Services & Resources / Wolfram Forums / MathGroup Archive
-----

MathGroup Archive 2011

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: position of sequence of numbers in list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg116219] Re: position of sequence of numbers in list
  • From: Armand Tamzarian <mike.honeychurch at gmail.com>
  • Date: Mon, 7 Feb 2011 06:05:19 -0500 (EST)
  • References: <iiltlt$355$1@smc.vnet.net>

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


  • Prev by Date: Re: Pattern matching
  • Next by Date: Re: Mathematica 8 Home Edition
  • Previous by thread: Re: position of sequence of numbers in list
  • Next by thread: Re: position of sequence of numbers in list