[R] Fit a sine to data

[Peter Dalgaard]

Juan Manuel Barreneche wrote:
> i guess nls or gnls should work....
>   
Yes. Notice though that this is really a linear problem because of the
trigonometric identity

sin(x+b) = cos(b) * sin(x) + sin(b) * cos(x)

which means that you can reparametrize c*sin(x+b) (you forgot the
multiplier, did you not?) as

A*sin(x)+B*cos(x)

It is easy to invert the reparametrization if you notice that A^2 + B^2
= c^2, but it may be expedient to use the nonlinear functions to get
confidence itervals for a,b,c.
> JM
>
> El Viernes, 23 de Mayo de 2008 10:47, Zroutik Zroutik escribió:
>   
>> Dear R-users,
>>
>> I'd like to fit a sine function to my data. The result should have a format
>> (and thus the formula, too)
>>
>> y ~ a + sin(x+b)
>>
>> where y and x are vectors, and a and b are (yet) unknown values.
>>
>> The data sets (vectors x and y) are OK, and I can do a simple lm fitting
>> lm(y~x), or lm(y~I(sin(2*pi*x/360))), succesfully  My issue is that I'm not
>> able to do the optional linear shift in x (e.g. x+b). What other
>> possibilities should I use instead of lm? I looked searched the R-seek for
>> terms: fit, sine, lm, and combinations, but I could not find anything
>> useful. Anybody would be so kind and point me to the right direction where
>> I can find other fitting possibilities?
>>
>> Thank you for listening and kind answers upfront.
>>
>> Regards,
>> Milan
>>
>> 	[[alternative HTML version deleted]]
>>
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>>     
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>   


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