roots(polynomial) drops zero roots if only one non-zero coeff in polynomial (Trac #323)

[numpy-gitbot]

Original ticket http://projects.scipy.org/numpy/ticket/323 on 2006-10-05 by trac user RyanGutenkunst, assigned to unknown.

roots(polynomial) drops zero roots if only one non-zero coeff in polynomial

Example (roots of x^{2} == 0)

numpy.roots((1,0,0))
array([], dtype=float64)

Also, roots fails if all coeffs are 0

numpy.roots((0,0))
Traceback (most recent call last):
File "", line 1, in ?
File "/afs/msc.cornell.edu/home/jsethna/ryang/installed/lib/python2.4/site-packages/numpy/lib/polynomial.py", line 96, in roots
trailing_zeros = len(p) - non_zero[-1] - 1
IndexError: index out of bounds

The attached patch fixes both issues. (For the all-zero polynomial, an empty array is returned. This is what matlab does.)

[numpy-gitbot]

Attachment added by trac user RyanGutenkunst on 2006-10-05: RGutenkunst_polynomial.patch

[numpy-gitbot]

trac user FabriciO wrote on 2006-10-14

Replying to [comment:1 oliphant]:
I don't agree that changeset 3265 closes ticket #921.
The modifications only solve the case when all coefficient are zero like

roots(poly1d([0,0,0])

but not when there is only one coefficient that is non-zero :

roots(poly1d([1,0,0])

[numpy-gitbot]

@teoliphant wrote on 2006-10-14

Please provide the error you are getting. This works fine for me.

roots(poly1d([1,0,0]))
array([0., 0.])

[numpy-gitbot]

trac user FabriciO wrote on 2006-10-14

Sorry, as I got numpy from bleeding-edge apt-get repository (cf http://deb-scipy.alioth.debian.org/), I was thinking that I have a version with the bug fixed.

But with the previous example, I have the following result:

>>> import numpy
>>> numpy.__version__
'1.0rc1'
>>> numpy.lib.polynomial.roots(numpy.lib.polynomial.poly1d([1,0,0]))
array([], dtype=float64)

I'll wait debian-scipy-team release...
Sorry for having reopened the ticket.