edit_histo.C

/* A function to read from a root file
   and save an edited version of one of 
   the histograms 

   gary.smith@ed.ac.uk
   01 Feb 2021
   
   This program was written for the EdMedPhysics 
   projects in 2021 to assist with data analysis.
   
   Input:
   A root file which is the output from
   the Geant4 simulation.
   
   Output: 
   1) a pdf file of a histogram with edited axis labels
   and a user-defined x-axis maximum; 
   2) a root file containing the same   
   
   How to run:
   
   From terminal command line
   $ root edit_histo.C
   
   From the root prompt
   $ root
   [0] .x edit_histo.C
   Or, setting x_max = 10 upon execution
   [0] .L edit_histo.C
   [1] edit_histo(10) 
   
*/

// Function declaration with optional argument
void edit_histo(double x_max = -1.){

  // The name of the file to input.
  // Ie the simulation output file name.
  char filename[128] = "EdMedPhysics.root";

  // Declare a TFile object to read in data from.
  // Use the filename character array from above.
  TFile * input_file = new TFile(filename,"");
  
  // Connect to a histogram in the file.
  // https://root.cern.ch/doc/master/classTH1D-members.html
  TH1D * Edep_vs_Z = (TH1D*)input_file->Get("Edep_vs_z"); 
  
  // Re-set the histogram object name.
  // This is not a title.
  Edep_vs_Z->SetName("Edep_vs_Z");

  // Fix the histogram title and axis labels.
  Edep_vs_Z->SetTitle(";Z in phantom (mm);Accumulated energy deposited (MeV)");
  
  // Fix the x-axis range.
  // Stay in the while loop until a valid answer is given.
  // (NB there is an option to skip this by setting the value as an 
  // input argument to this function - see function declaration at top)
  while(x_max < 0 || x_max > 500.){
    
    // Print a question to the user. IE to yourself.
    std::cout << std::endl;
    std::cout << " What do you want to set the x range maximum to (in cm)? " << std::endl;
    std::cout << " Enter a value between 0 and 50, e.g. for 12 cm enter 12." << std::endl;
    std::cout << " \t";
    
    // Read the user's answer in.
    std::cin >> x_max;
    
    // Scale from cm to mm.
    x_max *= 10.; 
  }
  std::cout << std::endl;
  
  // Set the x-axis range to the value specified above.
  // NB this applies to the pdf, the root file version
  // retains all of the data
  Edep_vs_Z->GetXaxis()->SetRangeUser(-5,x_max);

  // The stats box can be modified.
  // (https://root.cern.ch/doc/master/classTPaveStats.html)
  //
  // Draw stats box with: Entries and Integral only.
  // Entries is the number of hits in which some energy was deposited.
  // Intergal is total energy deposited in the phantom during the entire run.
  gStyle->SetOptStat(1000010);
  
  // To not draw stats box,
  // uncomment the line below.
  //gStyle->SetOptStat(0);
  
  // Dont show the title. 
  // To include the title, 
  // comment in the line below by putting // at the start of the line.
  gStyle->SetOptTitle(0);
  
  // Set max no. of digits on axes.
  // Display large numbers as exponentials 
  // Comment in the line below to implement by removing the //
  //TGaxis::SetMaxDigits(2);
  
  // Create a canvas to draw on
  // and later to save as a pdf
  TCanvas canvas;
  
  // Draw histogram as a simple histogram 
  // with no error bars.
  // This will only apply to the pdf image.
  Edep_vs_Z->Draw("hist");

  // Save the canvas as a pdf...
  canvas.SaveAs("my_edited_histo.pdf");

  // ... and also as a root file.
  Edep_vs_Z->SaveAs("my_edited_histo.root");
   
  // NB Several other file types can be specified
  // including image files (.png,.eps,.jpg); or even a .C macro.
  
  input_file->Close();

  // Quit root and return to the terminal command prompt.
  gApplication->Terminate();
  
  // "Wouldn't you like to be a pepper too?"
}