# Re: [R] Fit a sine to data

From: Zroutik Zroutik <zroutik_at_gmail.com>
Date: Mon, 26 May 2008 13:12:41 +0200

Thank you very much, Andy! I see I was lazy and I over-combined the problem. The idea of linearisation was pretty far from what I was thinking of, and it works :)

Happy day! Kind regards,
Milan

On Fri, May 23, 2008 at 5:19 PM, <apjaworski_at_mmm.com> wrote:

> Milan,
>
> This is a fairly standard trick. Let us generalize your equation slightly:
>
> y ~ a + c*sin(x+b)
>
> so the amplitude of the sine wave is adjustable (otherwise, you assume (or
> know) that the amplitude is 1). Then
>
> y ~ a + c*sin(b)*cos(x) + c*cos(b)*sin(x)
>
> or
>
> y ~ b0 + b1*x1 + b2*x2
>
> which is a linear regression form that you can do using the lm function.
> After you get b0, b1 and b2 you do
>
> a = b0
> b1^2 + b2^2 = c^2*(sin^2(b) + cos^2(b)) = c^2 ====> c = sqrt(b1^2 + b2^2)
> b1/b2 = tan(b) ====> b = arctan(b1/b2)
>
> Hope this helps,
>
> Andy
>
>
> __________________________________
> Andy Jaworski
> 518-1-01
> Process Laboratory
> 3M Corporate Research Laboratory
> -----
> E-mail: apjaworski_at_mmm.com
> Tel: (651) 733-6092
> Fax: (651) 736-3122
>
>
>
> "Zroutik Zroutik"
> <zroutik_at_gmail.co
> m> To
> Sent by: r-help_at_r-project.org
> r-help-bounces_at_r- cc
> project.org
> Subject
> [R] Fit a sine to data
> 05/23/2008 08:49
> AM
>
>
>
>
>
>
>
>
> 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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> and provide commented, minimal, self-contained, reproducible code.
>
>
>
>

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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. Received on Mon 26 May 2008 - 11:17:28 GMT

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