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df007_snapshot.C File Reference

Detailed Description

View in nbviewer Open in SWAN Write ROOT data with RDataFrame.

This tutorial shows how to write out datasets in ROOT format using the 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)
int i(0);
d.Define("b1", [&i]() { return i; })
[&i]() {
float j = i * i;
return j;
.Snapshot(treeName, fileName);
// 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")
[](float b2) {
std::vector<float> v;
for (int i = 0; i < 3; i++)
v.push_back(b2 * i);
return v;
// ## 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;
// 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;
// We can also get a fresh RDataFrame out of the snapshot and restart the
// analysis chain from it. The default columns are the one 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();
return 0;
#define d(i)
Definition RSha256.hxx:102
#define c(i)
Definition RSha256.hxx:101
#define h(i)
Definition RSha256.hxx:106
ROOT's RDataFrame offers a high level interface for analyses of data stored in TTrees,...
The Canvas class.
Definition TCanvas.h:23
A ROOT file is a suite of consecutive data records (TKey instances) with a well defined format.
Definition TFile.h:54
virtual const char * GetName() const
Returns name of object.
Definition TNamed.h:47
A TTree represents a columnar dataset.
Definition TTree.h:79
TF1 * f1
Definition legend1.C:11
April 2017
Danilo Piparo (CERN)

Definition in file df007_snapshot.C.