ROOT   Reference Guide
df001_introduction.C File Reference

## Detailed Description

Basic RDataFrame usage.

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.

// ## Preparation
// A simple helper function to fill a test tree: this makes the example
// stand-alone.
void fill_tree(const char *treeName, const char *fileName)
{
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
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.
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<T>. 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.
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.
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.
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.
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.
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.
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.
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:
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:
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:
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.
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 "rdfentry_" and "rdfslot_" respectively. Their types are an unsigned
// 64 bit integer and an unsigned integer.
auto printEntrySlot = [](ULong64_t iEntry, unsigned int slot) {
std::cout << "Entry: " << iEntry << " Slot: " << slot << std::endl;
};
d.Foreach(printEntrySlot, {"rdfentry_", "rdfslot_"});
return 0;
}
2 entries passed all filters
5 entries passed the string filter
The mean is always included between the min and the max: 1 <= 2 <= 3
Selected b1 entries
0 1 2 3 4
The type of b1Vec is vector<double>
Filled h 5 times, mean: 2
Filled h with 5 entries
Events passing cutb1: 5
Events passing cutb1b2: 2
Events passing both: 2
8
8
Entry: 0 Slot: 0
Entry: 1 Slot: 0
Entry: 2 Slot: 0
Entry: 3 Slot: 0
Entry: 4 Slot: 0
Entry: 5 Slot: 0
Entry: 6 Slot: 0
Entry: 7 Slot: 0
Entry: 8 Slot: 0
Entry: 9 Slot: 0
(int) 0
Date
December 2016

Definition in file df001_introduction.C.

TH1::GetEntries
virtual Double_t GetEntries() const
Return the current number of entries.
Definition: TH1.cxx:4301
ROOT::GetClass
TClass * GetClass(T *)
Definition: TClass.h:589
ROOT::RDataFrame
ROOT's RDataFrame offers a high level interface for analyses of data stored in TTrees,...
Definition: RDataFrame.hxx:42
ROOT::VecOps::Filter
RVec< T > Filter(const RVec< T > &v, F &&f)
Create a new collection with the elements passing the filter expressed by the predicate.
Definition: RVec.hxx:938
TH1::GetMean
virtual Double_t GetMean(Int_t axis=1) const
For axis = 1,2 or 3 returns the mean value of the histogram along X,Y or Z axis.
Definition: TH1.cxx:7085
df001_introduction
Definition: df001_introduction.py:1
h
#define h(i)
Definition: RSha256.hxx:124
double
double
Definition: Converters.cxx:921
sum
static long int sum(long int i)
Definition: Factory.cxx:2272
ULong64_t
unsigned long long ULong64_t
Definition: RtypesCore.h:74
TH1F
1-D histogram with a float per channel (see TH1 documentation)}
Definition: TH1.h:572
d
#define d(i)
Definition: RSha256.hxx:120