I have modified the example Ifit.C that I posted a few months ago to illustrate
how to get the final values of parameters and the errors.
Note that you can also get access to the "assymetric errors" by calling mnerrs.
For this, look an example in TH1::Fit.
Rene Brun
//
// Example of a program to fit non-equidistant data points
// =======================================================
//
// The fitting function fcn is a simple chisquare function
// The data consists of 5 data points (arrays x,y,z) + the errors in errorsz
// More details on the various functions or parameters for these functions
// can be obtained in an interactive ROOT session with:
// Root > TMinuit *minuit = new TMinuit(10);
// Root > minuit->mnhelp("*",0) to see the list of possible keywords
// Root > minuit->mnhelp("SET",0) explains most parameters
//
Float_t z[5],x[5],y[5],errorz[5];
//______________________________________________________________________________
void fcn(Int_t &npar, Double_t *gin, Double_t &f, Double_t *par, Int_t iflag)
{
const Int_t nbins = 5;
Int_t i;
//calculate chisquare
Double_t chisq = 0;
Double_t delta;
for (i=0;i<nbins; i++) {
delta = (z[i]-func(x[i],y[i],par))/errorz[i];
chisq += delta*delta;
}
f = chisq;
}
//______________________________________________________________________________
Double_t func(float x,float y,Double_t *par)
{
Double_t value=( (par[0]*par[0])/(x*x)-1)/ ( par[1]+par[2]*y-par[3]*y*y);
return value;
}
//______________________________________________________________________________
void Ifit()
{
// The z values
z[0]=1;
z[1]=0.96;
z[2]=0.89;
z[3]=0.85;
z[4]=0.78;
// The errors on z values
Float_t error = 0.01;
errorz[0]=error;
errorz[1]=error;
errorz[2]=error;
errorz[3]=error;
errorz[4]=error;
// the x values
x[0]=1.5751;
x[1]=1.5825;
x[2]=1.6069;
x[3]=1.6339;
x[4]=1.6706;
// the y values
y[0]=1.0642;
y[1]=0.97685;
y[2]=1.13168;
y[3]=1.128654;
y[4]=1.44016;
TMinuit *gMinuit = new TMinuit(5); //initialize TMinuit with a maximum of 5 params
gMinuit->SetFCN(fcn);
Double_t arglist[10];
Int_t ierflg = 0;
arglist[0] = 1;
gMinuit->mnexcm("SET ERR", arglist ,1,ierflg);
// Set starting values and step sizes for parameters
static Double_t vstart[4] = {3, 1 , 0.1 , 0.01};
static Double_t step[4] = {0.1 , 0.1 , 0.01 , 0.001};
gMinuit->mnparm(0, "a1", vstart[0], step[0], 0,0,ierflg);
gMinuit->mnparm(1, "a2", vstart[1], step[1], 0,0,ierflg);
gMinuit->mnparm(2, "a3", vstart[2], step[2], 0,0,ierflg);
gMinuit->mnparm(3, "a4", vstart[3], step[3], 0,0,ierflg);
// Now ready for minimization step
arglist[0] = 500;
arglist[1] = 1.;
gMinuit->mnexcm("MIGRAD", arglist ,2,ierflg);
// Print parameters values and errors
Double_t pvalue, perror, plow,phigh;
char pname[3];
for (Int_t i=0;i<4;i++) {
sprintf(pname,"a%d",i+1);
gMinuit->mnpout(i,pname,pvalue,perror,plow,phigh,ierflg);
printf("Par %d, name=%s = %g +- %g\n",i,pname,pvalue,perror);
}
// Print results using Minuit printing functions
Double_t amin,edm,errdef;
Int_t nvpar,nparx,icstat;
gMinuit->mnstat(amin,edm,errdef,nvpar,nparx,icstat);
gMinuit->mnprin(3,amin);
}