[TDF] Add ranges tutorial

tutorials/dataframe/tdf006_ranges.C

+/// \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;
+}