[Numpy-discussion] Polynomial evaluation inconsistencies
Eric Wieser
wieser.eric+numpy at gmail.com
Fri Jun 29 23:23:48 EDT 2018
Here's my take on this, but it may not be an accurate summary of the
history.
`np.poly<func>` is part of the original matlab-style API, built around
`poly1d` objects. This isn't a great design, because they represent:
p(x) = c[0] * x^2 + c[1] * x^1 + c[2] * x^0
For this reason, among others, the `np.polynomial` module was created,
starting with a clean slate. The core of this is
`np.polynomial.Polynomial`. There, everything uses the convention
p(x) = c[0] * x^0 + c[1] * x^1 + c[2] * x^2
It sounds like we might need clearer docs explaining the difference, and
pointing users to the more sensible `np.polynomial.Polynomial`
Eric
On Fri, 29 Jun 2018 at 20:10 Charles R Harris <charlesr.harris at gmail.com>
wrote:
> On Fri, Jun 29, 2018 at 8:21 PM, Maxwell Aifer <maifer at haverford.edu>
> wrote:
>
>> Hi,
>> I noticed some frustrating inconsistencies in the various ways to
>> evaluate polynomials using numpy. Numpy has three ways of evaluating
>> polynomials (that I know of) and each of them has a different syntax:
>>
>> -
>>
>> numpy.polynomial.polynomial.Polynomial
>> <https://docs.scipy.org/doc/numpy-1.13.0/reference/generated/numpy.polynomial.polynomial.Polynomial.html#numpy.polynomial.polynomial.Polynomial>:
>> You define a polynomial by a list of coefficients *in order of
>> increasing degree*, and then use the class’s call() function.
>> -
>>
>> np.polyval
>> <https://docs.scipy.org/doc/numpy-1.13.0/reference/generated/numpy.polyval.html>:
>> Evaluates a polynomial at a point. *First* argument is the
>> polynomial, or list of coefficients *in order of decreasing degree*,
>> and the *second* argument is the point to evaluate at.
>> -
>>
>> np.polynomial.polynomial.polyval
>> <https://docs.scipy.org/doc/numpy-1.12.0/reference/generated/numpy.polynomial.polynomial.polyval.html>:
>> Also evaluates a polynomial at a point, but has more support for
>> vectorization. *First* argument is the point to evaluate at, and
>> *second* argument the list of coefficients *in order of increasing
>> degree*.
>>
>> Not only the order of arguments is changed between different methods, but
>> the order of the coefficients is reversed as well, leading to puzzling bugs
>> (in my experience). What could be the reason for this madness? As polyval
>> is a shameless ripoff of Matlab’s function of the same name
>> <https://www.mathworks.com/help/matlab/ref/polyval.html> anyway, why not
>> just use matlab’s syntax (polyval([c0, c1, c2...], x)) across the board?
>>
>>
>>
> The polynomial package, with its various basis, deals with series, and
> especially with the truncated series approximations that are used in
> numerical work. Series are universally written in increasing order of the
> degree. The Polynomial class is efficient in a single variable, while the
> numpy.polynomial.polynomial.polyval function is intended as a building
> block and can also deal with multivariate polynomials or multidimensional
> arrays of polynomials, or a mix. See the simple implementation of polyval3d
> for an example. If you are just dealing with a single variable, use
> Polynomial, which will also track scaling and offsets for numerical
> stability and is generally much superior to the simple polyval function
> from a numerical point of view.
>
> As to the ordering of the degrees, learning that the degree matches the
> index is pretty easy and is a more natural fit for the implementation code,
> especially as the number of variables increases. I note that Matlab has
> ones based indexing, so that was really not an option for them.
>
> Chuck
> _______________________________________________
> NumPy-Discussion mailing list
> NumPy-Discussion at python.org
> https://mail.python.org/mailman/listinfo/numpy-discussion
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/numpy-discussion/attachments/20180629/82330581/attachment.html>
More information about the NumPy-Discussion
mailing list