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MathGroup Archive 2003

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Re: Is Sort stable?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg39743] Re: [mg39725] Is Sort stable?
  • From: BobHanlon at aol.com
  • Date: Wed, 5 Mar 2003 00:04:56 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

In a message dated 3/4/03 1:25:41 AM, rolle at ifm.liu.se writes:


> --Short form:
> Is the sorting algorithm implemented by Sort[] stable?
> 
> --Long form:
> I'm doing a thingy where I need to take out subsets of data and look at 
> them
> individually. I have a labeled data set, with labels being a vector of 1,2
> ... denoting class membership, and I've tried to pick out subset using
> either e.g.
> 
> Extract[data, Position[labels,1]]
> => the data points in "class 1"
> 
> or using Sort to sort data according to labels (so I get e.g. {1,1,1,1, ...
> 2,2,2, ... 3,3,3}, and which is nice for plotting data sets. This work ok,
> but it seems like Sort[] is rearranging the data points within classes
> different from Extract[]. Does anyone know i Is the Sort[] algorithm 
> stable?
> Could there be something else lurking here?
> 
> 

data = Table[

     {Random[Integer,{1,3}],Random[Integer,{1,3}],Random[]},

     {20}]



The default is a multi-position sort



Sort[data]



Sorting solely by the first position reverses the order of the elements 
within the individual groupings



Sort[data, #1[[1]]<#2[[1]]&]



To use Sort and leave the order within groupings untouched



Sort[Reverse[data], #1[[1]]<#2[[1]]&]



Verifying,



%==Flatten[Table[

       Extract[data,Position[data,{n,_,_}]],
 
       {n,3}], 1]


or alternatively

%%==Flatten[Table[

       Cases[data, {n,_,_}], {n,3}],1]



Bob Hanlon




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