ROOT   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"
18void 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}
#define h(i)
Definition: RSha256.hxx:106
int Int_t
Definition: RtypesCore.h:45
double Double_t
Definition: RtypesCore.h:59
float xmin
Definition: THbookFile.cxx:95
int nentries
Definition: THbookFile.cxx:91
Class to manage histogram axis.
Definition: TAxis.h:30
Double_t GetXmax() const
Definition: TAxis.h:134
virtual Double_t GetBinLowEdge(Int_t bin) const
Return low edge of bin.
Definition: TAxis.cxx:518
virtual Double_t GetBinWidth(Int_t bin) const
Return bin width.
Definition: TAxis.cxx:540
The Canvas class.
Definition: TCanvas.h:23
1-D histogram with a float per channel (see TH1 documentation)}
Definition: TH1.h:575
virtual void FillRandom(const char *fname, Int_t ntimes=5000, TRandom *rng=nullptr)
Fill histogram following distribution in function fname.
Definition: TH1.cxx:3525
virtual void Draw(Option_t *option="")
Draw this histogram with options.
Definition: TH1.cxx:3073
virtual TH1 * Rebin(Int_t ngroup=2, const char *newname="", const Double_t *xbins=0)
Rebin this histogram.
Definition: TH1.cxx:6222
return c1
Definition: legend1.C:41
Double_t y[n]
Definition: legend1.C:17
const double xbins[xbins_n]