{
//________________________________________________________________________________
/*
  makeCovMatrix.C
  
  A simple example ROOT script which turns a hard-coded matrix in the script (tDat)
  into a small ROOT file with just the matrix. The covariance matrix ROOT file
  is used as an input to grwghtnp, the covariance reweighting routine.

  The error matrix provided is for reweighting the four z-expansion parameters
  for the axial form factor. These values are UNPHYSICAL and are only for demonstration.

  Typical numerical values are included in Phys.Rev.D 93, 113015
  e-Print arXiv: 1603.03048 [hep-ph], which is calculated from combining
  Eqs. (32) and (33) to form an error matrix.

 */
#include <TMatrixD.h>

  int nP = 4; // matrix size
  TFile *fMat = new TFile("tmat.out.root","recreate");

  // UNPHYSICAL covariance matrix
  double tDat[nP][nP] =
    {{0.0154, 0.0, 0.0, 0.0},
     {0.0, 1.08, 0.0, 0.0},
     {0.0, 0.0, 6.54, 0.0},
     {0.0, 0.0, 0.0, 7.40} };

  // flatten matrix into 1-D array
  double tFlt[nP* nP] = {0.};
  for (int i=0;i<nP;i++) {
  for (int j=0;j<nP;j++) {
    if (tFlt[j*nP+i] != tDat[i][j] && i > j)
    {
      std::cout<< "Matrix needs to be symmetric!" << std::endl;
      std::cout<< i<<","<<j<<": "<<tFlt[j*nP+i] <<","<< tDat[i][j]<<std::endl;
      exit(1);
    }
    tFlt[i*nP+j] = tDat[i][j];
  }}
  
  // make into TMatrixD object and write to file
  TMatrixD tMat(nP,nP);
  tMat.SetMatrixArray(tFlt);
  tMat->Write("tMat",TObject::kOverwrite);

  fMat->Close();
  exit(0);
}