df007_snapshot.C File Reference

Detailed Description

Write ROOT data with RDataFrame.

This tutorial shows how to write out datasets in ROOT format using RDataFrame.

 
// A simple helper function to fill a test tree: this makes the example
// stand-alone.
void fill_tree(const char *treeName, const char *fileName)
{
   ROOT::RDataFrame d(10000);
   int i(0);
   d.Define("b1", [&i]() { return i; })
      .Define("b2",
              [&i]() {
                 float j = i * i;
                 ++i;
                 return j;
              })
      .Snapshot(treeName, fileName);
}
 
int df007_snapshot()
{
   // We prepare an input tree to run on
   auto fileName = "df007_snapshot.root";
   auto outFileName = "df007_snapshot_output.root";
   auto outFileNameAllColumns = "df007_snapshot_output_allColumns.root";
   auto treeName = "myTree";
   fill_tree(treeName, fileName);
 
   // We read the tree from the file and create a RDataFrame
   ROOT::RDataFrame d(treeName, fileName);
 
   // ## Select entries
   // We now select some entries in the dataset
   auto d_cut = d.Filter("b1 % 2 == 0");
   // ## Enrich the dataset
   // Build some temporary columns: we'll write them out
   auto d2 = d_cut.Define("b1_square", "b1 * b1")
                .Define("b2_vector",
                        [](float b2) {
                           std::vector<float> v;
                           for (int i = 0; i < 3; i++)
                              v.push_back(b2 * i);
                           return v;
                        },
                        {"b2"});
 
   // ## Write it to disk in ROOT format
   // We now write to disk a new dataset with one of the variables originally
   // present in the tree and the new variables.
   // The user can explicitly specify the types of the columns as template
   // arguments of the Snapshot method, otherwise they will be automatically
   // inferred.
   d2.Snapshot(treeName, outFileName, {"b1", "b1_square", "b2_vector"});
 
   // Open the new file and list the columns of the tree
   TFile f1(outFileName);
   auto t = f1.Get<TTree>(treeName);
   std::cout << "These are the columns b1, b1_square and b2_vector:" << std::endl;
   for (auto branch : *t->GetListOfBranches()) {
      std::cout << "Branch: " << branch->GetName() << std::endl;
   }
   f1.Close();
 
   // We are not forced to write the full set of column names. We can also
   // specify a regular expression for that. In case nothing is specified, all
   // columns are persistified.
   d2.Snapshot(treeName, outFileNameAllColumns);
 
   // Open the new file and list the columns of the tree
   TFile f2(outFileNameAllColumns);
   t = f2.Get<TTree>(treeName);
   std::cout << "These are all the columns available to this dataframe:" << std::endl;
   for (auto branch : *t->GetListOfBranches()) {
      std::cout << "Branch: " << branch->GetName() << std::endl;
   }
   f2.Close();
 
   // We can also get a fresh RDataFrame out of the snapshot and restart the
   // analysis chain from it. The default columns are the ones selected.
   // Notice also how we can decide to be more explicit with the types of the
   // columns.
   auto snapshot_df = d2.Snapshot<int>(treeName, outFileName, {"b1_square"});
   auto h = snapshot_df->Histo1D();
   auto c = new TCanvas();
   h->DrawClone();
 
   return 0;
}

Date

April 2017

Author

Danilo Piparo (CERN)

Definition in file df007_snapshot.C.