How to calculate the standard deviation over a list of histograms existing in one root file?

ROOT
,
8

Try:

{
  TFile *f = TFile::Open("ppepembbar_4FNS_LO_all_results.root");
  if ((!f) || f->IsZombie()) { delete f; return; }
  gROOT->cd();
  TProfile *hp = 0;
  for (int i = 0; i < 199; i++) {
    TH1 *h; f->GetObject(TString::Format("AFB_Mll_%d", i), h);
    if (h) {
      TAxis *a = h->GetXaxis();
      if (!hp) {
        // ... assuming all "AFB_Mll_*" are "akin" fix bin sizes histograms ...
        hp = new TProfile("AFB_Mll_profile", "AFB Mll profile;X;Y",
                          a->GetNbins(), a->GetXmin(), a->GetXmax(),
                          ""); // e.g., "", "s", "i" or "g"
        if (! hp->GetSumw2N()) hp->Sumw2(kTRUE);
      }
      for (int j = 0; j <= a->GetNbins() + 1; j++)
        hp->Fill(a->GetBinCenter(j), h->GetBinContent(j), h->GetBinError(j));
      delete h;
    }
  }
  delete f;
  if (hp) hp->Draw();
}
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