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MathGroup Archive 2002

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Re: Re: Pattern Matching in Lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg35595] Re: [mg35586] Re: Pattern Matching in Lists
  • From: Andrzej Kozlowski <andrzej at platon.c.u-tokyo.ac.jp>
  • Date: Mon, 22 Jul 2002 02:10:50 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Having quickly glanced through the avalanche of proposed solutions I did 
not see following one.(if there was I apologize for claiming it as my 
own):

Count[w - RotateLeft[w], 1]

According to my comparisons it is the fastest so far (the one that I 
sent originally is one of the slowest)

  In[1]:=
w=Table[Random[Integer],{200000}];

In[2]:=
Count[Partition[w,2,1],{1,0}]//Timing

Out[2]=
{0.86 Second,50131}

In[3]:=
Count[Drop[w,-1]-Drop[w,1],1]//Timing

Out[3]=
{0.34 Second,50131}

In[5]:=
Count[w-RotateLeft[w],1]//Timing

Out[5]=
{0.29 Second,50131}

Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/


On Sunday, July 21, 2002, at 02:01  PM, Allan Hayes wrote:

> [second posting in view of reported technical problem]
>
> Anthony,
> Take
>     w = Table[Random[Integer], {200000}
>
> My first thought was, and several posts used this,
>
>     Count[Partition[w, 2,1],{1,0}]//Timing
>
>         {3.24 Second,49851}
>
> Later it occured to me to use arithmetic, which turned out to be twice 
> as
> fast:
>
>      Count[ Drop[w,-1] - Drop[w,1],1]//Timing
>
>         {1.49 Second,49851}
>
> This is close to Selwyn Hollis's code
>
>     Count[Drop[w+2RotateRight[w],1],2]//Timing
>
>         {1.6 Second,49851}
>
> --
> Allan
>
> ---------------------
> Allan Hayes
> Mathematica Training and Consulting
> Leicester UK
> www.haystack.demon.co.uk
> hay at haystack.demon.co.uk
> Voice: +44 (0)116 271 4198
> Fax: +44 (0)870 164 0565
>
>
> "Anthony Mendes" <amendes at zeno.ucsd.edu> wrote in message
> news:ah5qce$59o$1 at smc.vnet.net...
>> Hello,
>>
>> Suppose w={1,1,1,0,0,1,0,1,0,0,1,0,0}.
>>
>> How can I count the number of occurrences of a 1 in w immediately
>> followed by a 0 in w?
>>
>> I have tried every incarnation of Count[] I can think of; for example,
>>
>> Count[w,{___,1,0,___}]
>>
>> does not seem to work.  In general, how can I count the number of
>> occurrences of a 1 followed by a 0 in a list of 1's and 0's?  Thank 
>> you!
>>
>>
>> --
>> Tony
>> _____________________
>> amendes at math.ucsd.edu
>>
>>
>
>
>
>
>
>
>
>



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