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))
and provide commented, minimal, self-contained, reproducible code.
Untested in absence of data.
David Winsemius, MD West Hartford, CT