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MathGroup Archive 2004

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Re: Sort into chain

  • To: mathgroup at smc.vnet.net
  • Subject: [mg45624] Re: Sort into chain
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Fri, 16 Jan 2004 19:57:01 -0500 (EST)
  • Organization: Universitaet Leipzig
  • References: <bu8ia8$aqj$1@smc.vnet.net>
  • Reply-to: kuska at informatik.uni-leipzig.de
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,


a //. {a___, {s_, t_}, b__, {t_, u_}, c___} :> {a, {s, t}, {t, u}, b, c}


???

Regards
  Jens

Goyder Dr HGD wrote:
> 
> Dear Mathgroup,
> 
> How do I sort a list of pairs, for example,
> 
> a = {{3, 1}, {2, 6}, {7, 3}, {8, 7}, {10, 8}, {6, 11},
> {12, 10}, {11, 14}, {15, 12}, {14, 18}, {19, 15},
> {18, 20}, {20, 19}, {1, 2}};
> 
> Into a list where the second entry in each pair is the same as the first
> entry in the next pair. Thus the effect of the sort will give
> 
> {{18, 20}, {20, 19}, {19, 15}, {15, 12},
> {12, 10}, {10, 8}, {8, 7}, {7, 3}, {3, 1},
> {1, 2}, {2, 6}, {6, 11}, {11, 14}, {14, 18}}
> 
> The first pair is not important because the pairs form a circular chain.
> 
> I have tried
> 
> Sort[a, (#1[[2]] == #2[[1]]) &]
> 
> but the answer is
> 
> {{1, 2}, {20, 19}, {14, 18}, {18, 20}, {19, 15},
> {15, 12}, {6, 11}, {11, 14}, {12, 10}, {10, 8},
> {8, 7}, {7, 3}, {2, 6}, {3, 1}}
> 
> which has a few correct entries but is not finished. I wondered if the
> difficulty was due to the circular nature of the chain but if you break the
> chain by dropping one pair it still does not work.
> 
> Thanks for any assistance
> 
> Hugh Goyder
> 
> --
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