1st working code!!!

TreeTest.C

+
+
+TFile* TPfile = new TFile("TP.root","update"); // Declared globally coz TPfile is closed in ain function after going through all the pixels
+TList* TPlist = new TList;
+
+void treeWrite(int c, int r){
+
+  int TPcol1, TPcol2, TPcol3;
+  double tot, tot_var, toa, toa_var,Ecol1,Ecol2,Ecol3,Ecol4;
+  string line_TP;
+  
+  std::vector<double>Energy_e = {200,300,400,500,600,700,800,900,1000,1100,1200,
+                                1300,1400,1500,1700,2200,2700,3200,4000,5000,6000,
+				7000,8000,9000,10000,11000,12000,13000,14000};
+  
+  //Creating TProot file with a TP tree
+  
+  //  TFile* TPfile = new TFile("TP.root","update");
+  std::string TPtreeName = "TPtree" + std::to_string(c) + "_" + std::to_string(r); 
+  TTree* TPtree = new TTree(TPtreeName.c_str()," TP tree");
+  // TList* list = new TList;
+
+  
+  TPtree->Branch("TPcol1",&TPcol1,"TPcol1/I");
+  TPtree->Branch("TPcol2",&TPcol2,"TPcol2/I");
+  TPtree->Branch("TPcol3",&TPcol3,"TPcol3/I");
+  TPtree->Branch("tot",&tot,"tot/D");
+  TPtree->Branch("tot_var",&tot_var,"tot_var/D");
+  TPtree->Branch("toa",&toa,"toa/D");
+  TPtree->Branch("toa_var",&toa_var,"toa_var/D");
+
+  //Creating second root file(Energy_table.dat) with a different tree
+  
+  /* TFile* friend = new TFile("TPtreeFriend.root","recreate");
+     TTree* Frirendtree = new TTree("Friendtree","friend tree");
+     Friendtree->Branch("Ecol4",&Ecol4,"Ecol4/D");*/
+
+  //Filling the trees
+
+  cout << "The tree is filled with values of col " << c << " and row "<< r << std::endl;
+  
+  for(int i= 0; i < Energy_e.size(); i++){
+    stringstream Sfile;
+    Sfile << Energy_e.at(i) << ".dat";
+    std::vector<std::string>Filenames = {std::string("tp_dat_" + Sfile.str())};
+
+    for(int j = 0; j < Filenames.size(); j++){ 
+      std::ifstream TPdata_file(Filenames.at(j).c_str());
+      
+      if(TPdata_file.is_open()){
+	std::cout << " The data file" <<  Filenames.at(j) << " is opened ..." <<std::endl;
+	
+	while(getline(TPdata_file,line_TP)){
+	  if(line_TP.rfind("#", 0) == 0) {
+	    continue;
+	  }
+	  istringstream ssTP(line_TP);
+	  ssTP >> TPcol1 >> TPcol2 >> TPcol3 >> tot >> tot_var >> toa >> toa_var;
+	  if(TPcol1==c && TPcol2 ==r){
+	    TPtree->Fill();
+	    break;
+	  }
+	}  //While loop getline
+      }   //if statement TPdatafile open
+    }    // for loop j filenames
+  }     // if statement energy_e.size()
+
+  // TPtree->Scan(); //Displays the data corresponding to all energy values of each pixel 
+  TPlist->Add(TPtree);
+  // list->Print(); //Prints the contents of the list
+  // std::cout <<" The size of the tree list is " << list->GetSize() << std::endl; // list size = 1, not growing!! 
+  TTree* newtree = TTree::MergeTrees(TPlist); 
+  newtree->SetName("MergedTree");
+  newtree->Write("",TObject::kWriteDelete);
+  //TPfile->Close();
+}
+
+
+void TreeTest(){
+  
+  int col, row ;
+  
+  for(col=0;col<2;col++){
+    for(row=0;row<2;row++){ 
+      treeWrite(col,row);
+    }
+  }
+
+  std::cout <<" The size of the tree list is " << TPlist->GetSize() << std::endl;
+  TPfile->Close();
+  //  treeRead();
+
+}