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MathGroup Archive 1999

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Re: Fast List-Selection

  • To: mathgroup at smc.vnet.net
  • Subject: [mg19915] Re: [mg19880] Fast List-Selection
  • From: "Arnold Knopfmacher" <arnoldk at cam.wits.ac.za>
  • Date: Tue, 21 Sep 1999 02:22:49 -0400
  • Organization: MS, University of the Witwatersrand
  • Sender: owner-wri-mathgroup at wolfram.com

Hi, this is faster than my last effort (also given  below)

dif[s_]:=Drop[s,1]-Drop[s,-1];
nConsecD2[l_,n_]:=With[{pat=Table[0,{n-1}]},
  Flatten[Position[Partition[dif[l],n-1,1],{x__} /;{x}==pat]]]

s=Table[Random[Integer,{1,4}],{50000}];

nConsecD2[s,7]//Timing

{3.35 Second,{37896,40088,40089,40703,42736,42737,42738,46784}}


nConsec[s,7]//Timing

{8.24 Second,{37896,40088,40089,40703,42736,42737,42738,46784}}


>Here is one approach to finding starting positions of n consecutive identical
>elements:

 nConsec[l_,n_]:=
  Flatten[Position[Partition[l,n,1],{x__} /;Length[Union[{x}]]==1]]

s={2,3,3,3,3,3,3,3,3,3,4,4,5,5,5,5,5,5,5,4}

nConsec[s,7]

{2,3,4,13}

Arnold Knopfmacher

> Date:          Sun, 19 Sep 1999 18:47:32 -0400
> From:          Hans Havermann <haver at total.net>
To: mathgroup at smc.vnet.net
> To:            mathgroup at smc.vnet.net
> Subject: [mg19915]       [mg19880] Fast List-Selection

> I have a list 's' composed of a large number of (small) integers. I wish to
> search this list for instances of 7 consecutive, identical elements.
> 
> My approach is:
> 
> Do[If[Count[t = Take[s, {i, i + 6}], t[[1]]] == 7,
>     Print[i]], {i, 1, Length[s] - 6}]
> 
> Can anyone think of a *faster* way of doing this?
> 
> 
> 
> 

Arnold Knopfmacher
Dept of Computational and Applied Maths
Witwatersrand University
Johannesburg 2050
South Africa
http://www.wits.ac.za/science/number_theory/arnold.htm
Fax: 2711-4039317
Phone: 2711- 7163353
email: arnoldk at gauss.cam.wits.ac.za


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