Add a new tutorial illustrating how to:

  -create a variable binwidth histogram with a binning such
   that the population per bin is about the same.
  -rebin a variable binwidth histogram into another one.


git-svn-id: http://root.cern.ch/svn/root/trunk@29921 27541ba8-7e3a-0410-8455-c3a389f83636

tutorials/hist/rebin.C

+//this tutorial illustrates how to:
+//  -create a variable binwidth histogram with a binning such
+//   that the population per bin is about the same.
+//  -rebin a variable binwidth histogram into another one.
+//Author: Rene Brun
+   
+#include "TH1.h"
+#include "TCanvas.h"
+void 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[1000];
+   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++;
+      }
+   }
+   //create a variable binwidth histogram out of fix bin histogram
+   //new rebinned histogram should have about 10 entries per bin
+   TH1F *hnew = new TH1F("hnew","rebinned",k-1,xbins);
+   hnew->FillRandom("gaus",10*nentries);
+   
+   //rebin hnew keeping only 50% of the bins
+   Double_t xbins2[500];
+   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-1,"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();
+}