Re: style question
- To: mathgroup at smc.vnet.net
- Subject: [mg79535] Re: style question
- From: Bill Rowe <readnewsciv at sbcglobal.net>
- Date: Sat, 28 Jul 2007 05:45:00 -0400 (EDT)
On 7/26/07 at 5:39 AM, carlw at wolfram.com (Carl Woll) wrote:
>Yaroslav Bulatov wrote:
>>What is the recommended way of counting the number of matches in
>>two lists?
>>The natural way would be to thread over Equal, but Equal will
>>evaluate before Thread gets to it. The method below works, but
>>somehow feels wrong
>>m1 = RandomInteger[{1}, {10^5, 2, 2}]; m2 = RandomInteger[{1},
>>{10^5, 2, 2}]; matches = Thread[temporary[m1, m2]] /. temporary ->
>>Equal; Count[matches, True]
>For this particular problem (in version 6), it's faster to use
>arithmetic and total:
>In[5]:= Count[Total[Unitize[m1 - m2], {2, 3}], 0] // Timing
>Out[5]= {0.391, 62300}
On my machine, it is somewhat faster to make use this idea and
SparseArray, i.e.,
In[15]:= k = 10^6;
m1 = RandomInteger[1, {k, 2, 2}];
m2 = RandomInteger[1, {k, 2, 2}];
In[18]:= Count[Total[Unitize[m1 - m2], {2, 3}], 0] // Timing
Out[18]= {1.12376,62590}
In[19]:= k - (SparseArray[Total[Unitize[m1 - m2], {2, 3}]] /.
SparseArray[_, _, _, a_] :> Length[Last@a]) // Timing
Out[19]= {0.929573,62590}
In[20]:= $Version
Out[20]= 6.0 for Mac OS X PowerPC (32-bit) (June 19, 2007)
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