void medfit(float x[], float y[], int ndata, float *a, float *b, float *abdev)
{
float rofunc(float b);
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;
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);
if (sigb > 0.0) {
b2=bb+SIGN(3.0*sigb,f1);
f2=rofunc(b2);
if (b2 == b1) {
*a=aa;
*b=bb;
*abdev=abdevt/ndata;
return;
}
while (f1*f2 > 0.0) {
bb=b2+1.6*(b2-b1);
b1=b2;
f1=f2;
b2=bb;
f2=rofunc(b2);
}
sigb=0.01*sigb;
while (fabs(b2-b1) > sigb) {
bb=b1+0.5*(b2-b1);
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;
}
int main(void)
{
long idum=(-11);
int i;
float b,rf,*x,*y;
x=vector(1,NDATA);
y=vector(1,NDATA);
ndatat=NDATA;
xt=x;
yt=y;
for (i=1;i<=NDATA;i++) {
x[i]=0.1*i;
y[i] = -2.0*x[i]+1.0+SPREAD*gasdev(&idum);
}
printf("%9s %9s %12s %10s\n","b","a","ROFUNC","ABDEVT");
for (i = -5;i<=5;i++) {
b = -2.0+0.02*i;
rf=rofunc(b);
printf("%10.2f %9.2f %11.2f %10.2f\n",
b,aa,rf,abdevt);
}
free_vector(y,1,NDATA);
free_vector(x,1,NDATA);
return 0;
}
