Notebook

FitHistoInFile¶

This example can be executed as: root > .x FitHistoInFile.C

Based on FittingDemo.C by Rene Brun

Author: Author E. von Toerne
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Monday, March 27, 2023 at 09:47 AM.

In [1]:

%%cpp -d

#include "TH1.h"
#include "TMath.h"
#include "TF1.h"
#include "TLegend.h"
#include "TCanvas.h"
#include "TFile.h"
#include "TStyle.h"

Function parameters are passed as an array to TF1. Here, we define the position of each parameter in this array. Note: N_PAR will give us the total number of parameters. Make sure it is always the last entry!

In [2]:

enum ParIndex_t {
   Bkg0=0, Bkg1=1, Bkg2,
   SigScale, SigSigma, SigMean,
   N_PAR};

Use this map t

In [3]:

%%cpp -d
o (re-)name parameters for the plot
const std::map<ParIndex_t,std::string> parNames{
   {Bkg0, "Bkg0"}, {Bkg1, "Bkg1"}, {Bkg2, "Bkg2"},
   {SigScale, "Gauss scale"}, {SigSigma, "Gauss #sigma"}, {SigMean, "Gauss #mu"}
};

input_line_53:1:1: error: C++ requires a type specifier for all declarations
o (re-)name parameters for the plot
^
input_line_53:1:4: error: use of undeclared identifier 're'
o (re-)name parameters for the plot
   ^
input_line_53:1:7: error: expected expression
o (re-)name parameters for the plot
      ^
input_line_53:1:8: error: expected ';' after top level declarator
o (re-)name parameters for the plot
       ^
       ;

Quadratic background function

In [4]:

%%cpp -d
double background(double *x, double *par) {
   return par[Bkg0] + par[Bkg1]*x[0] + par[Bkg2]*x[0]*x[0];
}

Gauss Peak function

In [5]:

%%cpp -d
double signal(double *x, double *par) {
   return par[SigScale]*TMath::Gaus(x[0], par[SigMean], par[SigSigma], true);
}

Sum of background and peak function. We pass x and the fit parameters down to the signal and background functions.

In [6]:

%%cpp -d
double fitFunction(double *x, double *par) {
   return background(x, par) + signal(x, par);
}

Fit "fitFunction" to the histogram, and draw results on the canvas c1.

In [7]:

%%cpp -d
void FitRoutine(TCanvas* c1, TH1* histo, float fitxmin, float fitxmax, TString filename){
   c1->cd();
   // create a TF1 with the range from 0 to 3 and N_PAR parameters (six by default)
   TF1 fitFcn("fitFcn",fitFunction,fitxmin,fitxmax,N_PAR);
   fitFcn.SetNpx(500);
   fitFcn.SetLineWidth(2);
   fitFcn.SetLineColor(kBlue);

   // Assign the names from the map "parNames". Optional, but makes a nicer plot.
   for (auto& idx_name : parNames) {
     fitFcn.SetParName(idx_name.first, idx_name.second.c_str());
   }

   // Fit. First set ok-ish starting values for the parameters
   fitFcn.SetParameters(30,0,0,50.,0.1,1.);
   histo->GetXaxis()->SetRange(2,40);
   histo->Fit("fitFcn","VR+","ep");

   // improve the picture:
   // Draw signal and background functions separately
   TF1 backFcn("backFcn",background,fitxmin,fitxmax,N_PAR);
   backFcn.SetLineColor(kRed);
   TF1 signalFcn("signalFcn",signal,fitxmin,fitxmax,N_PAR);
   signalFcn.SetLineColor(kBlue);
   signalFcn.SetNpx(500);

   // Retrieve fit parameters, and copy them to the signal and background functions
   double par[N_PAR];
   fitFcn.GetParameters(par);

   backFcn.SetParameters(par);
   backFcn.DrawCopy("same");

   signalFcn.SetParameters(par);
   signalFcn.SetLineColor(kGreen);
   signalFcn.DrawCopy("same");

   const double binwidth = histo->GetXaxis()->GetBinWidth(1);
   const double integral = signalFcn.Integral(0.,3.);
   std::cout << "number of signal events = " << integral/binwidth << " " << binwidth<< std::endl;

   // draw the legend
   TLegend legend(0.15,0.7,0.28,0.85);
   legend.SetTextFont(72);
   legend.SetTextSize(0.03);
   legend.AddEntry(histo,"Data","lpe");
   legend.AddEntry(&backFcn,"Bgd","l");
   legend.AddEntry(&signalFcn,"Sig","l");
   legend.AddEntry(&fitFcn,"Sig+Bgd","l");
   legend.DrawClone();
   histo->Draw("esame");
   c1->SaveAs(filename);
}

input_line_57:10:26: error: use of undeclared identifier 'parNames'
   for (auto& idx_name : parNames) {
                         ^

Create a file with example data

In [8]:

%%cpp -d
void CreateRootFile(){
   // The data in array form
   const int nBins = 60;
   double data[nBins] = { 6, 1,10,12, 6,13,23,22,15,21,
                           23,26,36,25,27,35,40,44,66,81,
                           75,57,43,37,36,31,35,36,43,32,
                           40,37,38,33,36,44,42,37,32,32,
                           43,44,35,33,33,39,29,41,32,44,
                           26,39,29,35,32,21,21,15,25,15};

   TFile* f = new TFile("exampleRootFile.root","RECREATE");
   TH1D *histo = new TH1D("histo", "Gauss Peak on Quadratic Background;x;Events/0.05",60,0,3);
   for(int i=0; i < nBins;  i++) histo->SetBinContent(i+1,data[i]);
   f->Write();
   f->Close();
}

In [9]:

gStyle->SetOptFit(1111);
gStyle->SetOptStat(0);

fit range from fitxmin to fitxmax

In [10]:

float fitxmin=0.2;
float fitxmax=2.7;

TCanvas *c1 = new TCanvas("c1","Fitting Demo of Histogram in File",10,10,700,500);
CreateRootFile();
TFile* f = new TFile("exampleRootFile.root");
TH1D* histo= nullptr;
f->GetObject("histo",histo);
if (!histo){
   std::cout << "histo not found" << std::endl;
   return;
}
histo->SetMarkerStyle(21);
histo->SetMarkerSize(0.8);

now call the fit routine

In [11]:

FitRoutine(c1,histo, fitxmin, fitxmax,"FitHistoInFile.pdf");

input_line_65:2:3: error: use of undeclared identifier 'FitRoutine'
 (FitRoutine(((*(class TCanvas **)0x7ff9281c2000)), ((*(class TH1D **)0x7ff9281c2018)), ((*(float*)0x7ff9281c2020)), ((*(float*)0x7ff9281c2024)), "FitHistoInFile.pdf"))
  ^
Error in <HandleInterpreterException>: Error evaluating expression (FitRoutine(((*(class TCanvas **)0x7ff9281c2000)), ((*(class TH1D **)0x7ff9281c2018)), ((*(float*)0x7ff9281c2020)), ((*(float*)0x7ff9281c2024)), "FitHistoInFile.pdf"))
Execution of your code was aborted.

Draw all canvases

In [12]:

gROOT->GetListOfCanvases()->Draw()