[R] bug in nls?

[Petr PIKAL]

Thank you Berwin. 

Ok, I take your point. I normally do nls modelling interactively but this 
time I was given a set of data so I tried to use an approach which I use 
quite often in lm or in plotting to pdf file. I obviously was not 
successful and there is nothing about it in documentation or at least I 
did not find it :-). 

I used Katharine's solution and will look on Gabor's too later.

Regards
Petr

Berwin A Turlach <berwin at maths.uwa.edu.au> napsal dne 26.06.2008 20:59:20:

> G'day Petr,
> 
> On Thu, 26 Jun 2008 13:57:39 +0200
> Petr PIKAL <petr.pikal at precheza.cz> wrote:
> 
> > 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.
> > 
> > [...]
> > 
> > 
> > I am not sure if this behaviour is a bug or feature.  [...]
> 
> It is definitely a feature. 
> 
> It is an error to believe that all modelling functions that use
> modelling formulae use the same syntax for their modelling formulae.
> This is, perhaps, easiest realised by observing how lm() and nls()
> interpret "*" and "/" in model formulae.
> 
> In nls(), "[..]" can be used to index parameters, if the parameter is
> allowed to change between groups in the data.  This seems to be a
> little known feature, though there is an example that uses that feature
> in MASS.  The contributed documentation "An Introduction to R: Software
> for Statistical Modelling & Computing" by Petra Kuhnert and Bill
> Venables, available from CRAN, also has such an example on pages 134
>  and 230. 
> 
> The fact that nls() allows you to use "[..]" to index parameters in the
> model formulae seems to conflict with the way you wanted to specify
> the observed values in the formula.  I guess Gabor's solution is a fix
> for your problem.
> 
> Cheers,
> 
>    Berwin
> 
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