From: Spencer Graves <spencer.graves_at_pdf.com>

Date: Wed 12 Oct 2005 - 10:54:43 EST

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Date: Wed 12 Oct 2005 - 10:54:43 EST

Have you considered writing a function to do the complex math and then calling nls referring to that function? Consider the following (not tried):

expg <- function(a., x, G0, R, T){

exp((a.[1]+(a.[2]*x)^(1/2)-G0)/(R*T))

}

mynls<-nls(formula=y~expg(a.=c(a, b), x=x, G0=G0, R=R, T=T),

data=mydata,...)

If "nls" stops prematurely, I then write another function, "SSE" to compute the sum of squares of deviations from y and then ask "optim" to minimize "SSE", using "hessian=TRUE". If you try this and have trouble making it work, please send another post.

spencer graves

Simple wrote:

> Thanks for your kind respond. Although the answer didn't solve my question

*> clearly,maybe I still not understand the art of R.
**>
**> I'm sorry that I had not talked the problem clearly, maybe a example with more
**> detail will be suitable as suggested in the the posting guide.
**>
**> In function fitting program, such as Sigmaplot, a fitting formula, can be
**> write in separate form:
**> G=a+(b*x)^(1/2)
**> k=exp((G-G0)/(R*T))
**> fit k to y
**>
**> of course,in R's nls, can write as:
**> mynls<-nls(formula=y~exp((a+(b*x)^(1/2)-G0)/(R*T)),data=mydata,...)
**>
**> In this example, the formula is simple and acceptable. However, when the
**> formula is more complexity,writing all in one formula,the readability will be
**> damaged.So I'm looking for a way to write simple and readable code in this
**> situation.
**>
**>
**> Spencer Graves wrote:
**>
*

>> I'm not certain what you are asking. >> >> You can build expressions in R as character strings and then execute >>them. Example: >> >>expr <- paste("two <-", 1, "+", 1) >>eval(parse(text=expr)) >>two >> >> If this does not answer your question, PLEASE do read the posting >>guide, "www.R-project.org/posting-guide.html". It can help increase the >>chances of a quick and useful reply. >> >> spencer graves >> >>Simple wrote: >> >>>hi, >>>I'm an newbie for R,I want do some fitting in R. >>> >>>I wander if it is possible to write a few of equations but only one >>>formual when fitting >>> >>>Currently,My problem is,in R, is there methods combination a few >>>equations into one formual? >>>For example, >>>y=f1(k); >>>k=f2(t); >>>t=f3(x); >>>although it is certain that the can be equations turn into one formual as >>>y~f(x),but write such a complexity string make me painful. >>> >>>I have searched the web and found out there were only examples with one >>>formual.any suggestion? >>> >>>I hope that I have omit something.

> R-help@stat.math.ethz.ch mailing list

-- Spencer Graves, PhD Senior Development Engineer PDF Solutions, Inc. 333 West San Carlos Street Suite 700 San Jose, CA 95110, USA spencer.graves@pdf.com www.pdf.com <http://www.pdf.com> Tel: 408-938-4420 Fax: 408-280-7915 ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.htmlReceived on Wed Oct 12 11:08:56 2005

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