myfit.C File Reference

Detailed Description

Get in memory an histogram from a root file and fit a user defined function.

Note that a user defined function must always be defined as in this example:

• first parameter: array of variables (in this example only 1-dimension)
• second parameter: array of parameters Note also that in case of user defined functions, one must set an initial value for each parameter.

 
****************************************
Minimizer is Minuit2 / Migrad
Chi2                      =      36.7428
NDf                       =           47
Edm                       =  2.03167e-06
NCalls                    =          101
Constant                  =      797.969   +/-   6.79742     
Mean_value                = -7.42918e-05   +/-   0.00734861  
Sigma                     =     0.998754   +/-   0.0071337   
Integral of function = 1907.36

 
#include <TCanvas.h>
#include <TF1.h>
#include <TFile.h>
#include <TH1F.h>
#include <TInterpreter.h>
#include <TROOT.h>
 
#include <cmath>
 
double fitf(double *x, double *par)
{
   double arg = 0;
   if (par[2] != 0) arg = (x[0] - par[1])/par[2];
 
   double fitval = par[0]*std::exp(-0.5*arg*arg);
   return fitval;
}
void myfit()
{
   TString dir = gROOT->GetTutorialDir();
   dir.Append("/hsimple.C");
   dir.ReplaceAll("/./","/");
   if (!gInterpreter->IsLoaded(dir.Data())) gInterpreter->LoadMacro(dir.Data());
   TFile *hsimpleFile = (TFile*)gROOT->ProcessLineFast("hsimple(1)");
   if (!hsimpleFile) return;
 
   TCanvas *c1 = new TCanvas("c1","the fit canvas",500,400);
 
   TH1F *hpx = (TH1F*)hsimpleFile->Get("hpx");
 
// Creates a Root function based on function fitf above
   TF1 *func = new TF1("fitf",fitf,-2,2,3);
 
// Sets initial values and parameter names
   func->SetParameters(100,0,1);
   func->SetParNames("Constant","Mean_value","Sigma");
 
// Fit histogram in range defined by function
   hpx->Fit(func,"r");
 
// Gets integral of function between fit limits
   printf("Integral of function = %g\n",func->Integral(-2,2));
}

Author

Rene Brun

Definition in file myfit.C.