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MathGroup Archive 1998

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Re: Split in Mathematica 2.2

  • To: mathgroup at smc.vnet.net
  • Subject: [mg12810] Re: [mg12777] Split in Mathematica 2.2
  • From: Andrzej Kozlowski <andrzej at tuins.ac.jp>
  • Date: Fri, 12 Jun 1998 04:05:40 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

>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."

Thanks. This is indeed pretty fast. Not as fast as the built-in Split
but much faster than my simple version.

w = Table[Random[Integer], {10000}]; using built-in Split:

Split[w];//Timing
Out[7]=
{0.433333 Second,Null}

Using Xah's split:

In[6]:=
split[w];//Timing
Out[6]=
{1.26667 Second,Null}

Unfortunatelly procedural programming seems to triumph again...

Andrzej

Dr. Andrzej Kozlowski
Professor of Mathematics
Toyama International University
Toyama, JAPAN
http://sigma.tuins.ac.jp/



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