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Re: position of sequence of numbers in list

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  • Subject: [mg116230] Re: position of sequence of numbers in list
  • From: Oliver Ruebenkoenig <ruebenko at>
  • Date: Tue, 8 Feb 2011 05:03:20 -0500 (EST)

On Mon, 7 Feb 2011, Armand Tamzarian wrote:

> On Feb 6, 9:37 pm, "Mr. Wizard" <gl... at> wrote:
>>> I just found another way that outperforms both of our solutions on a
>>> wide range of integer arrays I tested. It turns out that one can
>>> extract the position directly from SparseArray, without using
>>> ArrayRules. It took me some time to figure it out:
>>> extractPositionFromSparseArray[
>>>   HoldPattern[SparseArray[u___]]] := {u}[[4, 2, 2]]
>>> positionExtr[x_List, n_] :=
>>>  Flatten@extractPositionFromSparseArray[
>>>    SparseArray[1 - Unitize[x - n]]]
>>> Again, no unpacking.
>>> Best,
>>> Norbert
>> Oliver Ruebenkoenig recently gave an easier way to get these values
>> in the thread "Fast matrices comparison" started January 25th, 2011.
>> SparseArray[{0, 1, 0, 0, 1, 0, 1, 1, 0, 0}]["NonzeroPositions"]
>> SparseArray[{1}]["Properties"]
>> Paul
> I noted in his post that this stuff is undocumented. Hard to fathom
> why this would be. Wonder if this is a V8 thing or has been there
> since SparseArray was included.
> Oliver, if you're reading this, what other undocumented goodies can
> you show to the group?
> Mike


I filed a suggestions that the functionality used here is going to be 
documented. I can not, however, say if this will be approved and or when 
this will happen.

Please also keep in mind to use undocumented functionality very cautiously 
- there may be very good reasons that the functionality is undocumented; it 
may not have been tested as much as needed.

Concerning "goodies", let's see what computational issues come up.


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