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Round-Robins-art - 3/31/15

"Round-Robins on the Run" by Lord Cian Gillebhrath.

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NOTICE -

This article was submitted to me by the author for inclusion in this set of files, called Stefan's Florilegium.

These files are available on the Internet at: http://www.florilegium.org

While the author will likely give permission for this work to be reprinted in SCA type publications, please check with the author first or check for any permissions granted at the end of this file.

Thank you,

Mark S. Harris...AKA:..Stefan li Rous

stefan at florilegium.org

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This article may have been previously published in Cockatrice, the A&S newsletter for Lochac.

Round-Robins on the Run

by Lord Cian Gillebhrath

Ever needed to quickly sort out a round-robin for fighters and find that your trial-and-error method of working out the draws just keeps failing? Ever looked at those who seem to be able to work it out quickly with awe? I’m here to tell you that there is a science to it and it really is quite easy to work out a 30 person round-robin draw. Just don’t let your local fighters know!

The basic method is called by some, the Polygon method. One version is used for an odd number of contestants, and the other is for an even number of contestants.

ODD NUMBER DRAWS

If you have X contestants, then you will have X rounds, with 1 bye in each round. To work out the draw,

1) Number each of the contestants

2) Draw an X-sided polygon with a contestant number at each corner.

For instance, for 5 contestants, the polygon would look like this:

5 o o 2

4 o o 3

3) Draw horizontal lines across the polygon, with all but the top number connected to one other number. No line is allowed to cross another line.

5 o ----------- o 2

4 o ---- o 3

4) Those lines designate who will be competing against who in Round 1. The unattached number at the top represents the contestant who has the bye.

ie. Round 1 has Contestant 5 vs Contestant 2

Contestant 4 vs Contestant 3

Contestant 1 has the bye

5) For Round 2, remove the lines and rotate all the numbers clockwise around the polygon. Then draw new lines to give the draw for Round 2.

5o Round 2 has Contestant 4 vs Contestant 1

Contestant 3 vs Contestant 2

4 o ----------- o 1 Contestant 5 has the bye

3 o ---- o 2

6) For every other round, rotate and redraw the lines as in step 5. Remember, for X contestants, there are X rounds.

EVEN NUMBER DRAWS

Most people are stumped when working out round-robins for an even number of contestants, including me! You can’t use the same method as for odd number of contestants as that quickly runs into contestants doubling up on the same opposition. Instead, a similar but different polygon method is required.

If you have X contestants, you will have X-1 rounds, and no byes, as each contestant must face every other X-1 contestants once:

1) Number each of contestants

2) Place the number 1 on your paper, then draw a X-1 sided polygon around it with the numbers for the rest of the contestants.

For instance for 6 contestants, 1 is in the middle and the numbers 2, 3, 4, 5 and 6 form the 5 sided polygon:

2 o

6 o o 3

1 o

5 o o 4

3) Draw horizontal lines across the polygon, with all but the top number and number 1 connected to one other number. No line is allowed to cross another line.

2 o

6 o ---------- o 3

5 o ----- o 4

4) Now draw a vertical line to connect the number 1 to the top number. This is the only line that is allowed to cross other lines!

2 o

6 o -----|----- o 3

5 o ----- o 4

5) ALL of those lines designate who will be competing against who in Round 1, including the line between 1 and the top number. There are no byes in any round of a round-robin with an even number of contestants.

ie. Round 1 has Contestant 6 vs Contestant 3

Contestant 5 vs Contestant 4

Contestant 1 vs Contestant 2

6) For Round 2, remove the lines and rotate all the numbers clockwise around the polygon, except number 1. DO NOT MOVE NUMBER 1 FROM THE CENTRE.

5 o o 2

4 o o 3

7) Redraw the horizontal lines, and draw a new vertical line to connect 1 to the new top number. This gives the draw for Round 2.

6 o

| Contestant 5 vs Contestant 2

5 o -----|----- o 2 Contestant 4 vs Contestant 3

1o Contestant 1 vs Contestant 6

4 o ----- o 3

8) Continue deleting, rotating and redrawing the lines for all X-1 rounds. Remember, do not move number 1 from the centre.

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Copyright 2010 by Cian Gillebhrath. <cian at sorontar.com>. Permission is granted for republication in SCA-related publications, provided the author is credited. Addresses change, but a reasonable attempt should be made to ensure that the author is notified of the publication and if possible receives a copy.

If this article is reprinted in a publication, please place a notice in the publication that you found this article in the Florilegium. I would also appreciate an email to myself, so that I can track which articles are being reprinted. Thanks. -Stefan.