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Date:         Mon, 13 Dec 2010 23:09:02 -0800
Reply-To:     Daniel Nordlund <djnordlund@FRONTIER.COM>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Daniel Nordlund <djnordlund@FRONTIER.COM>
Subject:      Re: code, find all combinations
In-Reply-To:  <000201cb9b48$21a5b690$64f123b0$@com>
Content-Type: text/plain; charset="UTF-8"

> -----Original Message----- > From: SAS(r) Discussion [mailto:SAS-L@LISTSERV.UGA.EDU] On Behalf Of > bbser2009 > Sent: Monday, December 13, 2010 8:34 PM > To: SAS-L@LISTSERV.UGA.EDU > Subject: code, find all combinations > > I wrote a specific code finding all samples of size 3 from the population > consisting of 1, 2 ,3, 4. > The idea is direct: > First get data set like this, no repeated values in each observation. > 1 2 3 > 2 4 3 > 2 1 3 > ... > > Then bubble-sort each observation from small to big. > 1 2 3 > 2 3 4 > 1 2 3 > ... > > Next by-group observations that are identical with each other. > 1 2 3 > 1 2 3 > ... > 2 3 4 > 2 3 4 > ... > > At last, output the last observation of each by-group to get all the > samples. > 1 2 3 > 1 2 4 > 1 3 4 > 2 3 4 > (For code, see bottom) > > Apparently this algorithm/code is hard to generalize. > So I am curious about how people have addressed this issue in a more > general > way? > I guessed the general algorithm had to be complicated. > By any chance, however, does anyone know at least what key ideas lay > behind > it? > Any comments are welcome! > > Thanks. >

Max,

This question comes up regularly. The algorithm really isn't all that complicated. Here is one solution. It is a macro that writes a data step which outputs one record per combination into a dataset called COMB. This will work ok when n and k are not too big. Just be careful about combinatorial explosion. There are other solutions as well. If one has SAS STAT then I would probably recommend using PROC PLAN.

%macro comb(n,k); data comb; %let k0=; k0 = 0; %do i = 1 %to &k; do k&i = k%eval(&i-1) +1 to %eval(&n-&k+&i)%str(;); %end; output; %do i = 1 %to &k; end%str(;); %end; drop k0; run; %mend;

%comb(4,3)

Hope this is helpful,

Dan

Daniel Nordlund Bothell, WA USA


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