how to debug multifit_nlinear

[PICCA Frederic-Emmanuel]

Hello,

I am using

#include <gsl/gsl_multifit_nlinear.h>

in order to do a least square fitting of this function

int model_f (const gsl_vector *x, void *data, gsl_vector *f)
{
        size_t n = ((struct data *)data)->n;
        float *t = ((struct data *)data)->t;
        float *y = ((struct data *)data)->y;
        int* lista = ((struct data *)data)->lista;
        size_t n_lista = ((struct data *)data)->n_lista;
        float* parameters = ((struct data *)data)->parameters;
        size_t n_parameters = ((struct data *)data)->n_parameters;
        FitFunction funcs = ((struct data *)data)->funcs;

        float dyda;

        size_t i;

        for (i=0; i<n_lista; ++i){
                parameters[lista[i] - 1] = gsl_vector_get(x, i);
        }

        for(i=0; i<n; ++i){
                float ymod;

                (*funcs)(t[i], parameters, &ymod, &dyda, n_parameters, 1);
                ymod = t[i];
                fprintf(stdout, "f(i=%d, t=%f) = %f (%f)\n", i, t[i], ymod, 
y[i]);
                gsl_vector_set (f, i, ymod - y[i]);
        }
        fprintf(stdout, "\n");

        return GSL_SUCCESS;
}

with

        const gsl_multifit_nlinear_type *T = gsl_multifit_nlinear_trust;
        gsl_multifit_nlinear_parameters fdf_params = 
gsl_multifit_nlinear_default_parameters();
        struct data d = {
                .n = n,
                .t = xx,
                .y = yy,
                .wgt = wgt,
                .parameters = a,
                .n_parameters = ma,
                .lista = lista,
                .n_lista = p,
                .funcs = funcs,
        };

        gsl_rng_env_setup();
        r = gsl_rng_alloc(gsl_rng_default);

        /* define the function to be minimized */
        fdf.f = model_f;
        fdf.df = NULL;   /* set to NULL for finite-difference Jacobian */
        fdf.fvv = NULL;     /* not using geodesic acceleration */
        fdf.n = n;
        fdf.p = p;
        fdf.params = &d;

        /* allocate workspace with default parameters */
        w = gsl_multifit_nlinear_alloc (T, &fdf_params, n, p);

        /* initialize solver with starting point and weights */
        gsl_multifit_nlinear_winit (&x.vector, &wts.vector, &fdf, w);

        /* compute initial cost function */
        /* f = gsl_multifit_nlinear_residual(w); */
        /* gsl_blas_ddot(f, f, &chisq0); */

        /* solve the system with a maximum of 100 iterations */
        status = gsl_multifit_nlinear_driver(10000, xtol, gtol, ftol,
                                             callback, NULL, &info, w);


but when I fit the function (There is only 2 parameters and around 133
data points points), I have this error

iter  0: param0 = 0.2731, param1 = 0.000, cond(J) =   4.0000, |f(x)| = 566.7044
gsl: qrpt.c:345: ERROR: rank must have 0 < rank <= N
Default GSL error handler invoked.

Is it possible to explain the possible cause of this error. And  is
there a degug mode in order to observe the  internal of
gsl_multifit_nlinéar_driver during the fit.

thanks for your help

Frederic