Trouble with 2D array branch

ROOT
1

Hi,

I am trying store invariant mass combinations by using a 2D array as a branch

myTree->Branch("singlemuid_massCBOS", &Mysinglemuid_massCBOS, "singlemuid_massCBOS[mu_muid_n][mu_muid_n]/F");
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for (int i=0; i<mu_muid_n;++i){
       for (int j=i+1; j<mu_muid_n; ++j){
          /*TLorentzVector v1 = lv(Mymu_muid_pt[i],Mymu_muid_eta[i],Mymu_muid_phi[i]);
          TLorentzVector v2 = lv(Mymu_muid_pt[j],Mymu_muid_eta[j],Mymu_muid_phi[j]);
          TLorentzVector vSum = v1 + v2;
          Mymuid_massCB[index] = vSum.M()/1000.;*/


          if ((Mymu_muid_charge[i]*Mymu_muid_charge[j]) < 0){
          TLorentzVector v1OS = lv(Mymu_muid_pt[i],Mymu_muid_eta[i],Mymu_muid_phi[i]);
          TLorentzVector v2OS = lv(Mymu_muid_pt[j],Mymu_muid_eta[j],Mymu_muid_phi[j]);
          TLorentzVector vSumOS = v1OS + v2OS;

          //2D array that stores combinations of OS invMass 
          Mysinglemuid_massCBOS[i][j] = vSumOS.M()/1000.;
          }
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   myTree->Fill() ;

If I std::cout these values, they look fine. However, when I write them to a branch and try to draw, the histogram shows only values of 0. The same information written to a 1D array works fine. Does anyone know why 2D array branches won’t fill properly?

2

Hi,

The leaflist branch creation technique (the one you used) only support one variable array dimension. Instead you will either need to store a 1D array (and address it as a 2D array) or use a std:vector<std::vector > (for which you will need to provide a dictionary).

Cheers,
Philippe.

3

Thanks for your reply. This seems to work. But now I am trying to read this information into a small tree. After setting the branch address, I assume I can read the information with the syntax

std::vector< std::vector<float> > *muon_vec  ;
muon_vec->at(j)[k] ;

correct?

-Thomas

4

I seem to be having trouble properly accessing the 2D vector elements I previously wrote to a tree. I tried using the method

,where my event loop is over the exact same variables as I did to fill the original vector,
but if I cout << myMuon_eta << endl;, the output is garbage and doesn’t agree with the original eta distribution that I am trying to read in.

5

[quote]
std::vector< std::vector > *muon_vec ;
muon_vec->at(j)[k] ;
correct?[/quote]No, you must initialize the pointer. So something like: std::vector< std::vector<float> > *muon_vec = 0; tree->SetBranchAddress(branch_name,&muon_vec); for () { ... call GetEntry ... muon_vec->at(j)[k] ; }Cheers,
Philippe.