Re: My vectorized code is slower than a loop

[Nicholas Jankowski]

On Tue, Oct 18, 2016 at 1:38 PM, Andreas Weber <address@hidden> wrote:
> Am 17.10.2016 um 21:37 schrieb Dave Cottingham 2:
>> In the process of writing a routine to compute the Qn statistic of Croux and
>> Rousseeuw, I started from the Fortran code they published in their paper
>> (C&R, "Time-efficient algorithms for two highly robust estimators of scale")
>> and ran into unexpected trouble with one section. Here's a fairly literal
>> translation of the original, where x is an array of length n, and left and
>> right are also arrays of length n that contain integers:
>>
>> code snipped
>
> 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


Andy's got it, but for reference here's the profiler timing for your
code.  Andy's code appears to be too fast for the profiler :) Note the
additional operation count in your vectorized code.


vectorized
Elapsed time is 5.917 seconds.

   #            Function Attr     Time (s)   Time (%)        Calls
------------------------------------------------------------------
   5            binary -             0.131      43.52       400000
   4            binary +             0.121      40.20       400000
   3           binary <=             0.049      16.28       100000
   1                 tic             0.000       0.00            1
   2               zeros             0.000       0.00            1
   6                 toc             0.000       0.00            1
   7             profile             0.000       0.00            1
   8              nargin             0.000       0.00            1
   9           binary !=             0.000       0.00            1
  10               false             0.000       0.00            1
  11 __profiler_enable__             0.000       0.00            1

non-vectorized
Elapsed time is 3.594 seconds.

   #            Function Attr     Time (s)   Time (%)        Calls
------------------------------------------------------------------
   5            binary -             0.053      47.32       200000
   4            binary +             0.030      26.79       100000
   3           binary <=             0.029      25.89       100000
   1                 tic             0.000       0.00            1
   2               zeros             0.000       0.00            1
   6                 toc             0.000       0.00            1
   7             profile             0.000       0.00            1
   8              nargin             0.000       0.00            1
   9           binary !=             0.000       0.00            1
  10               false             0.000       0.00            1
  11 __profiler_enable__             0.000       0.00            1

Andy supercode
Elapsed time is 0.039 seconds.


   #            Function Attr     Time (s)   Time (%)        Calls
------------------------------------------------------------------
warning: division by zero
warning: called from
    profshow at line 85 column 17
    testy at line 59 column 3
   1                 tic             0.000        NaN            1
warning: division by zero
   2                 end             0.000        NaN            1
warning: division by zero
   3            prefix -             0.000        NaN            1
warning: division by zero
   4            binary -             0.000        NaN            1
warning: division by zero
   5                 toc             0.000        NaN            1
warning: division by zero
   6             profile             0.000        NaN            1
warning: division by zero
   7              nargin             0.000        NaN            1
warning: division by zero
   8           binary !=             0.000        NaN            1
warning: division by zero
   9               false             0.000        NaN            1
warning: division by zero
  10 __profiler_enable__             0.000        NaN            1
>>