[R] nlm() problem : extra parameters

[rolf at math.unb.ca]

Here's a partial answer to your question.

The argument ``n'' is getting lost because it ``matches'' one of the
formal arguments to nlm, namely ``ndigit''.  If you change the
argument list of ``derfs4'' to ``b,x,y,size'' (and replace references
to ``n'' in the body of derfs4 by references to ``size'') then nlm()
will work (?) --- but it seems to take 337 iterations, r.t. 3 (!!!)
to converge.

No idea what the problem is.

			cheers,

				Rolf Turner

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Original message:

Mervyn G Marasinghe wrote:

> Below is a toy logistic regression problem. When I wrote my own code,
> Newton-Raphson converged in three iterations using both the gradient
> and the Hessian  and the starting values  given below. But I can't
> get nlm() to work! I would much appreciate any help.
> 
>   > x
> [1] 10.2  7.7  5.1  3.8  2.6
>   > y
> [1] 9 8 3 2 1
>   > n
> [1] 10  9  6  8 10
> 
> 
> derfs4=function(b,x,y,n)
> {
>           b0 = b[1]
>           b1 = b[2]
>           c=b0+b1*x
>           d=exp(c)
>           p=d/(1+d)
>           e=d/(1+d)^2
>           f  = -sum(log(choose(n,y))-n*log(1+d)+y*c)
>           attr(f,"gradient")=c(-sum(y-n*p),-sum(x*(y-n*p)))
>           attr(f,"hessian")=matrix(c(sum(n*e),sum(n*x*e),sum(n*x*e),sum(n*x^2*e)),2,2)
>           return(f)
> }
> 
> 
>   > nlm(derfs4,c(-3.9,.64),hessian=T,print.level=2,x=x,y=y,n=n)
> Error in choose(n, y) : argument "n" is missing, with no default
>   >
> I tried a variety of other ways  too.  When I got it to work it did not
> converge in 100 iterations ;rather like the fgh example given on the lme
> help page.