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Re: Re: Counting Runs

• To: mathgroup at smc.vnet.net
• Subject: [mg52228] Re: [mg52194] Re: Counting Runs
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Mon, 15 Nov 2004 03:17:41 -0500 (EST)
• References: <001601c4c8d4$6aecc3e0$6400a8c0@Main> <opsheakrjtiz9bcq@monster.cox-internet.com> <006b01c4c998$29b9a220$6400a8c0@Main> <200411140930.EAA14521@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

>> I think Abs, Tr, and BitXor are very fast because they are probably
>> optimized for integer input and the use of packed arrays.
>
> I don't think an optimization for integers can be effective if 
> functions take time to CHECK whether the array contains only integers. 
> Is that precomputed for packed arrays, somehow?
>
> Bobby


Yes, that's precisely what packed arrays do. Actually you can check 
precisely the effect of packed arrays in this case:


<< "Developer`"


runs[int_, data_] := Module[{modlist},
    modlist = Sign[Abs[data - int]];
     Tr[BitXor[modlist, RotateRight[modlist]]]/2 + 1 -
      BitOr[modlist[[1]], modlist[[-1]]]]


runs1[int_, data_] :=
   Module[{modlist, dt = FromPackedArray[data]},
    modlist = FromPackedArray[Sign[Abs[dt - int]]];
     Tr[BitXor[modlist, RotateRight[modlist]]]/2 + 1 -
      BitOr[modlist[[1]], modlist[[-1]]]]


seq = Table[Random[Integer, 10], {10^6}];


Timing[runs[3, seq]]


{0.4299999999999997*Second, 83183}


Timing[runs1[3, seq]]


{0.9299999999999997*Second, 83183}

So just over 50% performance increase.


Andrzej Kozlowski
Chiba, Japan
http://www.akikoz.net/~andrzej/
http://www.mimuw.edu.pl/~akoz/

• References:
  • Re: Counting Runs
    • From: DrBob <drbob@bigfoot.com>