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Re: position of sequence of numbers in list


There have always been THOUSANDS of undocumented goodies in Mathematica,  
judging from the number that keep coming up.

Bobby

On Mon, 07 Feb 2011 05:05:19 -0600, Armand Tamzarian  
<mike.honeychurch at gmail.com> wrote:

> On Feb 6, 9:37 pm, "Mr. Wizard" <gl... at flashmail.com> 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
>


-- 
DrMajorBob at yahoo.com


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