tdf006_ranges.C

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/// \file
/// \ingroup tutorial_tdataframe
/// \notebook -nodraw
/// This tutorial shows how to express the concept of ranges when working with the TDataFrame.
/// \macro_code
///
/// \date March 2017
/// \author Danilo Piparo

#include "TFile.h"
#include "TH1F.h"
#include "TTree.h"

#include "ROOT/TDataFrame.hxx"

// A simple helper function to fill a test tree: this makes the example
// stand-alone.
void fill_tree(const char *filename, const char *treeName)
{
   TFile f(filename, "RECREATE");
   TTree t(treeName, treeName);
   int b1;
   float b2;
   t.Branch("b1", &b1);
   t.Branch("b2", &b2);
   for (int i = 0; i < 100; ++i) {
      b1 = i;
      b2 = i * i;
      t.Fill();
   }
   t.Write();
   f.Close();
   return;
}

int tdf006_ranges()
{

   // We prepare an input tree to run on
   auto fileName = "tdf006_ranges.root";
   auto treeName = "myTree";
   fill_tree(fileName, treeName);

   // We read the tree from the file and create a TDataFrame.
   ROOT::Experimental::TDataFrame d(treeName, fileName);

   // ## Usage of ranges
   // Now we'll count some entries using ranges
   auto c_all = d.Count();

   // This is how you can express a range of the first 30 entries
   auto d_0_30 = d.Range(0, 30);
   auto c_0_30 = d_0_30.Count();

   // This is how you pick all entries from 15 onwards
   auto d_15_end = d.Range(15, 0);
   auto c_15_end = d_15_end.Count();

   // We can use a stride too, in this case we pick an event every 3
   auto d_15_end_3 = d.Range(15, 0, 3);
   auto c_15_end_3 = d_15_end_3.Count();

   // The Range is a 1st class citizen in the TDataFrame graph:
   // not only actions (like Count) but also filters and new columns can be added to it.
   auto d_0_50 = d.Range(0, 50);
   auto c_0_50_odd_b1 = d_0_50.Filter("1 == b1 % 2").Count();

   // An important thing to notice is that the counts of a filter are relative to the
   // number of entries a filter "sees". Therefore, if a Range depends on a filter,
   // the Range will act on the entries passing the filter only.
   auto c_0_3_after_even_b1 = d.Filter("0 == b1 % 2").Range(0, 3).Count();

   // Ok, time to wrap up: let's print all counts!
   cout << "Usage of ranges:\n"
        << " - All entries: " << *c_all << endl
        << " - Entries from 0 to 30: " << *c_0_30 << endl
        << " - Entries from 15 onwards: " << *c_15_end << endl
        << " - Entries from 15 onwards in steps of 3: " << *c_15_end_3 << endl
        << " - Entries from 0 to 50, odd only: " << *c_0_50_odd_b1 << endl
        << " - First three entries of all even entries: " << *c_0_3_after_even_b1 << endl;

   return 0;
}