bug in nls?

Dear Petr,

I think it's a feature.  the formula interface also won't let you 

the slots of S4 objects in the model spec.

How about

coef(nls(y~a*x^b, data=list(x=DF[,1], y=DF[,2]), start=list(a=3, b=.7)))

?

On Thu, 26 Jun 2008, Petr PIKAL wrote:

Dear all

Nobody responded to my previous post so far so I try with more 

subject.

I just encountered a strange problem with nls formula. I tried to use 

in cycle but I was not successful. I traced the problem to some parse
command.

Here is an example

DF<-data.frame(x=1:10, y=3*(1:10)^.5+rnorm(10))

coef(lm(log(DF[,2])~log(DF[,1])))
 (Intercept) log(DF[, 1])
   0.7437320    0.6831726
# this works

coef(nls(y~a*x^b, data=DF, start=list(a=3, b=.7)))
        a         b
2.6412881 0.5545907
# OK, this works too

coef(nls(DF[,2]~a*DF[,1]^b, data=DF, start=list(a=3, b=.7)))
Error in parse(text = x) : unexpected end of input in "~ "
coef(nls(DF[,2]~a*DF[,1]^b, start=list(a=3, b=.7)))
Error in parse(text = x) : unexpected end of input in "~ "
# but this does not


Browse[1]>
debug: mf$formula <- as.formula(paste("~", paste(varNames[varIndex],
    collapse = "+")), env = environment(formula))
Browse[1]>
Error in parse(text = x) : unexpected end of input in "~ "

Actually the problem is that with calling nls with DF[,n]~... varNames 

varIndex is not correctly specified.

I am not sure if this behaviour is a bug or feature. If it is a 

please help me how to call variables from data frame
when using nls inside cycle

for (i in ....) result[i,] <- coef(nls( ...., ))

Thank you
Petr

sessionInfo()

R version 2.8.0 Under development (unstable) (2008-05-18 r45723)
i386-pc-mingw32

locale:
LC_COLLATE=Czech_Czech Republic.1250;LC_CTYPE=Czech_Czech
Republic.1250;LC_MONETARY=Czech_Czech
Republic.1250;LC_NUMERIC=C;LC_TIME=Czech_Czech Republic.1250

attached base packages:
[1] stats     grDevices datasets  utils     graphics  methods   base

other attached packages:
[1] nlme_3.1-88    lattice_0.17-7 fun_0.1

loaded via a namespace (and not attached):
[1] grid_2.8.0  tools_2.8.0


Petr Pikal
petr.pikal at precheza.cz
724008364, 581252140, 581252257