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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: [mg73684] Re: Custom sort a list of pairs
  • From: "Norbert Marxer" <marxer at mec.li>
  • Date: Sat, 24 Feb 2007 02:25:15 -0500 (EST)
  • References: <ermdtt$i3t$1@smc.vnet.net>

On 23 Feb., 10:59, planetmarshal... 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

Hi Andrew

You can use this:

data = {{1, a}, {2, a}, {3, a}, {1, b}, {2, b}, {1, c}}
Sort[data, ( ! OrderedQ[{#1[[1]], #2[[1]]}] ||  #1[[1]] == #2[[1]])
&&
    OrderedQ[{#1[[2]], #2[[2]]}] & ]

Best Regards
Norbert Marxer

www.mec.li



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