Re: position of sequence of numbers in list
- To: mathgroup at smc.vnet.net
- Subject: [mg116215] Re: position of sequence of numbers in list
- From: "Mr. Wizard" <gleam at flashmail.com>
- Date: Sun, 6 Feb 2011 05:37:39 -0500 (EST)
>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