[R] manipulating text to generate different formulas to use in nls()

[bartjoosen]

Take a look at ?eval

Bart


Héctor Villalobos-2 wrote:
> 
> Hello,
> 
> In doing a series of non-linear estimations of a function which is a sum
> of a varying number 
> of sinusoids, I would like to "autogenerate" the arguments needed by nls()
> depending on that 
> number.
> 
> For example, when there are two sinusoids: 
> 
>   > nls( y ~ mu + A1 * cos(2*pi*f1*x - P1) + A2 * cos(2*pi*f2*x - P2),
> data = some.xy.data, 
>             start = list( mu=some.value0, A1=some.value1, P1=some.value2,
> A2=some.value3,
>                          P2=some.value4, f1=some.value5, f2=some.value6  ) 
> )
> 
>  Adding a third sinusoid, the model and the starting parameters needed
> become:
> 
>   > nls( y ~ mu + A1 * cos(2*pi*f1*x - P1) + A2 * cos(2*pi*f2*x - P2) + A3
> * cos(2*pi*f3*x - P3), 
>             data = some.xy.data, start = list( mu=some.value0,
> A1=some.value1,
>             P1=some.value2, A2=some.value3, P2=some.value4,
> A3=some.value5,
>             P3=some.value6, f1=some.value7, f2=some.value8, f3=some.value9
> )
> 
> 
> So far, I managed to create the "building blocks" I need but when it comes
> to defining their 
> number I still haven't found a solution (see the following code).
> 
> ##  some fake starting values
> par.ini <- data.frame(period=1:5, invR=1:5, a0=1:5, Amplitude=1:5,
> Phase=1:5)
> 
>   n <- nrow(par.ini) - 1                        # I'm not interested in
> the last harmonic
>   inip <- par.ini[1:n, c(1, 3:5)]             # neither in the second
> column
>     amps <- paste("A", 1:n, sep="")
>     phas <- paste("P", 1:n, sep="")
>     freqs <- paste("f", 1:n, sep="")
> 
>  ( premodl <- paste(amps, " * cos(2*pi*", freqs, "*x - ", phas, ")",
> sep="") )
> 
> 
> ## In the two following instructions, "modl" and "initP" are defined by
> hand. This is what I 
> want to generate "automagically" depending on the number of harmonics (n).
> 
>  ( modl <- paste("y ~ mu", premodl[1],premodl[2], premodl[3], premodl[4],
> sep=" + ") ) 
> 
>  ( initP <- list(mu=sum(inip$a0), 
>           A1=inip$Amplitude[1], P1=inip$Phase[1], 
>           A2=inip$Amplitude[2], P2=inip$Phase[2], 
>           A3=inip$Amplitude[3], P3=inip$Phase[3],           
>           A4=inip$Amplitude[4], P4=inip$Phase[4], 
>           f1=1/inip$period[1], f2=1/inip$period[2],
>           f3=1/inip$period[3], f4=1/inip$period[4])  ) 
> 
> ## not run. These two will be used in
>   res <- nls(modl, data=data.frame(y, x), start=initP)
>  
> 
> 
> Thank you in advance for any insigth.
> 
> Héctor
> 
> 
>  
> -- 
> Héctor Villalobos <hvillalo at ipn.mx> 
>  CICIMAR - IPN
>  A.P. 592. Col. Centro 
>  La Paz, Baja California Sur, MÉXICO. 23000
>  Tels. (+52 612) 122 53 44; 123 46 58; 123 47 34  ext. 82425
>  Fax.  (+52 612) 122 53 22
> 
> ______________________________________________
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> 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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