Converting a variable bin with histo into a fixed width histo

[Ankur_Nath]

I have a histogram readable from a root file. This histogram has variable bin width over the x-range of 0-20 GeV. The bin width for the ranges 0-5 GeV, 5-10 GeV and 10-20 GeV are 0.5 GeV, 1GeV and 2 GeV respectively; viz. bin centered at 0.25, 0.75, 1.25…,3.75, 4.25, 4.75, 5.5, 6.5, 7.5, 8.5, 9.5, 11, 13, 15, 17 and 19 GeV consecutively.

I want to convert this histogram into a constant bin width of 0.05GeV over the whole range of 0-20 GeV and update the histogram back into the root file. This process will affect the BinContent too. How can I do that?

The BinCenter and the BinContent are as follows:

BinCenter BinContent
0.25; 0.141466
0.75; 0.300435
1.25; 0.356509
1.75; 0.327003
2.25; 0.277435
2.75; 0.215133
3.25; 0.165415
3.75; 0.131069
4.25; 0.115133
4.75; 0.102666
5.5; 0.094292
6.5; 0.0852912
7.5; 0.0750423
8.5; 0.0652837
9.5; 0.0524278
11; 0.0354956
13; 0.0196348
15; 0.0117284
17; 0.00716654
19; 0.00435495

[Wile_E_Coyote]

What you are trying to do doesn’t really make sense but here it is:

{
  TH1D *h_wide = new TH1D("h_wide", "wide bins;X;Y", 40, 0., 20.);
  h_wide->FillRandom("pol1", 10000);
  TH1D *h_narrow = new TH1D("h_narrow", "narrow bins;X;Y", 400, 0., 20.);
  //
#if 1 /* 0 or 1 */
  // note: we assume that the X-axis range of both histograms is the
  //       same and all bin edges of "h_wide" correspond to some bin
  //       edges of "h_narrow" and we neglect bin errors of "h_wide"
  h_narrow->AddBinContent(0, h_wide->GetBinContent(0)); // underflow bin
  h_narrow->AddBinContent(h_narrow->GetNbinsX() + 1, // overflow bin
                          h_wide->GetBinContent(h_wide->GetNbinsX() + 1));
  for (Int_t i = 1; i <= h_narrow->GetNbinsX(); i++) {
    Int_t b = h_wide->FindFixBin(h_narrow->GetBinCenter(i));
    Double_t w = h_narrow->GetBinWidth(i) / h_wide->GetBinWidth(b);
    h_narrow->AddBinContent(i, w * h_wide->GetBinContent(b));
  }
  h_narrow->ResetStats(); // reset the statistics including the number of entries
#else /* 0 or 1 */
  // note: the X-axis ranges and bin edges do not need to correspond
  //       but the contents of "h_narrow" will have some "random jitter"
  Int_t n = 10000;
  h_narrow->FillRandom(h_wide, n * h_wide->GetEntries());
  h_narrow->Scale(1. / n);
  h_narrow->Sumw2(kFALSE);
  h_narrow->ResetStats(); // reset the statistics including the number of entries
#endif /* 0 or 1 */
  //
  TCanvas *c = new TCanvas("c", "c");
  c->Divide(1, 2);
  c->cd(1); h_wide->Draw();
  c->cd(2); h_narrow->Draw();
  c->cd(0);
}

[Ankur_Nath]

That will serve the purpose. I would like to discuss in detail what I propose to do. May I have your email?

Thanks & Regards,
Ankur

[Wile_E_Coyote]

Note that the proper approach would be to ask the one who created and filled these histograms to provide you their new versions with fine binning.

[Ankur]

Yes, the published results are with finer binning. Thank you. I will do that.