Re: vectorization

[Ben Abbott]

On Jul 17, 2008, at 4:07 AM, dbateman wrote:

> Ben Abbott wrote:
> > I took a look at this as well and would have responded earlier but my
> > internet went down during a thunderstorm :-(
> >
> > David, of the three versions you posted only the third ran for me
> > without error. While the 3rd version does run, it does not give (what
> > I think) is the correct answer :-(
>
> Hey the third version worked!! None were tested which is why I had  
> so much
> trouble getting it right. However ...
>
> > octave:46> A = rand([1,9])
> > A =
> >
> >    0.90785   0.89152   0.74896   0.71078   0.21859   0.26943
> > 0.89706   0.63350   0.47987
> >
> > octave:47> q = newfilter (A, 3)
> > q =
> >
> >    0.89152   0.89152   0.89152   0.26943   0.26943   0.26943
> > 0.63350   0.63350   0.63350
> >
> > Unfortunately, a quick check of the first 3 elements gives a  
> > incorrect
> > result.
> >
> > octave:48> mean (A(1:3))
> > ans =  0.84944
> >
> > The repmat approach had not occurred to me. The approach I took was
> > to ...
> >
> >       a = mean (reshape(A, [npoints, numel(A)/npoints]));
> >       q = reshape (ones (npoints, 1) * a, [1, numel(A)]);
> >
> > Which (I believe) produces the desired result.
> >
> > octave:49> q = newfilter (A, 3)
> > q =
> >
> >    0.84944   0.84944   0.84944   0.39960   0.39960   0.39960
> > 0.67014   0.67014   0.67014
>
> The original code indexes back to a copy of the original array  
> elements and
> not the mean, so I think my version is correct. Or rather it should  
> have the
> same behavior as the original function.
>
> D.

opps ... of course, you are correct. My mind has a nasty habit of  
"filtering" text as I read it. I'd missed that he was extracting the  
value closest to the mean.

Ben