[Tutor] problem solving with lists

[Dennis Lee Bieber]

On Mon, 4 Apr 2022 20:53:51 +0200, <marcus.luetolf at bluewin.ch> declaimed
the following:

>Hello Experts,
>Francesco Loffredo's solution looks very clean.
>It returns 15 sublists/teams in final, but 20 are required, for each item in text has to combine with
>every other item in 4 sublists/teams  5 times.

	Which is the correct result IF one does not restart the combinations
for each week. Restarting the combinations allows subsequent weeks to
select "teams" that were excluded in previous weeks.

	I've never modified the ooREXX code to restart, and to treat "current
week" differently from "previous weeks"...

C:\Users\Wulfraed\Documents\_Hg-Repositories\REXX>letters_set.rex 16 4
Accepted items: 15
abcd aefg ahij aklm
anop behk bfil bgjm
ceim cfhn cgko cjlp
dejn dfkp dghl

C:\Users\Wulfraed\Documents\_Hg-Repositories\REXX>

	That (my ooREXX) code is only looking for common pairs over the entire
period, and does not account for "player can not repeat within current
week". I suspect the same is true for the referenced code.

	The Python version with both 

        Constraint: all players are assigned to groups each week;
                    ie, no player may appear in multiple groups in the week
        Constraint: no pairs of players may repeat from week to week
                    (a solution may mean some players are never paired)

C:\Users\Wulfraed\Documents\_Hg-Repositories\Python Progs\SGP>main.py 4 4 5
Week # 0:       abcd    efgh    ijkl    mnop
Week # 1:       aeim    bfjn    cgko    dhlp
Week # 2:       afkp    belo    chin    dgjm
Week # 3:       agln    bhkm    cejp    dfio
Week # 4:       ahjo    bgip    cflm    dekn

C:\Users\Wulfraed\Documents\_Hg-Repositories\Python Progs\SGP>

	The main computation (no input parsing/output formatting)
-=-=-
from itertools import combinations

LETTERS = string.ascii_letters + string.digits


def main(num_groups, group_size, num_weeks):
    schedule =[ [ set() for _ in range(num_groups)] for _ in
range(num_weeks)]
    for wn, week in enumerate(schedule):
        #initialize generator of groupings at start of each week, so items
        #skipped in previous week are again candidates
        group_gen = combinations(LETTERS[:group_size * num_groups],
group_size)
        for gn, group in enumerate(week):
            while True:
                accept = True
                candidate = set(next(group_gen))
                #first filter for no duplicate players in this week
                for agroup in week[:gn]:
                    if not candidate.isdisjoint(agroup):    #if candidate &
agroup:
                        accept = False
                        break
                #filter for no duplicate pairs in previous weeks
                if accept:
                    for aweek in schedule[:wn]:
                        for agroup in aweek:
                            if len(candidate & agroup) > 1:
                                #repeated pair
                                accept = False
                                break
                        if not accept: break
                if accept:
                    #candidate has not been rejected, add it to current
week
                    week[gn] = candidate
                    break
    return schedule
-=-=-


-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
	wlfraed at ix.netcom.com    http://wlfraed.microdiversity.freeddns.org/