making self-starting function for nls

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Hello.  Following pages 342-347 of Pinheiro & Bates, I am trying to
write a self-starting nonlinear function (a non-rectagular hyperbola) to
be used in nonlinear least squares regression (and eventually for a
mixed model).  When I use the getInitial function for my self-starting
function I get the following error message:
getInitial(photo~NRhyperbola(Irr,theta,Am,alpha,Rd),dat)
Error in tapply(y, x, mean, na.rm = TRUE) :

        arguments must have same length

Since I do not explicitly call tapply in my function that makes
NRhyperbola a self-starting function (called NRhyperbolaInit, see
below), I assume that the error is coming from within the mCall function
but I can't figure out where or how.
My guess is that it is occuring in the call to sortedXyData but I
won't be able to tell for sure without test data.

One of the things that sortedXyData does is to average the y values
for replicated x values.  It seems that in the call the lengths of the
x and y arguments are different.

Would someone who has successfully done this be willing to look at my
code and see where the problem arises?

NRhyperbolaInit
function(mCall,LHS,data)

{

xy<-sortedXyData(mCall[["x"]],LHS,data)

if(nrow(xy)<3){

 stop("Too few unique irradiance values")

}

theta<-0.75

Rd<-min(xy[,"y"])

Am<-max(xy[,"y"]) + abs(Rd)

if(sum(xy[,"x"]<50)>3)alpha<-coef(lm(y~x,data=xy,subset=x<50))[2]

if(sum(xy[,"x"]<50)<=3)alpha<-0.07

value<-c(theta,Am,alpha,Rd)

names(value)<-mCall[c("theta","Am","alpha","Rd")]

value

}

Bill Shipley

Bill.Shipley at USherbrooke.ca

 <http://callisto.si.usherb.ca:8080/bshipley/>
http://callisto.si.usherb.ca:8080/bshipley/

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