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MathGroup Archive 1997

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How to select unique elements in a list?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg7949] How to select unique elements in a list?
  • From: "C. Woll" <carlw at u.washington.edu>
  • Date: Fri, 25 Jul 1997 02:40:25 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Hi all,

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}

Of course, I can write a function to do this, but I am hoping someone can
do a better job them my feeble attempt. So, 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]!

It's likely that the way to go is to use a procedural method instead of a
functional method.

At any rate, can anyone come up with a solution which is competitive in
speed with Union?

Carl Woll



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