[R] Different result from nls in R-2.2.1 and R-2.3.1

[Prof Brian Ripley]

The way nls looks for variables in its formula is very unusual.  For 
non-parameters it has

     for(var in varNames[!varIndex])
         mf[[var]] <- eval(as.name(var), data)

inside nls, and that means that the scoping rules are first 'data' then 
the body of nls().  That's surely not what one would expect here: 
fitting functions using model.frame look in 'data' and then the 
environment of the formula (if non-null) or the parent frame.

So to avoid surprises, put fixed values in 'data'.  (Your example did not 
work in 2.2.1 when run from a function.)

I'll try to find a bug fix for 2.4.0.


On Thu, 21 Sep 2006, Frede Aakmann Tøgersen wrote:

>
>
> Short story: January 2006 I did some analysis in R-2.2.1 using nls. Repeating the exercise in R-2.3.1 yesterday produced somewhat different results.
>
> After some debugging I found that either nls is the problem or that mine understanding of environments or scoping rules is lacking something.
>
> This is a short reproducing example.
>
>
>
> x <- seq(0,5,len=20)
>
> n <- 1
> y <- 2*x^2 + n + rnorm(x)
>
> xy <- data.frame(x=x,y=y)
>
> myf <- function(x,a,b,n){
>  res <- a*x^b + n
>  ## a print for debugging purpose
>  print(n)
>  res
> }
>
> ## This works as I expect it to do in R-2.2.1 but doesn't work in R-2.3.1.
> ## n is somehow sat to nrow(xy) inside nls()
> ## Note that x and y is defined in the dataframe xy, whereas n is found in the global environment.
> fit <- nls(y ~ myf(x,a,b,n), data=xy, start=c(a=1,b=1), trace=TRUE)
>
> ## this works in both versions
> ## x,y,n found in the .GlobalEnv
> fit <- nls(y ~ myf(x,a,b,n), start=c(a=1,b=1), trace=TRUE)
>
> ## this works in both versions.
> ## x, y, n found in dataframe xyn
> xyn <- data.frame(xy,n=n)
> fit <- nls(y ~ myf(x,a,b,n), data=xyn, start=c(a=1,b=1), trace=TRUE)
>
> ## this works in both versions
> ## Now using the variable .n instead of n
> ## .n is found in .GlobaEnv
> .n <- 1
> fit <- nls(y ~ myf(x,a,b,.n), data=xy, start=c(a=1,b=1), trace=TRUE)
>
>
> In my real case and the example above, I do have three or more parameters of which fitting is done only on few of theme. Is this a problem? Or should I ask, why is this a problem in R-2.3.1 but not in R-2.2.1?
>
> Is my problem related to this difference between lines of code from nls:
>
> R-2.2.1:     mf <- as.list(eval(mf, parent.frame()))
>
> R-2.3.1:     mf <- eval.parent(mf)
>             n <- nrow(mf)
>             mf <- as.list(mf)
>
> where n is being defined in the scope of nls in the latest version?
>
> Best regards
>
> Frede Aakmann Tøgersen
>
>
>
> Danish Institute of Agricultural Sciences
> Research Centre Foulum
> Dept. of Genetics and Biotechnology
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> DK-8830 Tjele
>
> Phone:   +45 8999 1900
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>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
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)
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