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MathGroup Archive 1997

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Re: Re: How to select unique elements in a list?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg8416] Re: [mg8348] Re: How to select unique elements in a list?
  • From: jpk at max.mpae.gwdg.de
  • Date: Sat, 30 Aug 1997 00:42:23 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

> From nigelk at nortel.ca Tue Aug 26 15:10:46 1997
> Date: Tue, 26 Aug 1997 02:22:42 -0400
> From: "Nigel King" <nigelk at nortel.ca>
> Subject: [mg8348] Re: How to select unique elements in a list?
> 
> In article <5rmq5u$po1 at smc.vnet.net>,
> crose at c2.telstra-mm.net.au (Colin Rose) wrote:
> 
> >"Carl Woll" <carlw at u.washington.edu> wrote:
> >
> > First, let me say that Union does not do what I'm looking for! I am
> > interested in a function which returns the unique elements from a list.
> > Let's call the function Distinct. Then, I want
> >
> > Distinct[{i1,i2,i3,i2}]        to return        {i1,i3}
> >  
> >  RemoveDuplicates[a___,i_,i_,b___]:=RemoveDuplicates[a,b]
> >  Distinct[a_List]:=List@@RemoveDuplicates@@Sort[a]
> 
> I am not sure that people have noticed in this that the removal above does
> not remove odd numbers of duplicates
> 
> In[19]:=Distinct[{1,2,2,2,3}]
> Out[19]={1,2,3}
> 
> any solutions? As noticed pattern matching is not easy!
> 
> Nigel King
> Northern Telecom
> 
> 
> 
Yes, 

  what is with
  
  RemoveDuplicates[{a___,i_,i_,b___}] := 
    RemoveDuplicates[{a,b} /. i-> Sequence[]]
  
  Distinct[a_List]:=RemoveDuplicates[Sort[a]]
  
  Hope that helps
  Jens


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