[Numpy-discussion] Polynomial implementation in numpy.ma

Charles R Harris charlesr.harris at gmail.com
Sat Sep 17 12:40:22 EDT 2011

2011/9/14 Stéfan van der Walt <stefan at sun.ac.za>

> Hi all,
> There were some failures in the polynomial tests earlier today, and
> while investigating I saw that numpy.ma implements its own root
> finder.  It uses inversion of a Van der Monde matrix, which I believe
> may suffer from some numerical instability problems.  Given that
> Charles has gone to some length to implement good polynomial root
> finders, I think it would be best to employ those instead, and simply
> pre-filter the data that comes from the masked array module, if
> possible.
The test failure arises because the test compares against np.polyfit where
Travis changed the column scaling, which in turn changed the singular
values, and it is that comparison that fails.

The Vandermonde matrix needs to be used for the fitting so nothing should be
changed there. To fix the test failure, either the test could be modified or
the ma.polyfit routine can be changed. Probably the latter is the best
option, the new scaling code can be brought over and maybe the weight and
covariance options added.Travis' changes needs to be audited in any case to
make sure all the corner cases are covered.

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