Re: Is Sort stable?

• To: mathgroup at smc.vnet.net
• Subject: [mg39780] Re: Is Sort stable?
• From: "J.L.Garrido" <garrido at ruth.upc.es>
• Date: Fri, 7 Mar 2003 03:31:12 -0500 (EST)
• References: <b41bv1\$pea\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```
En/Na Roland Nilsson ha escrit:

> Hi,
>
> --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?
>
>    Regards,
>
> --
> ---------------------------------------------
> Roland Nilsson
> IFM Computational Biology
> rolle at ifm.liu.se

MySort[n_, data_] :=
Sort[Sort[Select[data, First[#] == n &], #1[[2]] <= #2[[2]] &],
Apply[Plus, Rest[#1]] <= Apply[Plus, Rest[#2]] &]

data = Table[{Random[Integer, {1, 3}], Random[Integer, {1, 3}],
Random[]}, {30}];

Flatten[Map[MySort[#, data] &, Union[Map[First, data]]], 1]

```

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