Re: My vectorized code is slower than a loop

[Nicholas Jankowski]

On Wed, Oct 19, 2016 at 10:26 AM, Dave Cottingham 2
<address@hidden> wrote:
> Andreas Weber-6 wrote
>> Your script can be simplified to
>>
>> w = x-x(end:-1:1);
>>
>> try it out!
>>
>> Since "if(left(i) <= right(i))" is always true. Perhaps you can provide
>> a realistic example?
>>
>> -- Andy
>>
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>
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>
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>
> I try to keep the examples short -- and the computation of a realistic
> example is a bit involved -- so rather than post the code to compute the
> example, here's an octave saveset with a realistic example, and a modified
> "looptest.m" that reads it and runs the timing comparison.
>
> Thanks,
> Dave Cottingham
>
> looptest.dat
> <http://octave.1599824.n4.nabble.com/file/n4680245/looptest.dat>
> looptest.m <http://octave.1599824.n4.nabble.com/file/n4680245/looptest.m>
>


Yes, so Andy's code no longer produces an equivalent w.  Also, I
didn't look to see if you changed any of the code, but there is now
less of a penalty for the vectorized function.

Again, use the profiler. it will show you where you're code is
spending time.  Your vectorized code is simply making almost twice as
many binary operation calls (500,000 vs 325000), eating up any of the
non-looping benefit. here's the summary:

vectorized
Elapsed time is 6.26725 seconds.
   #            Function Attr     Time (s)   Time (%)        Calls
------------------------------------------------------------------
   4            binary -             0.122      41.92       199956
   3            binary +             0.111      38.15       199956
   2           binary <=             0.058      19.93       100000
   1                 tic             0.000       0.00            1
   5                 toc             0.000       0.00            1
   6             profile             0.000       0.00            1
   7              nargin             0.000       0.00            1
   8           binary !=             0.000       0.00            1
   9               false             0.000       0.00            1
  10 __profiler_enable__             0.000       0.00            1

looped
j =  1
Elapsed time is 5.94019 seconds.
   #            Function Attr     Time (s)   Time (%)        Calls
------------------------------------------------------------------
   4            binary -             0.084      40.38       150344
   2           binary <=             0.062      29.81       100000
   3            binary +             0.062      29.81        75172
   1                 tic             0.000       0.00            1
   5                 toc             0.000       0.00            1
   6             profile             0.000       0.00            1
   7              nargin             0.000       0.00            1
   8           binary !=             0.000       0.00            1
   9               false             0.000       0.00            1
  10 __profiler_enable__             0.000       0.00            1