{
// >> example2_1.C
// takes a dipole sample reweighted with grwght1p (systematic named AxFFCCQEshape)
//  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
TFile* f2w = TFile::Open("wght.ma135.dpl.root"); //< dipole weight file

// extract the trees from the files
//
TTree* t1 = (TTree*) f1->Get("gst"); //< GENIE summary tree
TTree* t2 = (TTree*) f2->Get("gst");
// weight file has a different tree within it
// tree name is same as systematic which was reweighted
// to find this, try f2w->ls()
TTree* w2 = (TTree*) f2w->Get("AxFFCCQEshape");

// 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* hdr = new TH1F("hdr","",nbins,0,1); //< to hold reweighted dipole
TH1F* hdu = new TH1F("hdu","",nbins,0,1); //< to hold unweighted dipole
hz->Sumw2();
hdr->Sumw2();
hdu->Sumw2();

double wsum = 0; //< hold the sum of the weights

// create objects to hold data when loading
//
double q2z;
double q2d;
TArrayF* wght;
TArrayF* twk;

// 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);
w2->SetBranchAddress("weights",&wght); //< array of weights; i=0->garbage; i=1->dipole; i=2->zexp;
w2->SetBranchAddress("twkdials",&twk); //< array of tweak dials; i=0->dipole; i=1->zexp;

// loop over and read the trees
//
for(int i=0;i<t1->GetEntries();i++){
 t1->GetEntry(i);
 t2->GetEntry(i);
 w2->GetEntry(i);

 // fill the histograms with the data
 hz->Fill(q2z);
 hdu->Fill(q2d); //< unweighted dipole histogram
 hdr->Fill(q2d,wght.At(2)); //< fill the dipole histogram with the weighted value
 wsum += wght.At(2);
}

// scale reweighted dipole by the average weight
//
hdr->Scale(t2->GetEntries()/wsum);

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

// plot the histograms
//
hz->GetXaxis()->SetTitle("Q^{2}");
hz->SetLineColor(kRed); //< zexp nominal is red
hz->Draw("");
hdr->SetLineColor(kBlue); //< reweighted dipole is blue
hdr->Draw("psame");
hdu->SetLineColor(kBlack); //< unweighted dipole is black
hdu->Draw("psame");

}