[R] need suggestion about building formual

[Spencer Graves]

	  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.
> 
> 
> 
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-- 
Spencer Graves, PhD
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