all I want to create a round-robin tournament generator in c# where each player plays against each player exactly once. The total numebr of rounds is always (number of players - 1). If the number of players is odd, add player 0 (free win) to the list of players. I've been trying for days, on my own, and with the help of ChatGPT, but chatGPT throws the same bs and either breaks my results in one way or another. This code now generated 50 unique pairs for 12 players when it has to generate 66 unique pairs (6 per round). It generated 6 pairs for the first 3 rounds, but after that it's 4 pairs per round and I can't figure out why. You can test this code easily, just create winforms app, create 2 listboxes, 1 numbericupdown and 1 button and paste the code where the button click is
private void button_G_Click(object sender, EventArgs e)
{
int players = (int)numericUpDown1.Value;
int StartFrom = players % 2 == 0 ? 1 : 0; // if odd players, start from 0
players = players % 2 == 0 ? players : players + 1; // if odd players, make even (with the help of the dummy)
int Rounds = players - 1;
int Game = 0;
List<Pair> pairs = new List<Pair>();
int TotalPairs = 0;
List<int> WasPairedThisRound = new();
bool ContainsPair(int player1, int player2)
{
foreach (Pair pair in pairs)
{
if ((pair.Player1 == player1 && pair.Player2 == player2) ||
(pair.Player1 == player2 && pair.Player2 == player1))
{
return true;
}
}
return false;
}
for (int round = 0; round < Rounds; round++)
{
for (int player1 = StartFrom; player1 <= players; player1++)
{
if (WasPairedThisRound.Count == players) { break; }
for (int player2 = StartFrom; player2 <= players; player2++)
{
if (WasPairedThisRound.Count == players) { break; }
if (player1 == player2) { continue; }
if (WasPairedThisRound.Contains(player1)) { continue; }
if (WasPairedThisRound.Contains(player2)) { continue; }
if (ContainsPair(player1, player2)) { continue; }
Pair p = new Pair(player1, player2);
pairs.Add(p);
WasPairedThisRound.Add(player1);
WasPairedThisRound.Add(player2);
Game++;
TotalPairs++;
listBox1.Items.Add($"{TotalPairs}. Round {round + 1}, game {Game}: [player {player1}] vs [player {player2}]");
}
// Exit the loop when pairs for the round are generated
if (WasPairedThisRound.Count == players) { break; }
}
Game = 0;
listBox2.Items.Add(WasPairedThisRound.Count.ToString()); ; // how much were paired each round? (debug)
WasPairedThisRound.Clear(); // clear the list for the next loop of pairing
}
Console.WriteLine("Show each player matchup");
foreach(var player in Enumerable.Range(1,players))
{
Console.WriteLine($"Player: {player.ToString().PadLeft(2)}: {(String.Join(", ", pairs.Where(x => x.Player1 == player || x.Player2 == player).Select(x => (x.Player1 == player ? x.Player2 : x.Player1).ToString().PadLeft(2))))}");
}
}
public class Pair
{
public int Player1 { get; set; } = -1;
public int Player2 { get; set; } = -1;
public Pair(int p1, int p2)
{
Player1 = p1;
Player2 = p2;
}
}
When I dump the final player matchups out it is a little easier to see the problem:
Show each player matchup
Player: 1: 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12
Player: 2: 1, 4, 3, 6, 5, 8, 7, 10, 9, 12, 11
Player: 3: 4, 1, 2, 7, 8, 5, 6, 11, 12, 9, 10
Player: 4: 3, 2, 1, 8, 7, 6, 5, 12, 11, 10, 9
Player: 5: 6, 7, 8, 1, 2, 3, 4
Player: 6: 5, 8, 7, 2, 1, 4, 3
Player: 7: 8, 5, 6, 3, 4, 1, 2
Player: 8: 7, 6, 5, 4, 3, 2, 1
Player: 9: 10, 11, 12, 1, 2, 3, 4
Player: 10: 9, 12, 11, 2, 1, 4, 3
Player: 11: 12, 9, 10, 3, 4, 1, 2
Player: 12: 11, 10, 9, 4, 3, 2, 1
for 12 players there must be 66 pairs. Bascailly (number of players)/2 per round, but this code is only producing 50 pairings
I have created this simple fiddle to demonstrate your code: https://dotnetfiddle.net/GQwa02
Running with 6 players is enough to show the issues:
6 Players
1. Round 1, game 1: [player 1] vs [player 2]
2. Round 1, game 2: [player 3] vs [player 4]
3. Round 1, game 3: [player 5] vs [player 6]
6
4. Round 2, game 1: [player 1] vs [player 3]
5. Round 2, game 2: [player 2] vs [player 4]
4
6. Round 3, game 1: [player 1] vs [player 4]
7. Round 3, game 2: [player 2] vs [player 3]
4
8. Round 4, game 1: [player 1] vs [player 5]
9. Round 4, game 2: [player 2] vs [player 6]
4
10. Round 5, game 1: [player 1] vs [player 6]
11. Round 5, game 2: [player 2] vs [player 5]
4
Show each player matchup
Player: 1: 2, 3, 4, 5, 6
Player: 2: 1, 4, 3, 6, 5
Player: 3: 4, 1, 2
Player: 4: 3, 2, 1
Player: 5: 6, 1, 2
Player: 6: 5, 2, 1
As pointed out in the comments, the problem is that in the later rounds, in this case round 3 onwards, once the first 2 pairs are allocated, the remaining pairs have already played each other. It turns out that this approach will only work for 4 players after that you need something more robust.
As with the dummy for the odd numbers, you have already accepted there is a chance that some players will not have a matchup for some games, so if you are happy to accept that the number of pairings will not be equal, then we can skip past some players at the start of each round to start the pairing process at a different seed number.
I have called this "completing the square" because I want the final outcome to look more like a square, but I couldn't quickly find a universal value generator for this, instead I went with 2 for evens, and 1 for odds.
// 2 it turns out is not a bad general number for evens, 3 for odds
int completeTheSquare = players % 2 == 0 ? 2 : 3;
for (int round = 0; round < Rounds; round++)
{
//for (int player1 = StartFrom; player1 <= players; player1++)
foreach (var p1 in Enumerable.Range(1,players))
{
int startFrom = (p1 + round/completeTheSquare) % players;
if(StartFrom == 1) startFrom ++;
int player1 = startFrom;
if (WasPairedThisRound.Count == players) { break; }
for (int player2 = startFrom; player2 <= players; player2++)
...
You can see this in action here: https://dotnetfiddle.net/gLHy1N
This is the result for 6 players:
Show each player matchup
Player: 1: 6, 2, 4, 3
Player: 2: 3, 4, 1, 5
Player: 3: 2, 5, 4, 6, 1
Player: 4: 5, 2, 3, 1, 6
Player: 5: 4, 3, 6, 2
Player: 6: 1, 5, 3, 4
You could play with this by further staggering the second player loop, in this case I just used the same logic: https://dotnetfiddle.net/G6eZ48
//for (int player2 = startFrom; player2 <= players; player2++)
foreach (var p2 in Enumerable.Range(1,players))
{
int player2 = (p2 + ((round/completeTheSquare)*3)) % players;
if(StartFrom == 1) player2 ++;
...
With a final table like this:
Show each player matchup
Player: 1: 6, 4, 3, 2
Player: 2: 3, 4, 5, 1
Player: 3: 2, 5, 6, 1, 4
Player: 4: 5, 2, 1, 6, 3
Player: 5: 4, 3, 2, 6
Player: 6: 1, 3, 4, 5
But that doesn't really help much. If you need make sure that number of played matches is even, then for some numbers you will simply need more rounds.
To try this, take the restriction off the number of rounds and keep going until there are no more pairings that can be made: https://dotnetfiddle.net/qCsGUN
2 changes for this:
for (int round = 0; ; round++)
// if no pairs made this round, then exit the loop!
if(WasPairedThisRound.Count() == 0)
break;
Final output:
Simple Round Robin Generator
6 Players
1. Round 1, game 1: [player 2] vs [player 3]
2. Round 1, game 2: [player 4] vs [player 5]
3. Round 1, game 3: [player 6] vs [player 1]
6
4. Round 2, game 1: [player 2] vs [player 4]
5. Round 2, game 2: [player 3] vs [player 5]
4
6. Round 3, game 1: [player 3] vs [player 6]
7. Round 3, game 2: [player 4] vs [player 1]
8. Round 3, game 3: [player 5] vs [player 2]
6
9. Round 4, game 1: [player 3] vs [player 1]
10. Round 4, game 2: [player 4] vs [player 6]
4
11. Round 5, game 1: [player 4] vs [player 3]
12. Round 5, game 2: [player 5] vs [player 6]
13. Round 5, game 3: [player 1] vs [player 2]
6
14. Round 6, game 1: [player 5] vs [player 1]
15. Round 6, game 2: [player 6] vs [player 2]
4
Show each player matchup
Player: 1: 6, 4, 3, 2, 5
Player: 2: 3, 4, 5, 1, 6
Player: 3: 2, 5, 6, 1, 4
Player: 4: 5, 2, 1, 6, 3
Player: 5: 4, 3, 2, 6, 1
Player: 6: 1, 3, 4, 5, 2
In reality, when we need these additional rounds but don't want the other players "doing nothing" we can allow the pairing to allocate the remaining players from the start again. Yes, this means that they will be playing the same players again, but you can choose to count this as a "revenge" round and keep or average the result or simply ignore the result altogether.
Depending on the sport and the venue layout, there are practicalities to consider. For instance in a group of 66 players, you wouldn't want to play out a full round robin, you might consider swissing the playing group in stages and playing a finals series as a round robin at the end.
Another solution is to randomise and or brute force the pairings, at the end of the round if the optimal number is not reached, try again. This can work but often is not worth the effort over the simplest solution of allowing additional rounds or accepting that not all players will play each other.
When you make an app to generate these types of matchings, you would normally add a few extra parameters, like here we have explored altering the start number of the player for each round, or the ability to play additional rounds. You could also tick a box to enable random assignment and maybe best of X brute force attempts at each round.
A final practicality for many social clubs that play short games like card games is to have pre-set draws. You might have a set of given numbers of players that have the draws already defined, the result of algorithmic or brute force approaches, but it takes the guess work out of it and the "fairness" of each draw can be published for all to review.
Instead of asking ChatGPT to write the code, you should probably ask for guidance on the specific types of algorithms to use, or to explain the simple logic process that can be used.
The valuable step that you have missed here is understanding of these simple logic loops and importantly, how to debug them.
Another thing to ask is how it is done currently, not that you can't reinvent this wheel, but it can be helpful to understand the existing methodologies and to try and learn the lessons from them before deep diving into both code writing and algorithms.