[R] How can I fitting this function from values.

[Horacio Castellini]

Thanks for the help received, I will test the proposal.

> R is case sensitive ;-) so in your list of start values you must have B=3 and not b=3. Perhaps you have an object named b of a different length than your data somewhere giving rise to your error message.
>
> Also brute force wont work as Joerg point out. We have
>
> A*1/(1+t/B)*1/sqrt(1+t/C) = D/(B+t)/sqrt(C+t),
>
> where D = A*B*sqrt(C).
>
> We notice that we need B > -t and C > -t.
>
>
> Now one can use the "port" algorithme of nls() with constraints on parameters:
>
>
> foo <- function(t,ccc){
>   D <- ccc[1]
>   B <- ccc[2]
>   C <- ccc[3]
>   D/(B+t)/sqrt(C+t)
> }
>
> ttt <- seq(1,10,length=100)
> yyy <- foo(ttt,c(5*3*sqrt(2),3,2))
> set.seed(42)
>
> dat.test <- data.frame(t=ttt,y=yyy+rnorm(100,0,.1))
>
>
> fit.test <- nls(y~D/(B+t)/sqrt(C+t),data=dat.test,
>                 start=c(D=5*3*sqrt(2),B=3,C=2),
>                     lower=c(-Inf,-1,-1),alg="port")
> # -Inf could be 0, -1 could be -min(dat.test$t)
>
> with(dat.test,plot(t,y))
> with(dat.test,lines(t,foo(t,c(21.21,3,2)),col="green")) # Looks cool.
> lines(ttt,fitted(fit.test),col="red")
>
>
> Best regards
>
> Frede Aakmann Tøgersen
> Scientist
>
>
> UNIVERSITY OF AARHUS
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>
> > -----Oprindelig meddelelse-----
> > Fra: r-help-bounces en r-project.org
> > [mailto:r-help-bounces en r-project.org] På vegne af Rolf Turner
> > Sendt: 12. november 2007 21:16
> > Til: Horacio Castellini
> > Cc: r-help en r-project.org
> > Emne: Re: [R] How can I fitting this function from values.
> >
> >
> > On 13/11/2007, at 8:27 AM, Horacio Castellini wrote:
> >
> > > Hi all, I'd like fit this function:
> > >
> > > G(t)=A*1/(1+t/B)*1/sqrt(1+t/C)
> > >
> > > where A, B and C are fitting constants that one like search. I have
> > > got a fcs<-(t,G(t)) value table in a data frame.
> > >
> > > Any one can help me? Tahnks Horacio.
> >
> >
> > I ***thought*** that nls would solve your problem in a trice.
> > But I tried a toy example before replying to you, and it messed up
> > mightily:
> >
> > foo <- function(t,ccc){
> > ccc[1]*1/(1+t/ccc[2])*1/sqrt(1+1/ccc[3])
> > }
> > ttt <- seq(1,10,length=100)
> > yyy <- foo(ttt,c(5,3,2))
> > set.seed(42)
> > dat.test <- data.frame(t=ttt,y=yyy+rnorm(100,0,0.1))
> > with(dat.test,plot(t,y))
> > with(dat.test,lines(t,foo(t,c(5,3,2)))) # Looks cool.
> > fit.test <- nls(y~A*1/(1+t/B)*1/sqrt(1+t/C),start=list
> > (A=5,b=3,C=2),data=dat.test)
> > # Gives error; no idea what the implications are.
> >
> > The error message:
> >
> > Error in `[[<-.data.frame`(`*tmp*`, var, value =
> > c(0.70934153524875, 0.761288405463172,  :
> >    replacement has 65 rows, data has 100 In addition: Warning message:
> > In t/B : longer object length is not a multiple of shorter
> > object length
> >
> > I don't think I have *ever* had a good experience with nls();
> > it is always pretty flaky.
> > (Well, maybe it worked for me once ..., can't remember for sure. :-) )
> >
> >               cheers,
> >
> >                       Rolf Turner
> >
> >
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