Re: How to select unique elements in a list?

• To: mathgroup at smc.vnet.net
• Subject: [mg7986] Re: How to select unique elements in a list?
• From: crose at c2.telstra-mm.net.au (Colin Rose)
• Date: Wed, 30 Jul 1997 23:57:37 -0400
• Organization: Theoretical Research Institute
• Sender: owner-wri-mathgroup at wolfram.com

```"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}

Z[x_] := Select[ Split[Sort[x]], (Length[#] == 1)&]//Flatten

eg:

In[]   test=Table[ i[Random[Integer,1000]],{1000}];

In[]   Z[test]; // Timing

Out[]   {0.05 Second,Null}          PowerMac 8600/200

> my best functional solution so far is,
>
>      RemoveDuplicates[a___,i_,i_,b___]:=RemoveDuplicates[a,b]
>      Distinct[a_List]:=List@@RemoveDuplicates@@Sort[a]
>
> However, this solution is very slow. For example, with
>
> test=Table[i[Random[Integer,1000]],{1000}];
>
> it takes approximately 27 seconds to evaluate Distinct[test] on an SGI
> system. This is slow when compared to .01 seconds to evaluate Union[test]!

(i)  Z[test] == Distinct[test] yields FALSE, and
(ii) Z[test] appears to be correct.
That is the problem with pattern matching: you have to catch ALL
the possible patterns.

Hope this helps

Cheerio

Colin

--
Colin Rose
tr(I) - Theoretical Research Institute
______________________________________
crose at c2.telstra-mm.net.au
http://www.usyd.edu.au/su/tri/

```

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