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