Re: How to find the difference between two lists (opposite of union[]?)
- To: mathgroup at smc.vnet.net
- Subject: [mg119913] Re: How to find the difference between two lists (opposite of union[]?)
- From: Leo Ditolaghi <leoditolaghi at gmail.com>
- Date: Wed, 29 Jun 2011 05:29:15 -0400 (EDT)
Hi Paul,
Well, I'm still a Mathematica noob, but a walk around the help docs gave me
this solution,
which seems fairly straightforward (I used the range 1 to 10 for
illustration):
(* numbers from 1 to 10 not represented in a randomly generated list of
integers of the same range *)
randomNumbers = RandomInteger[{1,10},10]
Out[6]= {9,4,1,6,5,8,4,9,5,8}
In[7]:= sequence = Range[10]
Out[7]= {1,2,3,4,5,6,7,8,9,10}
In[8]:= Complement[sequence,randomNumbers]
Out[8]= {2,3,7,10}
Cheers,
Leo
On Tue, Jun 28, 2011 at 7:55 AM, McHale, Paul <Paul.McHale at excelitas.com>wrote:
> I have been working through a neat book called Mathematica(r): A
> Problem-Centered Approach<
> http://www.amazon.com/Mathematica%C2%AE-Problem-Centered-Approach-Undergraduate-Mathematics/dp/1849962502/ref=sr_1_2?ie=UTF8&qid=1309181549&sr=8-2>.
> I have a problem I would like to know if there is a more elegant solution.
> I would think involving set theory, but I can't seem to (embarrassingly)
> find it.
>
> I want to generate 1000 random integers between 1 and 1000. Then find out
> what numbers between 1 and 1000 were not generated.
>
> (* Generate random list of 1000 numbers with range 1-1000*)
> mList=RandomInteger[{1,1000},1000] //Sort;
>
> (* Find out what numbers were omitted in the range 1-1000*)
> (* Make a list of numbers 1-1000 with position of each within original
> array *)
> mList=Table[{i,Position[mList,i] //Flatten},{i,1,1000}];
>
> (* select numbers with no position in original array (musical chairs) *)
> Select[mList,Length[#[[2]]]==0&][[All,1]]
>
> This works creating a lot of extra array in the process. Just wanted to
> ask what are the methods you guys would use to solve this?
>
> A funny (non-working) way of stating the same thing would be:
>
> (* Generate random list of 1000 numbers with range 1-1000*)
> mList=RandomInteger[{1,1000},1000] //Sort;
>
> Solve[ Join[mList,mDiff]==Range[1000],mDiff]
>
> Thanks,
> Paul
>
>
> Paul McHale | Electrical Engineer, Energetics Systems | Excelitas
> Technologies Corp.
>
> Phone: +1 937.865.3004 | Fax: +1 937.865.5170 | Mobile: +1
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>
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