly enforces the problem constraints to prevent incorrect results.
Test for edge cases: Test the code for edge cases, including dates beyond the specified range, to ensure that it correctly handles such scenarios.

Q: How can we avoid missing test cases like this one?

A: We can avoid missing test cases like this one by following the best practices mentioned above. Additionally, we can:

Thoroughly review the code: Review the code to ensure that it properly enforces the problem constraints.
Test the code extensively: Test the code extensively to ensure that it correctly handles all possible scenarios.
Collaborate with team members: Collaborate with team members to ensure that everyone understands the problem constraints and the code.

Q: What are some common mistakes to avoid when working with date ranges?

A: Some common mistakes to avoid when working with date ranges include:

Failing to properly enforce constraints: Failing to properly enforce the problem constraints can lead to incorrect results.
Ignoring edge cases: Ignoring edge cases, such as dates beyond the specified range, can lead to incorrect results.
Not thoroughly reviewing the code: Not thoroughly reviewing the code can lead to missing test cases and incorrect results.

Q: How can we ensure that our code is accurate and reliable?

A: We can ensure that our code is accurate and reliable by:

Following best practices: Following best practices, such as clearly defining problem constraints and properly enforcing constraints.
Thoroughly reviewing the code: Thoroughly reviewing the code to ensure that it correctly handles all possible scenarios.
Testing the code extensively: Testing the code extensively to ensure that it correctly handles all possible scenarios.
Collaborating with team members: Collaborating with team members to ensure that everyone understands the problem constraints and the code.