From help-octave-request at bevo dot che dot wisc dot edu Thu Feb 13 16:47:44 2003
Subject: Re: polyfit weirdness
From: "Henry F. Mollet" <mollet at pacbell dot net>
To: Miquel dot Cabanas at uab dot es, Scott Lamb <slamb@slamb.org>
Cc: Octave_post <help-octave at bevo dot che dot wisc dot edu>
Date: Thu, 13 Feb 2003 14:49:27 -0800

Your commands produced exactly what Miguel calculated a little differently.
I'm using GNU Octave, version 2.1.35 (powerpc-apple-darwin6.2).
But please tell me if I'm fitting a polynomial of order 1 (= straight line)
thru the data and that my intercept (-1.49) and slope (4.2-15) appear in the
wrong order?
Henry

octave:1> l1 = 1e-9 * [ 578.45 545.88 435.87 404.71 365.26 ];
octave:2> v1 =        [   0.66   0.79   1.40   1.58   1.92 ];
octave:3> f1 = 299792458 ./ l1;
octave:4> [p1,cv1] = polyfit(f1,v1,1)

p1 =

    4.1679e-15
   -1.4948e+00

cv1 =

  0.66528  0.79417  1.37189  1.59260  1.92606




on 2/12/03 4:57 AM, Miquel Cabanas at Miquel dot Cabanas at uab dot es wrote:

> hi,
> 
> On Tue, Feb 11, 2003 at 02:08:37AM -0600, Scott Lamb wrote:
>> 
>> So first I'm calling polyfit to get the trendline:
>> 
>>     l1 = 1e-9 * [ 578.45 545.88 435.87 404.71 365.26 ]
>>     v1 =        [   0.66   0.79   1.40   1.58   1.92 ]
>>     f1 = 299792458 ./ l1
>>     [p1,cv1] = polyfit(f1,v1,1)
>> 
>> ...and it doesn't return what I'd expect at all:
> 
> I think we can narrow the problem to be related to the
> left-division in polyfit, could you try running (this is
> what polyfit does),
> 
>  n = 1;
>  l = 5;
> 
>  x = [ 5.1827e+14  5.4919e+14  6.8780e+14  7.4076e+14 ...
>  8.2076e+14 ]';
> 
>  y = [ 0.66000  0.79000  1.40000  1.58000  1.92000 ]';
> 
>  X = (x * ones (1, n+1)) .^ (ones (l, 1) * (0 : n));
>  p = X \ y;
>  yf = X * p;
> 
> In my working Octave I get
> 
>  p =
> 
>     -1.4948e+00
>      4.1680e-15
>      
> and (estimated y-values)
> 
>  yf =
> 
>    0.66529
>    0.79416
>    1.37188
>    1.59262
>    1.92605
> 
> Also, what are eps, realmax and realmin in your versions? Again,
> mines are,
> 
>  eps =  2.2204e-16
>  realmax =  1.7977e+308
>  realmin =  2.2251e-308
> 
> 
> Miquel



-------------------------------------------------------------
Octave is freely available under the terms of the GNU GPL.

Octave's home on the web:  http://www.octave.org
How to fund new projects:  http://www.octave.org/funding.html
Subscription information:  http://www.octave.org/archive.html
-------------------------------------------------------------