Re: Computing sets of equivalences

• To: mathgroup at smc.vnet.net
• Subject: [mg46444] Re: Computing sets of equivalences
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Thu, 19 Feb 2004 03:01:51 -0500 (EST)
• Organization: Universitaet Leipzig
• References: <c0uu8a\$e84\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi,

selectEquvalences[lst_] :=
FixedPoint[
Union[Flatten[#]] & /@
Split[#, Function[{x, y}, Or @@ (MemberQ[x, #1] & /@ y)]] & ,
lst]

at least the While[]-loop is removed :-)

Regards
Jens

Mariusz Jankowski wrote:
>
> Dear Mathgroup, I think this falls into the "classic algorithms" category,
> so I hope some of you will find this interesting. I checked archives and
> mathsource but did not find anything useful.
>
> I have a list of lists, each sublist implying an equivalence. I am trying to
> split the list into lists of equivalences (this is part of a connected
> components algorithm).  For example, given
>
> {{1,2},{1,5},{2,3},{3,4},{5,6},{7,8},{11,12},{12,13},{10,14}}
>
> I want
>
> {{1,2,3,4,5,6},{7,8},{10,14},{11,12,13}}.
>
> Here is my currently "best" attempt. I accumulate the equivalences by
> comparing pairs of list, merging them if they have common elements. At the
> end of each iteration I remove all merged pairs from original list and
> repeat.
>
>  iselectEquivalences[v_]:=Module[{x,y,tmp,pos},
>      x=v;y={};
>      While[x=!={},
>        tmp=x[[1]];
>        pos={{1}};
>        Do[
>          If[Intersection[tmp,x[[i]]]==={}, tmp, tmp=Union[tmp,x[[i]]];
>            pos=Join[pos, {{i}}]], {i, 2, Length[x]}];
>        x=Delete[x, pos];
>        y=Join[y, {tmp}] ];
>  y]
>
> Can you tell me if you have or know of a realization of this classic
> operation that works better/faster? Are there alternative paradigms for
> solving this kind of problem.
>
> Thanks, Mariusz

```

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