[R] Polynomial fitting

[Matti Jokipii]

Thank you Gerrit for the quick reply! And yes, i'm Matti.

I can get the coeffs now, though i'm not sure whether i'm doing 
something wrong or whether poly is just not the right method for what 
i'm trying to find. I will look into this more closely and give it 
another try.

Is poly best for fitting on noisy data that's been generated by a 
polynomial and not that good for approximating an arbitrary function? I 
tried a least squares fitting with a web applet and got all exited 
because the approximation looked quite promising. I understand that R is 
designed mainly for statistical computing and may not be the best tool 
for my purposes. Before i look elsewhere i would like to ask if there is 
some other R method i should try, perhaps a least squares approximation?

Thank you for your help!

Matti Jokipii

08.07.2011 08:25, Gerrit Eichner kirjoitti:
> Hello, mfa (Matti?),
>
> if x and y contain the coordinates of your data points and k is the
> wanted polynomial degree, then
>
> fit <- lm( y ~ poly( x, k))
>
> fits orthonormal polynomials up to degree k to your data. Using
>
> dummy.coef( fit)
>
> should give the coefficients you are interested in.
>
> Hth -- Gerrit
>
> On Thu, 7 Jul 2011, mfa wrote:
>
>> Hello,
>>
>> i'm fairly familiar with R and use it every now and then for math related
>> tasks.
>>
>> I have a simple non polynomial function that i would like to approximate
>> with a polynomial. I already looked into poly, but was unable to
>> understand
>> what to do with it. So my problem is this. I can generate virtually any
>> number of datapoints and would like to find the coeffs a1, a2, ... up
>> to a
>> given degree for a polynomial a1x^1 + a2x^2 + ... that approximates my
>> simple function. How can i do this with R?
>>
>> Your help will be highly appreciated!
>>
>> --
>> View this message in context:
>> http://r.789695.n4.nabble.com/Polynomial-fitting-tp3652816p3652816.html
>> Sent from the R help mailing list archive at Nabble.com.
>>
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>>
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