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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: [mg73654] Re: Custom sort a list of pairs
  • From: Paul Abbott <paul at physics.uwa.edu.au>
  • Date: Sat, 24 Feb 2007 02:09:03 -0500 (EST)
  • Organization: The University of Western Australia
  • References: <ermdtt$i3t$1@smc.vnet.net>

In article <ermdtt$i3t$1 at smc.vnet.net>, planetmarshalluk at hotmail.com 
wrote:

> 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}}

>From the Sort documentation, the default ordering function is  
OrderedQ[{#1, #2}]&. Since you want to sort by reverse canonical order 
on the first element, and in normal order on the second element, the 
following construct does what you want:

  Sort[ {{1,a},{2,a},{3,a},{1,b},{2,b},{1,c}}, 
    OrderedQ[{First[#2],First[#1]}] &&
    OrderedQ[{Last[#1],Last[#2]}]& ]

  {{3, a}, {2, a}, {2, b}, {1, a}, {1, b}, {1, c}}

Cheers,
Paul

_______________________________________________________________________
Paul Abbott                                      Phone:  61 8 6488 2734
School of Physics, M013                            Fax: +61 8 6488 1014
The University of Western Australia         (CRICOS Provider No 00126G)    
AUSTRALIA                               http://physics.uwa.edu.au/~paul


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