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MathGroup Archive 2007

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Re: Custom sort a list of pairs

  • To: mathgroup at smc.vnet.net
  • Subject: [mg73662] Re: Custom sort a list of pairs
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Sat, 24 Feb 2007 02:13:23 -0500 (EST)
  • Organization: Uni Leipzig
  • References: <ermdtt$i3t$1@smc.vnet.net>
  • Reply-to: kuska at informatik.uni-leipzig.de

Hi,

lst = {{1, a}, {2, a}, {3, a}, {1, b}, {2, b}, {1, c}};

Sort[lst,
   If[#1[[1]] == #2[[1]], #1[[2]] > #2[[2]], #1[[1]] > #2[[1]]] &]

??

Regards
   Jens

planetmarshalluk at hotmail.com wrote:
> Hi there,
> 
> I've searched through existing posts to no avail, what I want to do is
> this:
> Sort a nested list of pairs, so that each pair is sorted by reverse
> canonical order on its first element, and in normal order on its
> second element.
> 
> For example, I want some function
> 
> sortPairs[items_]
> 
> that when given the input
> 
> {{ 1,a},{2,a},{3,a},{1,b},{2,b},{1,c}}
> 
> produces the output
> 
> {{3,a},{2,a},{2,b}{1,a},{1,b},{1,c}}
> 
> Any help much appreciated.
> 
> Thanks,
> Andrew
> 
> 


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