Re: Custom sort a list of pairs
- To: mathgroup at smc.vnet.net
- Subject: [mg73688] Re: Custom sort a list of pairs
- From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
- Date: Sat, 24 Feb 2007 02:27:25 -0500 (EST)
- Organization: The Open University, Milton Keynes, UK
- References: <ermdtt$i3t$1@smc.vnet.net>
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
>
Hi Andrew,
You could try the following function:
In[1]:=
mySort[lst_] := Module[{iml},
iml = Split[Sort[lst, #1[[1]] > #2[[1]] & ], #1[[1]] == #2[[1]] & ];
Flatten[MapThread[Part,
{iml, (Ordering[#1[[All,2]]] & ) /@ iml}], 1]]
In[2]:=
mySort[{{1, a}, {2, a}, {3, a}, {1, b}, {2, b}, {1, c}}]
Out[2]=
{{3, a}, {2, a}, {2, b}, {1, a}, {1, b}, {1, c}}
Regards,
Jean-Marc