[R] nls and if statements

[David Winsemius]

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))

> and provide commented, minimal, self-contained, reproducible code.

Untested in absence of data.

-- 

David Winsemius, MD
West Hartford, CT