As has already been pointed out, the syntax of formulae in nls() is not
the same as in linear models. So things which are valid for linear models
are not necessarily valid for others.
Since nls() is sparsely documented on the help page, you need to look at
the references (and you can also learn a lot of tricks from Bill Venables'
account in MASS).
On Fri, 27 Jun 2008, Petr PIKAL wrote:
Thank you both.
Katharine approach seemed to me easier to implement so I used it
successfully. However I still wonder why in linear model using lm I can
use
for (i in ....) lm( data[,i]~data[,1])
but in nls the same construction fails. I understand that it is sometimes
difficult to set correct starting values especially if the values can by
quite different and then the cycle fails.
However it works in lm and probably in other regression models (I tested
it in ltsreg from MASS)
Petr
[EMAIL PROTECTED]
724008364, 581252140, 581252257
[EMAIL PROTECTED] napsal dne 26.06.2008 14:16:35:
Dear Petr,
I think it's a feature. the formula interface also won't let you
specify
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
offending
subject.
I just encountered a strange problem with nls formula. I tried to use
nls
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
and
varIndex is not correctly specified.
I am not sure if this behaviour is a bug or feature. If it is a
feature,
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
[EMAIL PROTECTED]
724008364, 581252140, 581252257
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--
Brian D. Ripley, [EMAIL PROTECTED]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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