[R] nls and if statements

[DWatts]

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))

Which returns the error:
Error in inherits(object, "formula") : object 'z' not found

Is there a better way to try and allow a conditional variable to, well,
vary?

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