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MathGroup Archive 1999

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Re: easiest way to sort a list?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg18324] Re: [mg18308] easiest way to sort a list?
  • From: Andrew Watson <abwatson at mail.arc.nasa.gov>
  • Date: Sat, 26 Jun 1999 19:08:03 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

At 7:24 PM -0700 6/25/99, Peter T. Wang wrote:

> Suppose you have a list of integers, not all distinct,
>say
>      {1, 5, 3, 5, 10, 10, 1},
>
>and you want to obtain the list
>
>      {1, 5, 3, 10}
>
>which is the set of distinct elements but with the order preserved;

How about:

In[3]:=
in = {1, 5, 3, 5, 10, 10, 1};

In[4]:=
elements = Union[input]

Out[4]=
{1, 3, 5, 10}

In[12]:=
positions = Flatten[First[Position[in, #]] & /@ elements]

Out[12]=
{1, 3, 2, 5}

In[13]:=
in[[positions]]

Out[13]=
{1, 3, 5, 10}

or, in one step,

In[14]:=
OrderedUnion[list_] :=
  list[[Flatten[First[Position[list, #]] & /@ Union[list]]]]

OrderedUnion[in]

Out[16]=
{3, 5, 7, 12}

Andrew B. Watson
MS 262-2
NASA Ames Research Center
Moffett Field, CA 94035-1000
(650) 604-5419	(650) 604-0255 fax
abwatson at mail.arc.nasa.gov	http://vision.arc.nasa.gov/


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