selection of ntuple and plot in a Graph

From: Ulrich Jost (jost@evalo1.ific.uv.es)
Date: Tue Apr 20 1999 - 10:57:30 MEST 

Hi,

I try to make a selection in an ntuple and plot the surviving points in a
graph, following some examples on the web. In the macro below the entire
ntuple is plotted, ntuple->GetSelectedRows() gives 0. So how do I make the
events 'selected'?
We still have version 2.00/13. 

Thanks, Ulrich


{
gROOT->Reset();

TFile* file = new TFile("data/test_repVI_17.4.1999_15:36:54.root", 
			"read");
TNtuple* ntuple = (TNtuple*) file->Get("ntuple");
TCanvas *canvas = new TCanvas("canvas", "ROOT Canvas", 500, 500);

static Float_t r;

TBranch *brep = ntuple->GetBranch("rep");
brep->SetAddress(&r);

Int_t nevent = ntuple->GetEntries();
Int_t nselected = 0;
Float_t repold = -1;

for (Int_t i=0; i<nevent; i++) {
  brep->GetEvent(i);    // read branch 'rep' only
  if (r == repold)    // make some cuts, whatever
    continue;
  else
    repold = r;
  ntuple->GetEvent(i);  //read complete accepted event in memory. This
                        // seems not to mark the event as 'selected'.
  nselected++;
}

cout<<"GetSelectedRows: "<<ntuple->GetSelectedRows()<<endl;
ntuple->SetEstimate(ntuple->GetSelectedRows());
ntuple->Draw("sy:rep", "", "goff");
TGraph *graph = new TGraph(ntuple->GetSelectedRows(),
			   ntuple->GetV2(), ntuple->GetV1());
graph->Draw("ac*");

}

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