medfit.c

Numerical Recipes 是国际公认的高水平的、关于数值计算的书

#include <math.h>

int ndatat=0;	/* defining declaration */
float *xt=0,*yt=0,aa=0.0,abdevt=0.0;	/* defining declaration */

void medfit(x,y,ndata,a,b,abdev)
float *x,*y,*a,*b,*abdev;
int ndata;
{
	int j;
	float bb,b1,b2,del,f,f1,f2,sigb,temp;
	float sx=0.0,sy=0.0,sxy=0.0,sxx=0.0,chisq=0.0;
	float rofunc();

	ndatat=ndata;
	xt=x;
	yt=y;
	for (j=1;j<=ndata;j++) {
		sx += x[j];
		sy += y[j];
		sxy += x[j]*y[j];
		sxx += x[j]*x[j];
	}
	del=ndata*sxx-sx*sx;
	aa=(sxx*sy-sx*sxy)/del;
	bb=(ndata*sxy-sx*sy)/del;
	for (j=1;j<=ndata;j++)
		chisq += (temp=y[j]-(aa+bb*x[j]),temp*temp);
	sigb=sqrt(chisq/del);
	b1=bb;
	f1=rofunc(b1);
	b2=bb+((f1 > 0.0) ? fabs(3.0*sigb) : -fabs(3.0*sigb));
	f2=rofunc(b2);
	while (f1*f2 > 0.0) {
		bb=2.0*b2-b1;
		b1=b2;
		f1=f2;
		b2=bb;
		f2=rofunc(b2);
	}
	sigb=0.01*sigb;
	while (fabs(b2-b1) > sigb) {
		bb=0.5*(b1+b2);
		if (bb == b1 || bb == b2) break;
		f=rofunc(bb);
		if (f*f1 >= 0.0) {
			f1=f;
			b1=bb;
		} else {
			f2=f;
			b2=bb;
		}
	}
	*a=aa;
	*b=bb;
	*abdev=abdevt/ndata;
}