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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: [mg73658] Re: Custom sort a list of pairs
  • From: "dimitris" <dimmechan at yahoo.com>
  • Date: Sat, 24 Feb 2007 02:11:12 -0500 (EST)
  • References: <ermdtt$i3t$1@smc.vnet.net>

Hi.

I apologize in advance I didn't understand well!

> 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.

> 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}}

3, 2,2,1,1,1 indeed sorted by reverse canonical order.
a,a,b,a,b,c sorted in normal order? I doubt.

Did you really mean something like the following?

In[69]:=
{{1, a}, {2, a}, {3, a}, {1, b}, {2, b}, {1, c}}
Transpose[%]
% /. {x_List, y_List} :> {Sort[x, #2 - #1 < 0 & ], Sort[y]}
Transpose[%]

Out[69]=
{{1,a},{2,a},{3,a},{1,b},{2,b},{1,c}}

Out[70]=
{{1,2,3,1,2,1},{a,a,a,b,b,c}}

Out[71]=
{{3,2,2,1,1,1},{a,a,a,b,b,c}}

Out[72]=
{{3,a},{2,a},{2,a},{1,b},{1,b},{1,c}}


Dimitris
=CF/=C7 planetmarshalluk at hotmail.com =DD=E3=F1=E1=F8=E5:
> 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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