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MathGroup Archive 1998

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Re: Split in Mathematica 2.2

  • To: mathgroup at smc.vnet.net
  • Subject: [mg12841] Re: Split in Mathematica 2.2
  • From: "Carl K.Woll" <carlw at fermi.phys.washington.edu>
  • Date: Wed, 17 Jun 1998 00:27:48 -0400
  • Organization: Department of Physics
  • References: <199806100704.DAA13675@smc.vnet.net.> <6lqqub$o9u@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Andrzej,

Here is a faster version of your idea:

split[li_,testQ_:SameQ] := Module[{r1,r2},
    r1=Flatten[Position[Apply[testQ,Partition[li,2,1],{1}],False]];
    r2=Transpose[{Join[{1},r1+1],Join[r1,{Length[li]}]}];
    Take[li,#]&/@r2]

At least, on my machine, the above version is more than twice as fast as
yours.

Cheers,

Carl Woll
Dept of Physics
U of Washington

Andrzej Kozlowski wrote:

> >
> >At 12:04 AM -0700 1998.06.10, Andrzej Kozlowski wrote:
> >>.. Can anyone suggest a really fast implementation
> >>of Split that works in version 2.2?
> >
> >Here's an emulated Split with speed considerations. Basically, it iterates
> >through the list, mark down where the boundaries should be, then map Take
> >to the boundary indexes to obtain the new list.
> >
> >split::"usage"=
> >  "split simulates the version 3 build-in function Split. Example:
> >Split[{1,2,3,4,4,3,2}]";
> >
> >split[li_,fQ_:SameQ]:=Module[{liLength,index,i},liLength=Length at li;
> >index={};For[i=1,i<liLength,i++,
> >      If[Not@(fQ@@Part[li,{i,i+1}]),index={index,i}]];
> >(Take[li,#]&)/@(Transpose[{Flatten[{1,#+1}],Flatten[{#,liLength}]}]&)@
> >        Flatten at index];
> >
> >
> >PS for those wondering what happened to my series of Mathematica expositions... I
> >got extremely busy with work. I will come back to it sometimes in the
> >future.
> >
> > Xah, xah at best.com
> > http://www.best.com/~xah/PageTwo_dir/more.html
> > "Perl: all unix's stupidity in one."
>
> I think I managed to construct an even faster program which avoids the
> dreaded For loop:
>
> split1[l_]:=
>   Module[{m},
>     Map[Take[l,#]&,
>       Partition[
>           Insert[Insert[
>
> Flatten[Position[Partition[l,2,1]/.{x_,y_}/;x=!=y->m,m]],1,1],
>             Length[l],-1],2,1]/.{a_,b_}/;a!=1->{a+1,b}]]
>
> Here are some test results (Mathematica 3.01, PowerMac G3 266) In[5]:=
> w = Table[Random[Integer], {10000}]; In[6]:=
> Split[w];//Timing
> Out[6]=
> {0.25 Second,Null}
> In[7]:=
> split[w];//Timing
> Out[7]=
> {1.16667 Second,Null}
> In[8]:=
> split1[w];//Timing
> Out[8]=
> {0.766667 Second,Null}
>
> Split is the built in mathematica 3.0 function, split is Xah's function
> and split1 is my new function. I am relieved; I hate the For loop!
>
> Andrzej
>
> Dr. Andrzej Kozlowski
> Professor of Mathematics
> Toyama International University
> Toyama, JAPAN
> http://sigma.tuins.ac.jp/




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