[R] fit nonlinear model with groups

[Bill Simpson]

I wanted to fit a nonlinear model using nls to two groups. Here is a toy
example.

library(nlme)
x<-rep(1:5,2)
cond<-c(rep("a",5),rep("b",5))
cond<-as.factor(cond)
y[cond=="a"]<-1/x +2
y[cond=="b"]<-2/x+3
df<-data.frame(x,y,cond)
fit<-nls(y~c(a1,b1)[cond]/x+c(a2,b2)[cond],data=df,
start=list(a1=1,b1=2,a2=2,b2=3))

I based above on an example posted by Bill Venables.

I get message:
maximum number of iterations exceeded
even if I reset maxiter using nls.control and even though I start the
params at the right place.

This example seems to work OK if I do
fit<-nls(y~c(a1,b1)[cond]/x+a2,data=df,
start=list(a1=1,b1=2,a2=2))
So maybe the trick using c()[cond] can be used only once per formula.

How to do it? Thanks for any help.

Bill

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