hist013_TH1_rebin.C

/// \file
/// \ingroup tutorial_hist
/// \notebook -js
/// \preview Rebin a variable bin-width histogram.
///
/// This tutorial illustrates how to:
///   - create a variable bin-width histogram with a binning such
///     that the population per bin is about the same.
///   - rebin a variable bin-width histogram into another one.
///
/// \macro_image
/// \macro_code
///
/// \date July 2016
/// \author Rene Brun

#include "TH1.h"
#include "TCanvas.h"
void hist013_TH1_rebin()
{
   // create a fix bin histogram
   TH1F *h = new TH1F("h", "test rebin", 100, -3, 3);
   Int_t nentries = 1000;
   h->FillRandom("gaus", nentries);
   Double_t xbins[1001];
   Int_t k = 0;
   TAxis *axis = h->GetXaxis();
   for (Int_t i = 1; i <= 100; i++) {
      Int_t y = (Int_t)h->GetBinContent(i);
      if (y <= 0)
         continue;
      Double_t dx = axis->GetBinWidth(i) / y;
      Double_t xmin = axis->GetBinLowEdge(i);
      for (Int_t j = 0; j < y; j++) {
         xbins[k] = xmin + j * dx;
         k++;
      }
   }
   xbins[k] = axis->GetXmax();
   // create a variable bin-width histogram out of fix bin histogram
   // new rebinned histogram should have about 10 entries per bin
   TH1F *hnew = new TH1F("hnew", "rebinned", k, xbins);
   hnew->FillRandom("gaus", 10 * nentries);

   // rebin hnew keeping only 50% of the bins
   Double_t xbins2[501];
   Int_t kk = 0;
   for (Int_t j = 0; j < k; j += 2) {
      xbins2[kk] = xbins[j];
      kk++;
   }
   xbins2[kk] = xbins[k];
   TH1F *hnew2 = (TH1F *)hnew->Rebin(kk, "hnew2", xbins2);

   // draw the 3 histograms
   TCanvas *c1 = new TCanvas("c1", "c1", 800, 1000);
   c1->Divide(1, 3);
   c1->cd(1);
   h->Draw();
   c1->cd(2);
   hnew->Draw();
   c1->cd(3);
   hnew2->Draw();
}