Re: [Matplotlib-users] Polyfit

[Jean-Baptiste C. <Jea...@de...>]

Thanks to both of you. It worked just fine

I will push my luck and ask if any of you knows of a module to fit a piecew=
ise polynomial to a list of (X,Y) points.
something like=20
p=3Dpiece-wiseFit([1,2,5,7,8],[3,4,2,5,5],2)=20
would return [[A0,B0,C0],[A1,B1,C1}[A2,B2,C2},[A3,B3,C3]}, coefficients for=
 the 4 polynoms=20
A0+B0.X+C0.X.X
A1+B1.X+C1.X.X
A2+B2.X+C2.X.X
A3+B2.X+C3.X.X

This is a classic and I expect the code to be written somewhere, eventhough=
 I could not find it even when I "Feel lucky" with Google.


Takk

Kve=F0ja

Jean-Baptiste




On Fri, 19 Mar 2004 09:45:10 +1000
"Gary Ruben" <ga...@em...> wrote:

> Hi Jean-Baptiste,
> Your problem is that polyfit (and polyval) expect Numeric array arguments=
, so you need to do:
>=20
> >>> x=3Darray([1,2,3])
> >>> y=3Darray([1,2,1])
> >>> p=3Dpolyfit(x,y,2)
> >>> p
> array([-1.,  4., -2.])
> >>> polyval(p,array([1,2,3]))
> array([ 1.,  2.,  1.])
>=20
> Note that your example only has 3 points, so can fit a 2nd order polynomi=
al exactly as I've done.
> If you try to fit a 3rd order, the method polyfit is using seems to fail.=
 ie.
> >>> p=3Dpolyfit(x,y,3)
> >>> p
> array([ 1.09375, -5.0625 ,  7.125  , -2.875  ])
> >>> polyval(p,array([1,2,3]))
> array([ 0.28125, -0.125  ,  2.46875])
>=20
> ideally p would have been array([0., -1.,  4., -2.]) so you'll have to be=
 careful.
>=20
> Gary
>=20
> ----- Original Message -----
> From: Jean-Baptiste Cazier <Jea...@de...>
> Date: Thu, 18 Mar 2004 14:36:45 +0000
> To: mat...@li...
> Subject: [Matplotlib-users] Polyfit
>=20
> > S=E6ll=20
> >=20
> > One mroe question from me today :)
> >=20
> > I have some trouble running polyfit with matplotlib-0.52
> > Can you please help me finding out what I do wrong=20
> > I give a list of x values and y values as well as the degree of the des=
ired polynome. But all I get is the following error
> >=20
> > >>> x=3D[1,2,3]
> > >>> y=3D[1,2,1]
> > >>> polyfit(x,y,3)
> > Traceback (most recent call last):
> >   File "<<console>>", line 1, in ?
> >   File "/usr/lib/python2.2/site-packages/matplotlib/mlab.py", line 341,=
 in polyfit
> >     X =3D Matrix(vander(x, N+1))
> >   File "/usr/lib/python2.2/site-packages/matplotlib/mlab.py", line 383,=
 in vander
> >     X =3D ones( (len(x),N), x.typecode())
> > AttributeError: 'list' object has no attribute 'typecode'
> >=20
> >=20
> > Any idea ?
> >=20
> > Thanks
> >=20
> > Jean-Baptiste
> >=20
> > --=20
> > -----------------------------
> > Jea...@de...
> >=20
> > Department of Statistics
> > deCODE genetics     Sturlugata,8
> > 570 2993          101 Reykjav=EDk
> >=20
> >=20
> >=20
> > -------------------------------------------------------
> > This SF.Net email is sponsored by: IBM Linux Tutorials
> > Free Linux tutorial presented by Daniel Robbins, President and CEO of
> > GenToo technologies. Learn everything from fundamentals to system
> > administration.http://ads.osdn.com/?ad_id=1470&alloc_id638&op=3Dclick
> > _______________________________________________
> > Matplotlib-users mailing list
> > Mat...@li...
> > https://lists.sourceforge.net/lists/listinfo/matplotlib-users
> >=20
>=20
> --=20
> ___________________________________________________________
> Sign-up for Ads Free at Mail.com
> http://promo.mail.com/adsfreejump.htm
>=20


--=20
-----------------------------
Jea...@de...

Department of Statistics
deCODE genetics     Sturlugata,8
570 2993          101 Reykjav=EDk