curve fitting - Problems with MPFIT and user-defined derivatives -

i trying use optimization library mpfit fit gaussian function data. code part of example code comes mpfit library. original code automatically calculates internally derivatives of function numerically , works perfectly. mpfit library allows user provide function derivatives. problem starts. here function used calculate residuals , function's first order partial derivatives.

int gaussfunc(int m, int n, double *p, double *dy, double **derivs, void *vars) {   int i,j;   struct vars_struct *v = (struct vars_struct *) vars;   double *x, *y, *ey;   double = p[1];   double b = p[2];   double c = p[3];   double d = p[0];   x = v->x;   y = v->y;   ey = v->ey;    (i=0; i<m; i++)   {     dy[i] = (y[i] - (a*exp(-(x[i]-b)*(x[i]-b)/(2*c*c))+d))/ey[i];   }    // code below point code added calculate derivatives.   if(derivs)   {       for(j = 0; j < n; j++)       {           if (derivs[j])           {               (i = 0; < m; i++)               {                   double da =     exp(-(x[i]-b)*(x[i]-b)/(2*c*c));                   double db = * exp(-(x[i]-b)*(x[i]-b)/(2*c*c)) * (x[i]-b)/(c*c);                   double dc = * exp(-(x[i]-b)*(x[i]-b)/(2*c*c)) * (x[i]-b)*(x[i]-b)/(c*c*c);                   double dd = 1;                   double foo;                   if     (j == 0) foo = dd;                   else if(j == 1) foo = da;                   else if(j == 2) foo = db;                   else if(j == 3) foo = dc;                   derivs[j][i] = foo;               }           }       }   }    return 0; } 

the code above line 'if (derivs)' original 1 refactored , code below code computing derivatives. believe maths correct, , verified https://math.stackexchange.com/questions/716545/calculating-the-first-order-partial-derivatives-of-the-gaussian-function/716553

has encountered same problem while using mpfit user-defined derivatives?

thank you.

because calculation of residuals (data-model)/sigma, derivatives should be:

[-d(model)/d(param)]/sigma.

so, line:

derivs[j][i] = foo; 

becomes:

derivs[j][i] = -foo/ey[i]; 

problem solved! thanks!