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Re: polyfit weirdness

 From: Henry F. Mollet Subject: Re: polyfit weirdness Date: Thu, 13 Feb 2003 14:49:27 -0800 User-agent: Microsoft-Entourage/10.1.1.2418

```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 address@hidden 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

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