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

```

• Prev by Date: fonts
• Next by Date: How to select unique elements in a list?