nls and if statements

On May 17, 2012, at 10:06 AM, DWatts wrote:

Hi All,

I have a situation where I want an 'if' variable to be  
parameterized. It's
entirely possible that the way I'm trying to do this is wrong,  
especially
given the error message I get that indicates I can't do this using  
an 'if'
statement.

Essentially, I have data where I think a relationship enters when a  
variable
(here Pwd) is below some value (z). I don't know that value, so I  
want to
fit it. The data is Pw, Tsoil, and Cfl.

nlstest=nls(if(Pw>z){Cfl~K*exp(-b*Pw)+c
	}else{
	Cfl~K*(exp(-a*Tsoil))*exp(-b*Pw)+c
	},start=c(K=5.5, a=0.1, b=0.1, c=2, z=5))

Perhaps recasting as Boolean equivalent:

nlstest== <- nls({Cfl~ (Pw>z)*K*exp(-b*Pw)+c) +

                      !(Pw>z)*(K*(exp(-a*Tsoil))*exp(-b*Pw)+c)},
            start=c(K=5.5, a=0.1, b=0.1, c=2, z=5))

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