multifit.C

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/// \file
/// \ingroup tutorial_fit
/// \notebook -js
///  Fitting multiple functions to different ranges of a 1-D histogram
///      Example showing how to fit in a sub-range of an histogram
///  A histogram is created and filled with the bin contents and errors
///  defined in the table below.
///  Three Gaussians are fitted in sub-ranges of this histogram.
///  A new function (a sum of 3 Gaussians) is fitted on another subrange
///  Note that when fitting simple functions, such as Gaussians, the initial
///  values of parameters are automatically computed by ROOT.
///  In the more complicated case of the sum of 3 Gaussians, the initial values
///  of parameters must be given. In this particular case, the initial values
///  are taken from the result of the individual fits.
///
/// \macro_image
/// \macro_output
/// \macro_code
///
/// \author Rene Brun
 
#include "TH1.h"
#include "TF1.h"
 
void multifit()
{
   const int np = 49;
   float x[np] = {1.913521,  1.953769,  2.347435,  2.883654,  3.493567,  4.047560,  4.337210,  4.364347,  4.563004,
                  5.054247,  5.194183,  5.380521,  5.303213,  5.384578,  5.563983,  5.728500,  5.685752,  5.080029,
                  4.251809,  3.372246,  2.207432,  1.227541,  0.8597788, 0.8220503, 0.8046592, 0.7684097, 0.7469761,
                  0.8019787, 0.8362375, 0.8744895, 0.9143721, 0.9462768, 0.9285364, 0.8954604, 0.8410891, 0.7853871,
                  0.7100883, 0.6938808, 0.7363682, 0.7032954, 0.6029015, 0.5600163, 0.7477068, 1.188785,  1.938228,
                  2.602717,  3.472962,  4.465014,  5.177035};
 
   // The histogram are filled with bins defined in the array x.
   TH1F *h = new TH1F("h", "Example of several fits in subranges", np, 85, 134);
   h->SetMaximum(7);
 
   for (int i = 0; i < np; i++) {
      h->SetBinContent(i + 1, x[i]);
   }
 
   // Define the parameter array for the total function.
   double par[9];
 
   // Three TF1 objects are created, one for each subrange.
   TF1 *g1 = new TF1("g1", "gaus", 85, 95);
   TF1 *g2 = new TF1("g2", "gaus", 98, 108);
   TF1 *g3 = new TF1("g3", "gaus", 110, 121);
 
   // The total is the sum of the three, each has three parameters.
   TF1 *total = new TF1("total", "gaus(0)+gaus(3)+gaus(6)", 85, 125);
   total->SetLineColor(2);
 
   // Fit each function and add it to the list of functions. By default,
   // TH1::Fit() fits the function on the defined histogram range. You can
   // specify the "R" option in the second parameter of TH1::Fit() to restrict
   // the fit to the range specified in the TF1 constructor. Alternatively, you
   // can also specify the range in the call to TH1::Fit(), which we demonstrate
   // here with the 3rd Gaussian. The "+" option needs to be added to the later
   // fits to not replace existing fitted functions in the histogram.
   h->Fit(g1, "R");
   h->Fit(g2, "R+");
   h->Fit(g3, "+", "", 110, 121);
 
   // Get the parameters from the fit.
   g1->GetParameters(&par[0]);
   g2->GetParameters(&par[3]);
   g3->GetParameters(&par[6]);
 
   // Use the parameters on the sum.
   total->SetParameters(par);
   h->Fit(total, "R+");
}