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Reference Guide
rebin.C
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1 /// \file
2 /// \ingroup tutorial_hist
3 /// \notebook -js
4 /// Rebin a variable bin-width histogram.
5 ///
6 /// This tutorial illustrates how to:
7 /// - create a variable bin-width histogram with a binning such
8 /// that the population per bin is about the same.
9 /// - rebin a variable bin-width histogram into another one.
10 ///
11 /// \macro_image
12 /// \macro_code
13 ///
14 /// \author Rene Brun
15 
16 #include "TH1.h"
17 #include "TCanvas.h"
18 void rebin() {
19  //create a fix bin histogram
20  TH1F *h = new TH1F("h","test rebin",100,-3,3);
21  Int_t nentries = 1000;
22  h->FillRandom("gaus",nentries);
23  Double_t xbins[1001];
24  Int_t k=0;
25  TAxis *axis = h->GetXaxis();
26  for (Int_t i=1;i<=100;i++) {
27  Int_t y = (Int_t)h->GetBinContent(i);
28  if (y <=0) continue;
29  Double_t dx = axis->GetBinWidth(i)/y;
30  Double_t xmin = axis->GetBinLowEdge(i);
31  for (Int_t j=0;j<y;j++) {
32  xbins[k] = xmin +j*dx;
33  k++;
34  }
35  }
36  xbins[k] = axis->GetXmax();
37  //create a variable bin-width histogram out of fix bin histogram
38  //new rebinned histogram should have about 10 entries per bin
39  TH1F *hnew = new TH1F("hnew","rebinned",k,xbins);
40  hnew->FillRandom("gaus",10*nentries);
41 
42  //rebin hnew keeping only 50% of the bins
43  Double_t xbins2[501];
44  Int_t kk=0;
45  for (Int_t j=0;j<k;j+=2) {
46  xbins2[kk] = xbins[j];
47  kk++;
48  }
49  xbins2[kk] = xbins[k];
50  TH1F *hnew2 = (TH1F*)hnew->Rebin(kk,"hnew2",xbins2);
51 
52  //draw the 3 histograms
53  TCanvas *c1 = new TCanvas("c1","c1",800,1000);
54  c1->Divide(1,3);
55  c1->cd(1);
56  h->Draw();
57  c1->cd(2);
58  hnew->Draw();
59  c1->cd(3);
60  hnew2->Draw();
61 }
float xmin
Definition: THbookFile.cxx:93
virtual Double_t GetBinLowEdge(Int_t bin) const
Return low edge of bin.
Definition: TAxis.cxx:504
return c1
Definition: legend1.C:41
THist< 1, float, THistStatContent, THistStatUncertainty > TH1F
Definition: THist.hxx:285
virtual Double_t GetBinContent(Int_t bin) const
Return content of bin number bin.
Definition: TH1.cxx:4770
TVirtualPad * cd(Int_t subpadnumber=0)
Set current canvas & pad.
Definition: TCanvas.cxx:688
1-D histogram with a float per channel (see TH1 documentation)}
Definition: TH1.h:567
int Int_t
Definition: RtypesCore.h:41
virtual void FillRandom(const char *fname, Int_t ntimes=5000)
Fill histogram following distribution in function fname.
Definition: TH1.cxx:3421
Class to manage histogram axis.
Definition: TAxis.h:30
virtual void Draw(Option_t *option="")
Draw this histogram with options.
Definition: TH1.cxx:2974
virtual TH1 * Rebin(Int_t ngroup=2, const char *newname="", const Double_t *xbins=0)
Rebin this histogram.
Definition: TH1.cxx:5759
#define h(i)
Definition: RSha256.hxx:106
The Canvas class.
Definition: TCanvas.h:31
double Double_t
Definition: RtypesCore.h:55
int nentries
Definition: THbookFile.cxx:89
Double_t y[n]
Definition: legend1.C:17
virtual Double_t GetBinWidth(Int_t bin) const
Return bin width.
Definition: TAxis.cxx:526
virtual void Divide(Int_t nx=1, Int_t ny=1, Float_t xmargin=0.01, Float_t ymargin=0.01, Int_t color=0)
Automatic pad generation by division.
Definition: TPad.cxx:1162
Double_t GetXmax() const
Definition: TAxis.h:134
TAxis * GetXaxis()
Get the behaviour adopted by the object about the statoverflows. See EStatOverflows for more informat...
Definition: TH1.h:315