make_read_TTree.txt

//Shiv Subedi
//This snippet of code is written to demonstrate how to write a TTree in a .root file and read it later


//A. For writing TTree and root file


//1. defining the root file
TFile *root_file = new TFile ("name_of_root_file.root", "RECREATE");
TTree *My_tree= new TTree("My_tree","tree with events");  //creating a tree



//2. Define the branches in TTree
Double_t Branch1, Branch2;
My_tree->Branch("Branch1", &Branch1);
My_tree->Branch("Branch2", &Branch2);
... (Do this for as many branches you want to make) ...

//3. Assign data values to the branches in a loop and fill the TTree
for (Int_t i=0; i<n; i++)
{
 datafile>>col[i];
 Branch1 = col[i];

 My_tree->Fill(); //filling the tree with parameters
}

//4. Write the TTree
My_tree->Write();

//5. Close the root file and datafile
root_file->Close();
datafile.close();


//***************************************************************************************************

B. For Reading the root file and TTree

//1. Define the path to the root file
TFile *my_root_file = new TFile("Path of the root file");

//2. Get the tree defined previously
TTree *my_tree = (TTree*)my_root_file -> Get("My_tree");

//3. Set the address of the branches defined previously
Double_t Branch1, Branch2;
my_tree->SetBranchAddress("Branch1", &Branch1);
my_tree->SetBranchAddress("Branch2", &Branch2);

//4. Get the total entries in the root file
Int_t  n_entries = my_tree-> GetEntries();

//5. For accessing the entries of the root file, loop over to n_entries and Get entry
//If want to fill histogram of two branches, defile the histogram here
for (Int_t j = 1; j<=n_entries; j++)
{
  my_tree->GetEntry(j);
  hist->Fill(Branch1,Branch2);
}

//6. Close the TFile
my_root_file->Close();





//******************SOME AUTOMATE TIPS in writing root file and TTree**************************

//Let's say you have a case where you have several data files
//And you want to make each data file as one branch in a root file
//In such a case, we follow follow steps:

//a. Make a ".txt" file with information of the run number of data files in a single row
eg: for runs names "runLG001", "runLG002" etc, text file can include information of 001, 002 etc.
However, to not be misread as an octal number, defile these run# as 'TString'

//b. Open the ".txt" file as follows:
ifstream run_list;
run_list.open(Form("path to text file"));

//c. for automating purpose, you can also give a common path to the folder including all the data runs
TString dir_path = Form("path to the base directory");

//d. Run the loop to read the ".txt"  file which runs along for 1 row
runlist>>run1>>run2;

//e. Inside the loop, for each data runs, open the datafile as follows:
ifstream data_run1;
data_run1.open(Form("%s/runLG%s.Spe",dir_path.Data(), run1.Data()));

//f. for the data in each of the datafiles (eg: data_file1), fill it inside the branch of a TTree


//g. Defining tree name for successive data files (automate)
char data_filename[runs];  //While in loop, runs defines the number of rows of datafile
sprintf(data_filename,"res_scan%s_Ener%s.root",scan_no.Data(), energy.Data());
TFile *root_file = new TFile (data_filename, "RECREATE");
TTree *my_tree= new TTree("Name of tree","tree with events");  //creating a tree



//***************SOME AUTOMATE TIPS in reading tree **********************************
//If you want to automate, reading information from each branch in a loop and plotting histograms
//follow following steps

//a. Follow steps 1-4 of reading a TTree file above

//b. Get the list of all the branches as follows:
TObjArray *branchList;
branchList  = my_tree->GetListOfBranches();

//c. Get the total number of branches as follows:
Int_t nBranch     = my_tree->GetNbranches();
cout<<"No_of_branches: "<<nBranch<<endl;

//d. Define the following variables
TString varnames[nBranch];
TLeaf *var[nBranch];
TH1D *h1_master[nBranch];  //for drawing histograms of each branch
char name[nBranch];
char title[nBranch];
Double_t value_br [n_chan]; //n_chan is the total number of channels

//e. Run a loop of variable 'br' through 'nBranch' and operate following inside the loop
varnames[br] = branchList->At(br)->GetName();
var[br] = my_tree->GetBranch(varnames[br])->GetLeaf(varnames[br]);

sprintf(name,"hist_branch%d",br);  //automate name of histogram for each branch
sprintf(title,"Overall #gamma spectrum for %s",varnames[br].Data()); //automate title of histogram for each branch
h1_master[br] = new TH1D(name,title,n_chan,0,n_chan-1);  //defile the histogram for each branch


  //Inside the loop of number e, run a loop of the entries (ent) goint to max number of entries (n_entries)
  my_tree-> GetEntry(ent);
  value_br[ent] = var[br]->GetValue();
  cout<<Channel<<"     "<<varnames[br]<<"        "<<value_br[ent]<<endl;
  h1_master[br] -> SetBinContent(Channel, value_br[ent]);


//f. Later, running a different loop through 'nBranch', above histograms 'h1_master[br]' can be drawn for each branch