Polyfit

Charles R Harris charlesr.harris at gmail.com
Thu Oct 12 17:03:04 CDT 2006


On 10/12/06, Greg Willden <gregwillden at gmail.com> wrote:
>
> On 10/12/06, Charles R Harris <charlesr.harris at gmail.com> wrote:
> >
> > I'm guessing that the rcond number in the lstsq version (default 1e-10)
> > is the difference. Generally the lstsq version should work better than the
> > MPL version because at*a is not as well conditioned and vandermonde matrices
> > are notoriously ill conditioned anyway for higher degree fits. It would help
> > if you attached the data files in ascii form unless they happen to contain
> > thousands of data items. Just the x will suffice, zip it up if you have to.
> >
>
>
> Here are the files.
>
> Since the two algorithms behave differently and each has it place then can
> both be included in numpy?
> i.e. numpy.polyfit(x,y,N, mode='alg1')
> numpy.polyfit (x,y,N, mode='alg2')
>
> replacing alg1 and alg2 with meaningful names.
>

The polyfit function looks seriously busted. If I do the fits by hand I get
the same results using the (not so hot) MPL version or lstsq. I don't know
what the problem is. The docstring is also incorrect for the method. Hmmm...

Chuck
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