R help - Apr 2008 - for loop in nls function

Hello,
I am trying to apply a least squares non-linear regression to my  
dataset, dem16. I can apply it to a subset based on a single site  
fine, but I want to apply it to each (of 197) sites. I am stumbling  
over a "for" loop. Site and Mean_Air are variables in my dataset and  
I would like to estimate alpha, beta, gamma, mu and Tw for each site.  
I have over 50,000 observations in my dataset.


results <- c()
for (i in Site) {
results[i]<- nls(Tw ~ mu + ((alpha - mu)/(1 + exp(gamma*(B -  
Mean_Air)))),
	data = dem16,
	start = list(mu = 0.0001, alpha = 21.8, gamma = 0.22, B = 12.8))
	}

Thank you in advance,
Ingrid

> results[i]<- nls(Tw ~ mu + ((alpha - mu)/(1 + exp(gamma*(B -  
> Mean_Air)))),
> 	data = dem16,
> 	start = list(mu = 0.0001, alpha = 21.8, gamma = 0.22, B = 12.8))
> 	}
If you have a variable that codes "site" then you can try something
like this to get the parameters over sites.  Pinheiro and Bates (2000)
has several examples. 

nlsList(Tw ~ mu + ((alpha - mu)/(1 + exp(gamma*(B - Mean_Air)))) | site)

Yuelin.

 
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