Fitting a 2-D histogram This tutorial illustrates :
This example can be executed via the interpreter or ACLIC
root > .x fit2.C
root > .x fit2.C++
Author: Rene Brun
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Wednesday, April 17, 2024 at 11:08 AM.
Definition of a helper function:
%%cpp -d
#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));
}
Definition of a helper function:
%%cpp -d
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;
}
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");
**************************************** 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
input_line_55:2:2: warning: 'ratio' shadows a declaration with the same name in the 'std' namespace; use '::ratio' to reference this declaration float ratio = 4*nentries/100000; ^ Info in <TCanvas::MakeDefCanvas>: created default TCanvas with name c1
Draw all canvases
gROOT->GetListOfCanvases()->Draw()