Re: selection of ntuple and plot in a Graph

Rene Brun (Rene.Brun@cern.ch)
Tue, 20 Apr 1999 14:41:31 +0000

Hi Ulrich.
The number of selectedRows is only computed by TTree::Draw, not by
GetEvent.
If you implement the selection logic yourself, there is no need to call
TTree::Draw. You should modify your program below to set the addresses
for the branches "sy" and "rep" and fill directly the TGraph object.
Below is a sketch of a possible macro.

Rene Brun

{
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 rep;
static Float_t sy;

ntuple->SetBranchAddress("sy",&sy);
ntuple->SetBranchAddress("rep",&rep);
TBranch *brep = ntuple->GetBranch("rep");

Int_t nevent = ntuple->GetEntries();
Int_t nselected = 0;
Float_t repold = -1;
Float_t *gsy = new Float_t[nevent];
Float_t *grep = new Float_t[nevent];

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; //here put the brackets at teh right place!!!
ntuple->GetEvent(i); //read complete accepted event in memory. This
// seems not to mark the event as 'selected'.
{
gsy[nselected] = sy;
grep[nselected] = rep;
nselected++;
}

}
TGraph *graph = new TGraph(nselected, gsy, grep);
graph->Draw("ac*");

}

Ulrich Jost wrote:
>
> 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*");
>
> }