Re: How to find the difference between two lists (opposite of union[]?)

• To: mathgroup at smc.vnet.net
• Subject: [mg119920] Re: How to find the difference between two lists (opposite of union[]?)
• From: Leonid Shifrin <lshifr at gmail.com>
• Date: Wed, 29 Jun 2011 05:30:34 -0400 (EDT)

```Use Complement:

Complement[Range[1000], mList]

Regards,
Leonid

On Tue, Jun 28, 2011 at 3:55 PM, McHale, Paul <Paul.McHale at excelitas.com>wrote:

> I have been working through a neat book called Mathematica(r): A
> Problem-Centered Approach<
>  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.
>
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> Paul.McHale at Excelitas.com<mailto:Paul.McHale at perkinelmer.com>
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>
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```

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