Your first problem in 15 minutes

You will build the "Sum of Two Numbers" (A + B) problem on LCOJ end to end: write the statement, upload tests, pick a checker, test it with real submissions, and make it public.

⏱ ~15 min · 👤 New problem setters, teachers · 🔑 judge.add_problem (to create); making it public needs an admin or judge.change_public_visibility

What you'll do

• Create the problem aplusb in the web UI, with a Markdown statement and math.
• Prepare 5 tiny tests, zip them, and upload them in the test data editor.
• Pick the standard checker and save, so the site generates init.yml.
• Submit two correct solutions (C++ and Python) and one deliberately wrong one to see WA.
• Make the problem public so everyone can solve it.

Before you start

• You are signed in to luyencode.net and your account has judge.add_problem. If /problems/create refuses you, ask an administrator to grant it (see Permissions).
• You have a zip tool (built into Windows, macOS, and most Linux distributions).
• (Optional) A C++ compiler or Python to run the solutions locally before submitting.

Step 1: Create the problem

Goal: a private problem with the code aplusb.

1. Open https://luyencode.net/problems/create (or go to Problems and click the Create new problem tab).

2. Fill in:

   Field	Value
   Problem code	aplusb
   Problem name	Sum of Two Numbers
   Time limit	1 (seconds)
   Memory limit	keep the default 262144 (KB = 256 MB)
   Points	1
   Allows partial points	leave it checked (on by default)

3. Leave the statement field for now; you will write it in step 2 on this same form.

Problem code rules

A problem code may contain only lowercase letters, digits, and underscores (^[a-z0-9_]+$), at most 32 characters, and must be unique across LCOJ. APlusB, a-plus-b, and a+b are all rejected. If aplusb is taken, add a suffix such as aplusb_class10a and use that code in every later step.

✅ Result: the form has the code, name, and limits filled in; you have not clicked Create yet.

Step 2: Write the statement

Goal: a clear statement with math and a sample.

1. Scroll to the Problem body field. It is pre-filled with a sample statement; delete it.
2. Paste this statement:

You are given two integers ~a~ and ~b~. Compute ~a + b~.

## Input

A single line containing two integers ~a~ and ~b~ (~-10^9 \le a, b \le 10^9~).

## Output

Print a single integer, the value of ~a + b~.

## Example

### Input

```
3 5
```

### Output

```
8
```

### Explanation

~3 + 5 = 8~.

3. Use the live preview next to the editor to check that the math renders.
4. Click Create at the bottom of the page.

Math uses ~

On LCOJ, inline math is ~a+b~ and display math is $$...$$. $a+b$ is shown as plain text. You do not need to write the time and memory limits in the statement; the problem sidebar shows them.

✅ Result: you land on /problem/aplusb. The right sidebar shows Edit problem and Edit test data. The problem is private: only you (as curator) and administrators can see it.

Step 3: Prepare the tests

Goal: a file aplusb.zip with 5 input/output pairs named in Themis style (N.inp / N.out) so the test data editor fills in the table for you.

Create these 10 files in an empty folder:

Test	N.inp	N.out	What it checks
1	3 5	8	The sample from the statement
2	-7 2	-5	Negative numbers
3	0 0	0	Zero
4	1000000000 1000000000	2000000000	Sum beyond the int range
5	-1000000000 -1000000000	-2000000000	Largest negative sum

Create them quickly and zip them:

Linux / macOS

mkdir aplusb-tests && cd aplusb-tests
printf '3 5\n'                     > 1.inp; printf '8\n'           > 1.out
printf -- '-7 2\n'                 > 2.inp; printf -- '-5\n'       > 2.out
printf '0 0\n'                     > 3.inp; printf '0\n'           > 3.out
printf '1000000000 1000000000\n'   > 4.inp; printf '2000000000\n'  > 4.out
printf -- '-1000000000 -1000000000\n' > 5.inp; printf -- '-2000000000\n' > 5.out
zip ../aplusb.zip *.inp *.out

Windows (PowerShell)

mkdir aplusb-tests; cd aplusb-tests
"3 5"                     | Out-File -Encoding ascii 1.inp; "8"           | Out-File -Encoding ascii 1.out
"-7 2"                    | Out-File -Encoding ascii 2.inp; "-5"          | Out-File -Encoding ascii 2.out
"0 0"                     | Out-File -Encoding ascii 3.inp; "0"           | Out-File -Encoding ascii 3.out
"1000000000 1000000000"   | Out-File -Encoding ascii 4.inp; "2000000000"  | Out-File -Encoding ascii 4.out
"-1000000000 -1000000000" | Out-File -Encoding ascii 5.inp; "-2000000000" | Out-File -Encoding ascii 5.out
Compress-Archive -Path *.inp,*.out -DestinationPath ..\aplusb.zip

Naming rules for automatic filling

• Use one naming style for the whole zip: here 1.inp + 1.out. aplusb.1.in + aplusb.1.out also works, but do not mix styles.
• Put the files at the root of the zip, not inside a folder.
• Inputs and outputs are sorted naturally (1, 2, …, 10) and paired in order, so there must be as many .inp files as .out files.

✅ Result: you have aplusb.zip containing exactly 10 files.

Step 4: Upload the tests and pick the checker

Goal: save the tests so the site generates init.yml for the judges.

1. On the problem page, click Edit test data (URL /problem/aplusb/test_data).
2. In Data zip file, choose aplusb.zip. If you only have loose files, click or click here to build zip file and pick all 10 files; the browser zips them for you.
3. The table fills in 5 Normal case rows worth 1 point each, and a yellow notice Test cases have been filled automatically! appears. The table is not saved yet.
4. Check the Input file / Output file columns: row 1 is 1.inp / 1.out, …, row 5 is 5.inp / 5.out.
5. In the Checker drop-down, keep Standard (standard): it compares token by token and ignores extra whitespace. That is the right choice for a problem that prints one integer.
6. Click Save.

✅ Result: the page reloads without errors, and a View YAML link appears next to the title. It shows:

archive: aplusb.zip
checker: standard
test_cases:
- in: 1.inp
  out: 1.out
  points: 1
- in: 2.inp
  out: 2.out
  points: 1
- in: 3.inp
  out: 3.out
  points: 1
- in: 4.inp
  out: 4.out
  points: 1
- in: 5.inp
  out: 5.out
  points: 1

(The zip name on the archive line may differ slightly depending on how the site stores the file; the test_cases part should match.)

Step 5: Test with real submissions

Goal: make sure correct solutions get AC and the tests catch a wrong one.

1. On the problem page, click Submit solution.
2. Choose the language, paste the code, and click Submit!. Do this for each of the three solutions below (wait for each one to finish; by default you can have at most 2 submissions waiting to be judged).

Correct: C++ (CPP17)

#include <bits/stdc++.h>
using namespace std;

int main() {
    long long a, b;
    cin >> a >> b;
    cout << a + b << '\n';
    return 0;
}

Correct: Python 3 (PY3)

a, b = map(int, input().split())
print(a + b)

Deliberately wrong: uses int

#include <bits/stdc++.h>
using namespace std;

int main() {
    int a, b;          // wrong: a + b can exceed 2^31 - 1
    cin >> a >> b;
    cout << a + b << '\n';
    return 0;
}

✅ Result:

Solution	What you see
C++ with long long	All 5 tests Accepted (AC), score 5/5
Python 3	All 5 tests Accepted (AC), score 5/5
C++ with int	Test 4 is Wrong Answer (WA) because of overflow (it prints a negative number), the rest are AC; score 4/5, i.e. 0.8 of the problem's 1 point, because partial points are allowed

If the wrong solution still passes every test, your tests are too weak; add edge cases. For how to read the results page, see Submitting and judging.

Step 6: Make the problem public

Goal: everyone can see the problem in the list and submit.

A problem created on the site always starts private, and the site's edit form has no public checkbox. Publishing happens in the admin, by an administrator or someone with judge.change_public_visibility (the account must be staff):

1. Open /admin/judge/problem/ and find aplusb.
2. Option A: open the problem, tick Publicly visible, and click Save. Option B: tick the problem's checkbox in the list, choose the action Mark problems as public and set publish date to now, and run it.

No permission to publish?

Send the problem code (aplusb) to an LCOJ administrator and ask them to publish it. If you plan to use the problem in a contest, do not publish it first: anyone can read a public problem without joining the contest. See Your first contest.

✅ Result: sign out (or use a private window) and open https://luyencode.net/problem/aplusb: the statement shows normally.

Verify

• /problem/aplusb shows the statement with rendered math (no ~ around a + b).
• View YAML lists 5 tests and checker: standard.
• At least one C++ and one Python submission are AC 5/5.
• The int submission is WA on test 4.
• A guest (signed out) can open the problem page after publishing.

Troubleshooting

Symptom	Cause	Fix
The form says the problem code is invalid	Uppercase letters, -, spaces, or the code already exists	Use lowercase letters, digits, _; add a suffix if taken
Dialog "Files are not in the same format!" after choosing the zip	Mixed naming styles, e.g. 1.inp with aplusb.2.in	Rename everything to one style and zip again
Dialog "The number of input files (…) do not match the number of output files (…)!"	A missing .out file or a stray extra file	Check the folder; every N.inp needs an N.out
Dialog "No input/output files. Make sure your files are following themis/polygon/cms test format"	File extensions not recognized (.inp, .in, .out, .ok, .ans)	Rename files as described in Managing problems
No View YAML after saving, with an error at the top	A row is missing a file or points	Read the error, fix the row, and click Save again
A submission stays queued forever	No judge has picked up the problem or language	Tell an administrator; see Judge setup
A submission gets IE	The judge cannot read the problem data	Check that View YAML exists; tell an administrator if it persists
A correct solution gets WA	The expected output is wrong, or the .out file has odd bytes (a BOM)	Reopen the .out file and save it as ASCII / UTF-8 without BOM
No public checkbox on the edit form	The site form does not have it (except for organization problems)	Publish in the admin as in Step 6

Next steps

• Managing problems: every problem field, subtasks (batches), rejudging.
• Problem format: writing init.yml by hand.
• Checkers: floating-point output, multiple answers, custom checkers.
• Graders and Generators: interactive problems and generated tests.
• Example problems: complete sample problems.
• Your first contest: use the problem in a contest.