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    <p>Okay, I've given it a shot. I think that all_teams_paired/2 will provide a list of all teams paired as you described, and a leftover team (if there are an odd-number of teams).</p> <pre><code>% list of all regions regions(Regions) :- findall(Region, region(_, Region), UnsortedRegions), sort(UnsortedRegions, Regions). % list of all teams in a region teams_in_region(Region, Teams) :- findall(Team, (team(Team, _), region(Team, Region)), Teams). % bottom team in a list bottom_team(TeamList, BottomTeam) :- member(BottomTeam, TeamList), findall(Losses, (team(Team, Losses), member(Team, TeamList)), AllLosses), team(BottomTeam, Losses), min_member(Losses, AllLosses). % top team in a list top_team(TeamList, TopTeam) :- member(TopTeam, TeamList), findall(Losses, (member(Team, TeamList), team(Team, Losses)), AllLosses), team(TopTeam, Losses), max_member(Losses, AllLosses). % teams are paired with the top loser playing the bottom loser and there can be a leftover % if there is an odd number of teams paired_teams([], [], []). paired_teams([LonelyTeam], [], [LonelyTeam]). paired_teams(TeamList, PairedTeams, UnpairedTeam) :- top_team(TeamList, TopTeam), bottom_team(TeamList, BottomTeam), TopTeam \= BottomTeam, subtract(TeamList, [TopTeam, BottomTeam], RemainingTeams), paired_teams(RemainingTeams, RemainingPairedTeams, UnpairedTeam), append([TopTeam-BottomTeam], RemainingPairedTeams, PairedTeams). % a list of regions has an associated list of paired teams and leftover teams region_paired_teams([], [], []). region_paired_teams(Regions, PairedTeams, UnpairedTeams) :- Regions = [Region|RemainingRegions], teams_in_region(Region, TeamList), paired_teams(TeamList, RegionPairedTeams, RegionUnpairedTeam), region_paired_teams(RemainingRegions, RemainingPairedTeams, RemainingUnpairedTeams), append(RegionPairedTeams, RemainingPairedTeams, PairedTeams), append(RegionUnpairedTeam, RemainingUnpairedTeams, UnpairedTeams). % a list of all teams paired with priority given to region and a leftover team (if any) all_teams_paired(TeamsPaired, Leftover) :- regions(Regions), region_paired_teams(Regions, RegionPairedTeams, UnpairedTeams), paired_teams(UnpairedTeams, NonRegionPairedTeams, Leftover), append(RegionPairedTeams, NonRegionPairedTeams, TeamsPaired). </code></pre> <p>Have I understood your requirements correctly?</p>
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    1. POPairing off teams
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    1. USPeter Hude
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    1. CO+1. I have shown how I would approach the pairing off problem in my own answer below, which I thought you might find interesting.
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      1. USDaniel Lyons
    2. COBoth of your suggestions are much nicer than my implementation above. I know you mention that the first one (the "match" predicate) wouldn't be as efficient but it does make for some very nice looking Prolog code!
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      1. USPeter Hude
    3. COI think nice looking code is very important! I believe one should try hard to sequester as much of the efficiency concern in small corners of the program so the bulk of it will still read well. But it does seem like the longer one uses Prolog the greater one's concern for efficiency seems to become.
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