[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

`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`


On Fri, 29 Jun 2018 at 20:10 Charles R Harris <charlesr.harris at gmail.com>

> 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
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