Thread: nonlinear curve fitting with a fixed point

Hi,

I have found this function that will calculate the best fit polynomial for a set of dx's and dy's and then save the coefficients in the 1D store[] array using the GSL GNU Scientific Library. I'm not familiar with GSL so I'm not sure exactly what the gsl_* functions are doing, but I've gotten it to work for my purposes.

However, now I would like to fit my data with a polynomial and use an initial fixed point. Is there a way I can modify this to use an initial fixed point? If so, do you have any suggestions (or resources you could point me towards) about how I would go about doing this?

Thanks!

Code:

int polynomialfit(int obs, int degree, double *dx, double *dy, double *store, int degree_max) /* n, p */
{
  gsl_multifit_linear_workspace *ws;
  gsl_matrix *cov, *X;
  gsl_vector *y, *c;
  double chisq;
 
  int i, j;
  int coeff_num = degree+1; //number of coeff is one more than the degree of the polynomial
 
  X = gsl_matrix_alloc(obs, coeff_num);
  y = gsl_vector_alloc(obs);
  c = gsl_vector_alloc(coeff_num);
  cov = gsl_matrix_alloc(coeff_num, coeff_num);
 
  for(i=0; i < obs; i++) {
    gsl_matrix_set(X, i, 0, 1.0);
    for(j=0; j < coeff_num; j++) {
      gsl_matrix_set(X, i, j, pow(dx[i], j));
    }
    gsl_vector_set(y, i, dy[i]);
  }
 
  ws = gsl_multifit_linear_alloc(obs, coeff_num);
  gsl_multifit_linear(X, y, c, cov, &chisq, ws);
 
  /* store result ... */
  for(i=0; i < degree_max; i++)  {
	if (i<coeff_num){
		store[i] = gsl_vector_get(c, i); //coeff[consnt, co*x, c1*x^2....etc]
	}
	if (i>=coeff_num){
		store[i] = 0.0;
	}	
  }

 
  gsl_multifit_linear_free(ws);
  gsl_matrix_free(X);
  gsl_matrix_free(cov);
  gsl_vector_free(y);
  gsl_vector_free(c);
  return 0; 
}

This probably isn't the best forum for your question, since it's more of a math and GSL question rather than a pure C question. Below are the links to the GSL documentation and mailing list to get you started.

GNU Scientific Library -- Reference Manual
GNU Scientific Library - Mailing Lists [Savannah]