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Re: Occurrences in Mathematica

  • To: mathgroup at
  • Subject: [mg106373] Re: [mg106350] Occurrences in Mathematica
  • From: Murray Eisenberg <murray at>
  • Date: Mon, 11 Jan 2010 05:28:04 -0500 (EST)
  • Organization: Mathematics & Statistics, Univ. of Mass./Amherst
  • References: <>
  • Reply-to: murray at

Here's one way, via an example:

   lis = RandomChoice[Characters["abcdefghijklmnopqrstuvwxyz"], 50];

   Select[Union[lis], Count[lis, #] == 2 &]

Your results will vary according to what RandomChoice gives you. But the 
code does exactly what you asked: it selects (Select) from the unique 
elements of the list (Union[lis]) those elements for which the Count of 
their appearances in the list is 2.

I make no claims for the efficiency of the code. That may depend upon 
the length and perhaps even the elements of your list, among other things.

On 1/10/2010 3:28 AM, George wrote:
> Hi,
> I have a very big list of elements and I need Mathematica to give me
> only those elements which occure only once...
> Could anybody advise me please how to write the code for that?
> Thks much
> George

Murray Eisenberg                     murray at
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305

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