[R] Trouble with optim on a specific problem

[Stu @ AGS]

Thanks for your response!
No, my basic equation does not use matrices at all.  It takes scalar values and returns a scalar.
What I am trying to accomplish is to find the "best-fit" coefficients to the equation as follows:
y ~ c1 * x1 * x2^c2 * x3^c3
where y, x1, x2, and x3 are observed data and c1, c2, and c3 are regression coefficients.

I thought the correct way to use optim() to find the coefficients was to put the data observations into a data.frame object, then call optim()

Maybe I am not using optim() correctly?

My observed data are (as vectors where each row of observations are located at the same index):
y <- c(0.111111111111111, 0.0921052631578947, 0.0564516129032258, 0.0608108108108108, 0.0128205128205128, 0.0136078431372549)
x3 <- c(3600, 169200, 185400, 255600, 271800, 342000)
x2 <- c(1, 3, 4, 5, 6, 7)
x1 <- c(0, 0.978723404255319, 0.087378640776699, 0.549295774647887, 0.0596026490066225, 0.61578947368421)

Thank you again,
Stu

> -----Original Message-----
> From: Liviu Andronic [mailto:landronimirc at gmail.com]
> Sent: Tuesday, June 16, 2009 2:35 AM
> To: Stu @ AGS
> Cc: r-help at r-project.org
> Subject: Re: [R] Trouble with optim on a specific problem
> 
> Hello,
> 
> On 6/16/09, Stu @ AGS <stu at agstechnet.com> wrote:
> >  Error in optim(c(0.66, 0.999, 0.064), pe, NULL, method = "L-BFGS-B")
> :
> >
> >   objective function in optim evaluates to length 6 not 1
> >
> <skip>
> >
> >  > pe <- function(c) c[1]*x1*x2^c[2]*x3^c[3]
> >
> I would suspect a matrix multiplication issue. In order to minimise
> your function optim expects the result of your specific function to be
> a scalar. Are you sure that after all the multiplications pe results
> in a vector of dimension 1x1?
> Liviu