Problem Checker

A problem checker is a special program used during the testing of solutions. It runs after the participant's program completes and determines whether their answer is correct. The checker has access to three files: the input file (input.txt), the expected answer file (answer.txt), and the participant's output file (output.txt).

Eolymp offers several built-in checkers, and you can also write your own custom checker in one of the supported programming languages.

How to Сonfigure a Сhecker

To set up a checker for your problem, follow these steps:

1. Open the console and select the space where your problem is hosted.

2. Navigate to the Problems section from the left menu.

3. Select the problem you want to modify.

4. Go to the Testing section within the problem menu.

Click on Checker to configure it.

Fig 1. Checker configuration form

Types of Сheckers

No Сhecker

If you are using an interactor for grading, you can opt to disable the checker entirely. In this case, the interactor will determine whether the answer is correct. If no checker or interactor is set up, the testing process will result in an invalid outcome.

Line-by-line Сhecker

This checker compares the answer.txt and output.txt files line by line. If even a single character (byte) differs, the participant's answer will be marked as incorrect.

To make formatting easier for participants, the checker ignores:

• Spaces and tabs at the end of each line.

• Empty lines at the end of the file.

Use this checker when the participant's output needs to exactly match the expected answer, with no deviations (output.txt and answer.txt must be identical).

Token Сhecker

The token checker compares the answer.txt and output.txt files word by word or number by number. It determines the comparison method based on the data type in answer.txt. If answer.txt contains a number, output.txt must also contain a number; similarly, if answer.txt contains a word (string), output.txt must contain a word. Numbers are compared with a specified precision, and words are compared case-insensitively (e.g., "AA" == "aa").

When using this checker, you need to specify:

• The precision for number comparisons.

• Whether string comparisons should be case-sensitive.

The following formula is used to compare numbers:

Where:

• $A$ is the participant's answer.

• $B$ is the correct answer.

• $ϵ$ is the precision parameter specified in the problem.

If $B=0$, the formula becomes $|A|\le ϵ$. The system uses the long double (float64) data type for comparing numbers.

This checker is ideal for problems where you only need to compare meaningful tokens (words or numbers), ignoring whitespace and formatting. However, it may not be suitable for problems with large datasets, as it can be slow. In such cases, consider using the Line-by-line checker or a custom checker.

Custom Program Checker

You can also write your own custom checker using the "Compare using program" option. This checker runs after the participant's solution completes successfully, ensuring the system receives a complete output without errors, timeouts, or memory violations.

Your custom checker is compiled and run with the following arguments:

1. Input file (input.txt).

2. Participant's output file (output.txt).

3. Expected answer file (answer.txt).

For example, the checker would be executed like this:

checker input.txt output.txt answer.txt

In C++, you can access these arguments via the main function's second parameter.

The checker should return an exit code of 0 if the participant's output is correct. Any other exit code will indicate an incorrect answer.