[Help-gsl] A gsl_vector of pointers?

[John Gehman]

Hello All,

I've got a general question regarding gsl_vector(s) and (in particular) 
non linear least squares fitting. Several of the associated functions 
used for nonlinear least squares fitting (the functions whose pointers 
are arguments to gsl_multifit_function_fdf, as well as 
gsl_multifit_fdfsolver_set) require as an argument the gsl_vector of 
values which correspond to those parameters one wishes to float in a 
nonlinear least squares fit. This is straightforward and reasonable in 
situations such as the example provided in the gsl-ref.ps 
documentation.

I, however, have now two unrelated situations for which I do not (yet) 
have analytic expressions. I want to test the model numerically before 
I get too carried away with the pencil and paper. Calculating the 
residuals and gradients are fairly involved (calculating multiple roots 
in one case, numeric integration in the other case, and using 
gsl_deriv_central in both cases to fill the Jacobian). The complexity 
of the problem drives me to organize the problem into data struct(s), 
and the parameters that I want to float are actually components of 
these structures.

At the moment it's clumsy, as I can only figure to write two functions 
which read and write updated values back and forth between the 
structure components and the gsl_vector of double-precision numbers 
which is needed to pass into the nonlinear fitting functions. Even if 
this is an OK workaround, it is prone to human error, as I seem to be 
fouling unrelated values by implementing the scheme incorrectly (and 
the application that I'm further along on isn't working either, but 
that could be for other reasons).

Now, finally, the question: is there actually a way that I can fill a 
gsl_vector with pointers to doubles (those within my structures) 
instead of explicit doubles? I might guess there could be some trick 
with "stride", but I can't find enough explanation to hack with.

Heaps of gratitude for anybody who can help!

John Gehman
Research Fellow
School of Chemistry
University of Melbourne
Australia