Df 0 0 1_Introduction

This tutorial illustrates the basic features of the RDataFrame class, a utility which allows to interact with data stored in TTrees following a functional-chain like approach.

Author: Enrico Guiraud
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Sunday, January 19, 2020 at 12:44 AM.

Preparation

A simple helper function to fill a test tree: this makes the example stand-alone.

In [ ]:
%%cpp -d
void fill_tree(const char *treeName, const char *fileName)
{
   ROOT::RDataFrame d(10);
   int i(0);
   d.Define("b1", [&i]() { return (double)i; })
      .Define("b2",
              [&i]() {
                 auto j = i * i;
                 ++i;
                 return j;
              })
      .Snapshot(treeName, fileName);
}

We prepare an input tree to run on

In [ ]:
auto fileName = "df001_introduction.root";
auto treeName = "myTree";
fill_tree(treeName, fileName);

We read the tree from the file and create a rdataframe, a class that allows us to interact with the data contained in the tree. We select a default column, a branch to adopt ROOT jargon, which will be looked at if none is specified by the user when dealing with filters and actions.

In [ ]:
ROOT::RDataFrame d(treeName, fileName, {"b1"});

Operations on the dataframe

We now review some actions which can be performed on the data frame. All actions but ForEach return a TActionResultPtr. The series of operations on the data frame is not executed until one of those pointers is accessed. If the Foreach action is invoked, the execution is immediate. But first of all, let us we define now our cut-flow with two lambda functions. We can use free functions too.

In [ ]:
auto cutb1 = [](double b1) { return b1 < 5.; };
auto cutb1b2 = [](int b2, double b1) { return b2 % 2 && b1 < 4.; };

Count action

The Count allows to retrieve the number of the entries that passed the filters. Here we show how the automatic selection of the column kicks in in case the user specifies none.

In [ ]:
auto entries1 = d.Filter(cutb1) // <- no column name specified here!
                   .Filter(cutb1b2, {"b2", "b1"})
                   .Count();

std::cout << *entries1 << " entries passed all filters" << std::endl;

Filters can be expressed as strings. the content must be c++ code. the name of the variables must be the name of the branches. The code is just in time compiled.

In [ ]:
auto entries2 = d.Filter("b1 < 5.").Count();
std::cout << *entries2 << " entries passed the string filter" << std::endl;

Min, Max and Mean actions

These actions allow to retrieve statistical information about the entries passing the cuts, if any.

In [ ]:
auto b1b2_cut = d.Filter(cutb1b2, {"b2", "b1"});
auto minVal = b1b2_cut.Min();
auto maxVal = b1b2_cut.Max();
auto meanVal = b1b2_cut.Mean();
auto nonDefmeanVal = b1b2_cut.Mean("b2"); // <- Column is not the default
std::cout << "The mean is always included between the min and the max: " << *minVal << " <= " << *meanVal
          << " <= " << *maxVal << std::endl;

Take action

The Take action allows to retrieve all values of the variable stored in a particular column that passed filters we specified. The values are stored in a list by default, but other collections can be chosen.

In [ ]:
auto b1_cut = d.Filter(cutb1);
auto b1Vec = b1_cut.Take<double>();
auto b1List = b1_cut.Take<double, std::list<double>>();

std::cout << "Selected b1 entries" << std::endl;
for (auto b1_entry : *b1List)
   std::cout << b1_entry << " ";
std::cout << std::endl;
auto b1VecCl = ROOT::GetClass(b1Vec.GetPtr());
std::cout << "The type of b1Vec is " << b1VecCl->GetName() << std::endl;

Histo1D action

The Histo1D action allows to fill an histogram. It returns a TH1D filled with values of the column that passed the filters. For the most common types, the type of the values stored in the column is automatically guessed.

In [ ]:
auto hist = d.Filter(cutb1).Histo1D();
std::cout << "Filled h " << hist->GetEntries() << " times, mean: " << hist->GetMean() << std::endl;

Foreach action

The most generic action of all: an operation is applied to all entries. In this case we fill a histogram. In some sense this is a violation of a purely functional paradigm - C++ allows to do that.

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TH1F h("h", "h", 12, -1, 11);
d.Filter([](int b2) { return b2 % 2 == 0; }, {"b2"}).Foreach([&h](double b1) { h.Fill(b1); });

std::cout << "Filled h with " << h.GetEntries() << " entries" << std::endl;

Express your chain of operations with clarity!

We are discussing an example here but it is not hard to imagine much more complex pipelines of actions acting on data. Those might require code which is well organised, for example allowing to conditionally add filters or again to clearly separate filters and actions without the need of writing the entire pipeline on one line. This can be easily achieved. We'll show this re-working the Count example:

In [ ]:
auto cutb1_result = d.Filter(cutb1);
auto cutb1b2_result = d.Filter(cutb1b2, {"b2", "b1"});
auto cutb1_cutb1b2_result = cutb1_result.Filter(cutb1b2, {"b2", "b1"});

Now we want to count:

In [ ]:
auto evts_cutb1_result = cutb1_result.Count();
auto evts_cutb1b2_result = cutb1b2_result.Count();
auto evts_cutb1_cutb1b2_result = cutb1_cutb1b2_result.Count();

std::cout << "Events passing cutb1: " << *evts_cutb1_result << std::endl
          << "Events passing cutb1b2: " << *evts_cutb1b2_result << std::endl
          << "Events passing both: " << *evts_cutb1_cutb1b2_result << std::endl;

Calculating quantities starting from existing columns

Often, operations need to be carried out on quantities calculated starting from the ones present in the columns. We'll create in this example a third column the values of which are the sum of the b1 and b2 ones, entry by entry. The way in which the new quantity is defined is via a runable. It is important to note two aspects at this point:

  • The value is created on the fly only if the entry passed the existing filters.
  • The newly created column behaves as the one present on the file on disk.
  • The operation creates a new value, without modifying anything. De facto, this is like having a general container at disposal able to accommodate any value of any type. Let's dive in an example:
In [ ]:
auto entries_sum = d.Define("sum", [](double b1, int b2) { return b2 + b1; }, {"b1", "b2"})
                      .Filter([](double sum) { return sum > 4.2; }, {"sum"})
                      .Count();
std::cout << *entries_sum << std::endl;

Additional columns can be expressed as strings. the content must be c++ code. The name of the variables must be the name of the branches. The code is just in time compiled.

In [ ]:
auto entries_sum2 = d.Define("sum2", "b1 + b2").Filter("sum2 > 4.2").Count();
std::cout << *entries_sum2 << std::endl;

It is possible at any moment to read the entry number and the processing slot number. The latter may change when implicit multithreading is active. The special columns which provide the entry number and the slot index are called "tdfentry" and "tdfslot" respectively. Their types are an unsigned 64 bit integer and an unsigned integer.

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auto printEntrySlot = [](ULong64_t iEntry, unsigned int slot) {
   std::cout << "Entry: " << iEntry << " Slot: " << slot << std::endl;
};
d.Foreach(printEntrySlot, {"tdfentry_", "tdfslot_"});

return 0;