Re: Position of Sign change in long list
- To: mathgroup at smc.vnet.net
- Subject: [mg18706] Re: [mg18691] Position of Sign change in long list
- From: Jean-Marie Thomas <jmt at agat.net>
- Date: Sat, 17 Jul 1999 02:36:31 -0400
- Organization: TeA
- References: <199907150546.BAA15968@smc.vnet.net.>
- Sender: owner-wri-mathgroup at wolfram.com
Try this :
Timing at Catch@Module[{i,s=0},
s=Sign at First@test;
Scan[(++i,If[Sign at #!=s,Throw at i])&,test]]
Hope this helps,
jmt
On Thu, 15 Jul 1999, Martin Rommel wrote:
> I have a long list of data points and need to find the position where the
> sign changes for the first time.
> My first attempt was
>
> test=Table[Sin[Pi i/5000. +1],{i,10000}];
>
> With[{a=Abs[test]},Position[a,Min[a]]]
>
> which works, but is slow. It turns out that Min is relatively slow and
> avoiding it can save time. The following is almost a factor 4 faster:
>
> With[{st=Sign[test]},Length[Split[st][[1]]]]
>
> If you can think of a still faster or more elegant solution please let me
> know!
>
> Martin
- References:
- Position of Sign change in long list
- From: "Martin Rommel" <rommel@semitest.com>
- Position of Sign change in long list