R help - Sep 2012 - optim and "the function should not" advice

Dear R users, 

I'm using optim to optimize a pretty complicated function. This function
takes the parameter vector "theta" and within its body I use
instructions like

sigma<-theta[a:b]; computations with sigma... 
out<-c() 
for (i in 1:d){ 
a<-theta[(3*d+i):c] 
out[i]<-evaluation of an expression involving 'a' (I use symbolic
differentiation)
} 

Unfortunately for certain problems 'optim' returns a parameter vector
which didn't move at all from the initial parameters, and the output says
that although the function has been evaluated a high number of times, the
gradient (which I fed the function with) has been evaluated only one time. I
used the BFGS method.

By chance I looked at the help and I read "The parameter vector passed to
fn has special semantics and may be shared between calls: the function should
not change or copy it" . Could the instructions above be the cause of the
failure? If so, how to deal with symbolic differentation?

Thanks in advance, 
Gildas 



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On 20/09/2012 09:24, Gildas Mazo wrote:
> Dear R users,
>
> I'm using optim to optimize a pretty complicated function. This
function takes the parameter vector "theta" and within its body I use
instructions like
>
> sigma<-theta[a:b]; computations with sigma...
> out<-c()
> for (i in 1:d){
> a<-theta[(3*d+i):c]
> out[i]<-evaluation of an expression involving 'a' (I use
symbolic differentiation)
> }
>
> Unfortunately for certain problems 'optim' returns a parameter
vector which didn't move at all from the initial parameters, and the output
says that although the function has been evaluated a high number of times, the
gradient (which I fed the function with) has been evaluated only one time. I
used the BFGS method.
On face value that means it is unable to find a small step that goes 
downhill consistent with the gradient, and usually indicates an error in 
the gradient function or using numerical derivatives on a 
non-differentiable function.
> By chance I looked at the help and I read "The parameter vector passed
to fn has special semantics and may be shared between calls: the function should
not change or copy it" . Could the instructions above be the cause of the
failure? If so, how to deal with symbolic differentation?
None of the code you show us changes 'theta'.  It would be a very 
unusual thing to do, but has happened in error when people have used 
compiled code.
> Thanks in advance,
> Gildas

-- 
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)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595