How to make custom binning

ROOT
1

Hi

I am trying to hadd two ttree’s , ( I am using a modified version of $ROOTSYS/tutorials/io/hadd.C) but what I would like to have at the end, is a “merged” TH1F plot what a custom binning, for example specifying that each bin should include at least n events (TAxis for example ?)… Is there any way to do that ?

Thanks in advance

2

Hi,

you could simply post-process the result of the merging of the histograms, and combine adjacent bins until they satisfy your minimum entries requirement.

Cheers, Axel.

3

Hi Axel

Would it be possible to give me a small script how to implement this ?

Thanks in advance

4

So, I was looking on

root.cern.ch/root/html/tutorials … bin.C.html

but could not understand, where you define the #events per bin in the rebinned histo…

Furthermore, I tried to do exactly the same thing to another histo, using the above script, but it failed… You need some .root file ?

Thanks in advance

5

So, I was looking on

root.cern.ch/root/html/tutorials … bin.C.html

but could not understand, where you define the #events per bin in the rebinned histo…

Furthermore, I tried to do exactly the same thing to another histo, using the above script, but it failed…

#include "TH1.h"
#include "TCanvas.h"
#include "TFile.h"
#include "TH1F.h"
#include "TRoot.h"
#include <iostream>

using namespace std;
void rebin() {

   //open a fixed bin histogram
  TFile *  fout = new TFile ("jets.root", "read");

   TH1F *h = (TH1F*) fout->Get("AllJetsPt");
   
   Double_t xbins[100000];
   Int_t k=0;
   TAxis *axis = h->GetXaxis();
   for (Int_t i=1;i<=h->GetNbinsX();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);
    // if (dx>0){
      for (Int_t j=0;j<y;j++) {
         xbins[k] = xmin +j*dx;
         k++;
         cout<<k<<"  "<<xbins[k]<<"  "<<dx<<"  "<<j<<"  "<<j*dx<<"  "<<xmin<<"  "<<h->GetNbinsX()<<endl;
      }
    //  }
   }
   //create a variable binwidth histogram out of fix bin histogram
   
      /
   TH1F *hnew = new TH1F("hnew","rebinned",k-1,xbins);

   hnew->Rebin(k-1,"h",xbins);
   //rebin hnew keepingonly 50% of the bins
   Double_t xbins2[100000];
   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",800,600);
   c1->Divide(1,3);
   c1->cd(1);
   h->Draw();
   c1->cd(2);
   hnew->Draw();
   c1->cd(3);
  // hnew2->Draw();
   
}

Thanks in advance
jets.root (5.35 KB)

6

Hi,
that tutorial creates a binning with on average one entry per bin: a bin with a content of 4 gets split into four bins.

You want to merge bins and that’s different. It’s actually a common problem; I agree that it would be good to provide the optimal bin edges given a histogram - right now ROOT doesn’t. A good implementation isn’t completely trivial; I will post a script that does it once it’s ready.

Cheers, Axel.

7

Actually I am trying something like


TAxis* MergeBins(TH1F* h, Int_t minEntriesPerBin){
  Int_t nbins = (Int_t) (h->Integral() / minEntriesPerBin);

   Double_t xq[nbins+1];
   TAxis *axis = new TAxis(nbins,xq);
   axis = h->GetXaxis();

   Double_t xbins[1000];
   Int_t k=0;
   for (Int_t i=0; i<=nbins; i++)
   xq[i] = ((Double_t) i) / nbins;
  Double_t yq[nbins+1];
  yq[0] = h->GetBinLowEdge(1);
  yq[nbins] = h->GetBinLowEdge(h->GetNbinsX()+1);
  return axis;
}

but does not work as expected, ie merge bins with a requierment of minEvents/bin

Cheers

8

Hi,

TAxis *axis = new TAxis(nbins,xq);copies immediately the content of xq to the TAxis object.
Any modification to xq and yq done after this creation have no effect on the TAxis object.

Philippe.

9

Hi Philippe - So, if I understand well, you suggest something like

TAxis *axis; 
   axis = h->GetXaxis();

   Double_t xbins[1000];
   Int_t k=0;
   for (Int_t i=0; i<=nbins; i++)
   xq[i] = ((Double_t) i) / nbins;
  Double_t yq[nbins+1];
  yq[0] = h->GetBinLowEdge(1);
  yq[nbins] = h->GetBinLowEdge(h->GetNbinsX()+1);
  axis= TAxis(nbins,xq);
  return axis;

it will give correctly the bins I need ?

Thanks in advance!

10

[quote]it will give correctly the bins I need ?[/quote]I still don’t think so, but at least it is now because the algorithm might be wrong and not because there is no information passed to the TAxis.

I have no clue why you hav yq array that you set but do not use.
The xq array seems to ask for evenly spaced bins from 0 to 1.

Philippe.