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MathGroup Archive 2006

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Re: subsets of a set

  • To: mathgroup at smc.vnet.net
  • Subject: [mg70030] Re: subsets of a set
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Sun, 1 Oct 2006 04:08:11 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <efld41$d3i$1@smc.vnet.net>

dimmechan at yahoo.com wrote:
> In page 226 of John Gray's Mastering Mathematica
> (unfortunately I had the 1994's edition)
> there is the following exercise:
> 
> "Given a finite set (presented as a list) and an integer k
> find all k-element subsets of the set".
> 
> There is one solution however for practicing (no there is no professor
> that asked me this for homeworks! I work individually...) I search for
> other
> 
> So considering the following list
> 
> lst = Tuples[Range[7], 3];
> 
> I can erase the sublists containg at least two same elements
> 
> Cases[lst, {x_, y_, z_} /; x ß?  y && x ß?  z && y ß?  z]
> 
> but after I cannot erase the permutations of e.g. {1,2,3};
> that is in the output there are the sublists {1,2,3},
> {2,1,3},{3,1,2} e.t.c. for other triplets.
> 
> I look for both functional as well pattern matching approaches.
> 
> Thanks
> 
Hi Dimitris,

You could try

Sort /@ lst // Sort // Union

Regards,
Jean-Marc


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