pylab, matplotlib ... roots function question

[=?ISO-8859-1?Q?Sch=FCle_Daniel?=]

Hello NG,

given this call to roots funtion from pylab

In [342]: roots([0,2,2])
Out[342]: array([-1.])

as far as I understand it [a0,a1,a2] stands for a0+a1*x+a2*x^2
in the above case it yields 2x^2+2x = 2x(1+x)
and the roots are 0 and -1
I am wondering why roots function gives me only the -1

second try

In [343]: roots([1,0,0])
Out[343]: array([], dtype=float64)

ok, as it should be

In [344]: roots([0,0,1])
Out[344]: array([], dtype=float64)

here again, 0 is the root of x^2

Do I miss something important?


Regards, Daniel

 

[Guest]

Hi,

[...]

No, that's actually wrong. What version of numpy are you using? With a recent
SVN checkout of numpy, I get the correct answer:

In [3]: roots([1,0,0])
Out[3]: array([ 0., 0.])

In [17]: import sys, numpy

In [18]: sys.version
Out[18]: '2.5 (r25:51908, Sep 23 2006, 01:23:14) \n[GCC 4.1.1]'

In [19]: numpy.version.version
Out[19]: '1.0rc1'


moon:/pool/PROG/python # uname -a
Linux moon 2.6.16.13-4-smp #1 SMP Wed May 3 04:53:23 UTC 2006 x86_64
x86_64 x86_64 GNU/Linux

I think I will get and compile newer version of source

BTW, I also look for good (more or less complete and/or interessting)
tutoruals on signal processing with python
something like low/high-pass filtering/ploting of wave files etc
I would appreciate pointers very much.

I am learning this at the moment, when it all starts to make sense to
me, I will write such a tutorial on my own later

Regards, Daniel

 

[Robert Kern]

Hello NG,

given this call to roots funtion from pylab

It's actually from numpy and numpy questions are best asked (and best answered!)
on numpy-discussion.

http://www.scipy.org/Mailing_Lists

In [342]: roots([0,2,2])
Out[342]: array([-1.])

as far as I understand it [a0,a1,a2] stands for a0+a1*x+a2*x^2
in the above case it yields 2x^2+2x = 2x(1+x)
and the roots are 0 and -1
I am wondering why roots function gives me only the -1

No, it's the other way around.

In [1]: from numpy import *

In [2]: roots?
Type: function
Base Class: <type 'function'>
Namespace: Interactive
File:
/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/numpy-1.0.2.dev3507-py2.5-macosx-10.4-i386.egg/numpy/lib/polynomial.py
Definition: roots(p)
Docstring:
Return the roots of the polynomial coefficients in p.

The values in the rank-1 array p are coefficients of a polynomial.
If the length of p is n+1 then the polynomial is
p[0] * x**n + p[1] * x**(n-1) + ... + p[n-1]*x + p[n]


So you were really solving 2*x + 2 = 0, the single root of which is -1.

second try

In [343]: roots([1,0,0])
Out[343]: array([], dtype=float64)

ok, as it should be

No, that's actually wrong. What version of numpy are you using? With a recent
SVN checkout of numpy, I get the correct answer:

In [3]: roots([1,0,0])
Out[3]: array([ 0., 0.])

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco