fit2.C File Reference

Detailed Description

Fitting a 2-D histogram This tutorial illustrates :

• how to create a 2-d function
• fill a 2-d histogram randomly from this function
• fit the histogram
• display the fitted function on top of the histogram

This example can be executed via the interpreter or ACLIC

root > .x fit2.C
root > .x fit2.C++

 
****************************************
Minimizer is Minuit2 / Migrad
Chi2                      =      1048.29
NDf                       =         1139
Edm                       =  3.90056e-06
NCalls                    =          519
p0                        =      392.558   +/-   2.07088     
p1                        =     -2.99838   +/-   0.0116072   
p2                        =      2.98484   +/-   0.00840711  
p3                        =     -3.00201   +/-   0.0115172   
p4                        =      2.97271   +/-   0.00841038  
p5                        =      601.133   +/-   10.5562     
p6                        =   0.00614073   +/-   0.0119548   
p7                        =      0.81626   +/-   0.0107847   
p8                        = -0.000781266   +/-   0.0134062   
p9                        =     0.911288   +/-   0.0119899   
p10                       =      146.899   +/-   5.12261     
p11                       =       3.9882   +/-   0.0182639   
p12                       =     0.727561   +/-   0.0142962   
p13                       =      4.02637   +/-   0.0175896   
p14                       =     0.703077   +/-   0.0140242   

 
#include "TF2.h"
#include "TH2.h"
#include "TMath.h"
 
double g2(double *x, double *par) {
   double r1 = double((x[0]-par[1])/par[2]);
   double r2 = double((x[1]-par[3])/par[4]);
   return par[0]*TMath::Exp(-0.5*(r1*r1+r2*r2));
}
double fun2(double *x, double *par) {
   double *p1 = &par[0];
   double *p2 = &par[5];
   double *p3 = &par[10];
   double result = g2(x,p1) + g2(x,p2) + g2(x,p3);
   return result;
}
 
void fit2() {
   const int npar = 15;
   double f2params[npar] =
      {100,-3,3,-3,3,160,0,0.8,0,0.9,40,4,0.7,4,0.7};
   TF2 *f2 = new TF2("f2",fun2,-10,10,-10,10, npar);
   f2->SetParameters(f2params);
 
   //Create an histogram and fill it randomly with f2
   TH2F *h2 = new TH2F("h2","from f2",40,-10,10,40,-10,10);
   int nentries = 100000;
   h2->FillRandom("f2",nentries);
   //Fit h2 with original function f2
   float ratio = 4*nentries/100000;
   f2params[ 0] *= ratio;
   f2params[ 5] *= ratio;
   f2params[10] *= ratio;
   f2->SetParameters(f2params);
   h2->Fit("f2");
}

Author

Rene Brun

Definition in file fit2.C.