SAS: Coffee anyone? - sas

Question

I tried this in C# but have not had much success. So I am now trying in SAS. Using an EG session and my SAS code, we work with the list of students in SASHELP.CLASS.
These people want to get to know each other and have a monthly random pairing to go on a Coffee Date.
Rules:
A random Coffee Date List is Generated monthly;
I store each months pairing into a Historical Dataset, which I append monthly.
One person cannot have coffee with the same person within a 6 month period. So we keep a separate dataset for historical purposes with 3 Vars:
LastDate,InviterID,InvitedID
We check each pairing against the Historical list of which we only load the most recent 6 months data into a temp dataset for checking purposes.
If no recent matched pair is found, a new matched pair is added to a new Paired Dataset, and the 2 names (Rows) are removed from the original Participants dataset until the dataset has less than 2 rows. (a single person cannot be paired with another)
Unfortunately we have 19 people in this list so one person will be left out until we can add a new participant. Is anyone interested in joining our coffee club? :-)
So I start by deriving and ID (n) from the dataset, and I only keep the Name
Data Participants(Keep=ID Name);
FORMAT ID 8.;
set SASHelp.class;
ID=_n_;
run;
These 19 People will be my Participants in the Coffee Club.
I more or less follow the line of thought:
data _null_;
randvar = ceil(rand('UNIFORM') * 100000);
call symput('RANDSEED', randvar);
run;
data CR.names2(keep=MEMID randid);
set CR.MasterNames;
randid = rand('UNIFORM');
run;
proc sort data=CR.names2 ; by randid; run;
data CR.pairs(keep=pairgrp MEMID);
set CR.names2 nobs=num_peeps;
pairgrp+1;
if pairgrp > floor(num_peeps/2) then pairgrp=1;
run;
proc sort data=CR.pairs; by pairgrp;run;
proc transpose data=CR.pairs
out=CR.pairs2 (drop=_NAME_);
var memid;
by pairgrp;
run;
Data CR.Pairs3;
set CR.pairs2;
rename COL1=InviterID COL2=InvitedID;
run;
But I get stuck :-(
I need help with the rest please...
Has anyone else done this type of random pairing successfully before? I am grasping straws here...
Any help much appreciated.
Len

Answer 1

Here is my idea. This is far from efficient. Esp. when NOBS is getting big, as there is a cartesian product involved. Also I cheated on the odd number by adding another row in that case.
Prepare data and generate empty result table.
Create a list of all possible pairings (combinations) excluding recent pairings.
Random sort and descend through the list until every element has been picked once.
Append to result table.
There is a drawback as there might be members who will not get pairings as all possible partners are already picked. To avoid that we could iterate until we get a maximum of pairings.
EDIT: Added iteration. Now the program makes draws randomly until everyone is matched or a threshold is reached.
This problem should probably be implemented in a matrix orientated language like IML or R.
data Participants(Keep=ID Name) ;
set SASHelp.class nobs = num_peeps ;
ID=_n_ ;
output ;
if _n_ = 1 and mod(num_peeps,2) then do ; /* get even number of members: empty ID to pair with last participant*/
name = 'empty' ;
id = 0 ;
output ;
end ;
run ;
data list_of_meetings ;
length iteration InviterID InvitedID 8. ;
run ;
/****
iter = number of club meetings
hist = length of memory for pairings
tries = number of iterations to pair everyone
****/
%macro loop_coffee (iter=, hist=6, tries= 10) ;
proc sql noprint ;
select max(0,max(iteration)) + 1 into :base
from list_of_meetings ;
quit ;
%do i = &base. %to &iter. ; /* loop through number of meetings */
proc sort data = list_of_meetings (where=(iteration >= &i - &hist )) out = lookup nodupkey ; by InviterID InvitedID ; run ; /* get memory of pairings */
proc sql ; /* list all acceptable pairs */
create table all_pairs as
select a.ID as InviterID, b.ID as InvitedID
from Participants a
inner join Participants b
on a.ID lt b.ID
left join lookup c /* exclude the memory */
on a.ID eq c.InviterID and b.ID eq c.InvitedID
where c.InviterID is NULL ;
quit ;
%let j = 0 ;
%let all_pairs = 0 ;
%do %until (&all_pairs | &j > &tries) ; /* iterate and random sort until all members are paired */
%let j = %eval( &j + 1 ) ;
data all_pairs;
set all_pairs;
randnum = ranuni(12345 + &i + &j);
run;
proc sort data = all_pairs ; by randnum ; run ; /* random sort */
data out_pairs ; /* select the pairs: no. of IDs/2 */
declare hash h() ;
h.defineKey("ID") ;
h.defineDone() ;
do until ( eof1 ) ;
set Participants (keep= ID) end = eof1 ;
rc = h.add () ; /* populate list of members */
end ;
do until ( eof2 ) ;
set all_pairs (keep= InviterID InvitedID) end = eof2 ;
rc1 = h.check (key:InviterID) ;
rc2 = h.check (key:InvitedID) ;
if rc1 = 0 and rc2 = 0 then do ;
rc = h.remove (key:InviterID) ; /* delete member from list if paired */
rc = h.remove (key:InvitedID) ;
output ;
end ;
if h.num_items = 0 then do ;
call symput('all_pairs', 1 ) ;
stop ;
end;
end ;
stop ;
keep InviterID InvitedID ;
run ;
%end ;
data list_of_meetings ;
set list_of_meetings (where=(iteration ne .))
Out_pairs (in=pairs) ;
if pairs then iteration = &i. ;
run ;
%end ;
%mend ;
%loop_coffee (iter=10,hist=6,tries=10) ;