Notebook

df009_FromScratchVSTTree¶

Compare creation of a ROOT dataset with RDataFrame and TTree.

This tutorial illustrates how much simpler it can be to use a RDataFrame to create a dataset with respect to the usage of the TTree interfaces.

Author: Danilo Piparo (CERN)
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Thursday, March 23, 2023 at 10:44 AM.

This is the classic way of creating a ROOT dataset¶

The steps are:

Create a file
Create a tree associated to the file
Define the variables to write in the entries
Define the branches associated to those variables
Write the event loop to set the right value to the variables
Call TTree::Fill to save the value of the variables
Write the TTree
Close the file

In [1]:

%%cpp -d
void classicWay()
{
   TFile f("df009_FromScratchVSTTree_classic.root", "RECREATE");
   TTree t("treeName", "treeName");
   double b1;
   int b2;
   t.Branch("b1", &b1);
   t.Branch("b2", &b2);
   for (int i = 0; i < 10; ++i) {
      b1 = i;
      b2 = i * i;
      t.Fill();
   }
   t.Write();
   f.Close();
}

This is the RDF way of creating a ROOT dataset¶

Few lines are needed to achieve the same result. Parallel creation of the TTree is not supported in the classic method. In this case the steps are:

Create an empty RDataFrame
If needed, define variables for the functions used to fill the branches
Create new columns expressing their content with lambdas, functors, functions or strings
Invoke the Snapshot action

Parallelism is not the only advantage. Starting from an existing dataset, filtering it, enriching it with new columns, leaving aside some other columns, and writing a new dataset become very easy to do.

In [2]:

%%cpp -d
void RDFWay()
{
   ROOT::RDataFrame df(10);
   auto b = 0.;
   df.Define("b1", [&b]() { return b++; })
     .Define("b2", "(int) b1 * b1")  This can even be a string
     .Snapshot("treeName", "df009_FromScratchVSTTree_df.root");
}

input_line_44:6:36: error: expected ';' after expression
     .Define("b2", "(int) b1 * b1")  This can even be a string
                                   ^
                                   ;
input_line_44:6:38: error: unknown type name 'This'
     .Define("b2", "(int) b1 * b1")  This can even be a string
                                     ^
input_line_44:6:46: error: expected ';' at end of declaration
     .Define("b2", "(int) b1 * b1")  This can even be a string
                                             ^
                                             ;

In [3]:

classicWay();
RDFWay();

input_line_46:2:3: error: use of undeclared identifier 'RDFWay'
 (RDFWay())
  ^
Error in <HandleInterpreterException>: Error evaluating expression (RDFWay())
Execution of your code was aborted.