# Converting a variable bin with histo into a fixed width histo

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

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(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->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);
}
``````
2 Likes

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

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.

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

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