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Reference Guide
fit2.C File Reference

Detailed Description

View in nbviewer Open in SWAN 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++
FCN=1048.29 FROM MIGRAD STATUS=CONVERGED 384 CALLS 385 TOTAL
EDM=1.71323e-07 STRATEGY= 1 ERROR MATRIX UNCERTAINTY 1.7 per cent
EXT PARAMETER STEP FIRST
NO. NAME VALUE ERROR SIZE DERIVATIVE
1 p0 3.92557e+02 1.80794e+00 -8.93086e-04 1.08685e-04
2 p1 -2.99839e+00 1.13352e-02 -6.49024e-06 -1.81171e-02
3 p2 2.98485e+00 7.41044e-03 3.61030e-06 -5.86707e-02
4 p3 -3.00202e+00 1.11595e-02 -5.78996e-06 -1.75293e-02
5 p4 2.97271e+00 7.15832e-03 -3.28538e-06 -4.12677e-02
6 p5 6.01136e+02 9.83774e+00 3.21497e-03 -2.69040e-05
7 p6 6.14587e-03 1.13579e-02 5.13989e-06 2.41418e-02
8 p7 8.16263e-01 9.52518e-03 3.01650e-06 -8.27896e-03
9 p8 -7.76134e-04 1.27854e-02 5.13131e-06 -5.37304e-03
10 p9 9.11281e-01 1.09052e-02 -6.70503e-06 2.34952e-02
11 p10 1.46899e+02 4.66962e+00 -3.42267e-04 5.38648e-05
12 p11 3.98822e+00 1.79131e-02 1.99376e-05 9.00611e-03
13 p12 7.27558e-01 1.32237e-02 -2.78350e-06 5.94843e-03
14 p13 4.02638e+00 1.71292e-02 4.21853e-06 5.32476e-03
15 p14 7.03078e-01 1.29939e-02 1.10618e-06 -3.47421e-02
#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");
f2->Draw("cont1 same");
}
Option_t Option_t TPoint TPoint const char GetTextMagnitude GetFillStyle GetLineColor GetLineWidth GetMarkerStyle GetTextAlign GetTextColor GetTextSize void char Point_t Rectangle_t WindowAttributes_t Float_t Float_t Float_t Int_t Int_t UInt_t UInt_t Rectangle_t result
int nentries
Definition: THbookFile.cxx:91
virtual void SetParameters(const Double_t *params)
Definition: TF1.h:649
A 2-Dim function with parameters.
Definition: TF2.h:29
void Draw(Option_t *option="") override
Draw this function with its current attributes.
Definition: TF2.cxx:260
virtual TFitResultPtr Fit(const char *formula, Option_t *option="", Option_t *goption="", Double_t xmin=0, Double_t xmax=0)
Fit histogram with function fname.
Definition: TH1.cxx:3904
2-D histogram with a float per channel (see TH1 documentation)}
Definition: TH2.h:257
void FillRandom(const char *fname, Int_t ntimes=5000, TRandom *rng=nullptr) override
Fill histogram following distribution in function fname.
Definition: TH2.cxx:677
Double_t x[n]
Definition: legend1.C:17
Double_t Exp(Double_t x)
Returns the base-e exponential function of x, which is e raised to the power x.
Definition: TMath.h:707
Author
Rene Brun

Definition in file fit2.C.