{
// >> example1.C
// takes a dipole sample and a nominal z-expansion sample
//  and plots histograms of Q2 to compare
//
gStyle->SetOptStat(0); //< remove statistics box

// import the gst files and the weight file
//
TFile* f1  = TFile::Open("gntp.1.gst.root"); //< zexp file
TFile* f2  = TFile::Open("gntp.ma135.gst.root"); //< dipole file

// extract the trees from the files
//
TTree* t1 = (TTree*) f1->Get("gst"); //< GENIE summary tree
TTree* t2 = (TTree*) f2->Get("gst"); //< to find this, try f1->ls()

// create histograms to fill
//
int nbins=8; //< number of bins in histograms
TH1F* hz = new TH1F("hz","",nbins,0,1);  //< bins in range (0,1), zexp sample
TH1F* hd = new TH1F("hd","",nbins,0,1);  //< to hold dipole
hz->Sumw2();
hd->Sumw2();

// create objects to hold data when loading
//
double q2z;
double q2d;

// set trees to point to data objects
// > now when GetEntry is called, it puts the data in the variables below
//
t1->SetBranchAddress("Q2",&q2z);
t2->SetBranchAddress("Q2",&q2d);

// loop over and read the trees
//
for(int i=0;i<t1->GetEntries();i++){
 t1->GetEntry(i);
 t2->GetEntry(i);

 // fill the histograms with the data
 hz->Fill(q2z);
 hd->Fill(q2d);
}

// create canvas to plot on
//
TCanvas* c1 = new TCanvas("c1","");

// plot the histograms
//
hz->GetXaxis()->SetTitle("Q^{2}");
hz->SetLineColor(kRed);  //< zexp is red
hz->Draw("");
hd->SetLineColor(kBlue); //< dipole is blue
hd->Draw("psame");

}