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Reference Guide
df015_LazyDataSource.C File Reference

Detailed Description

Use the lazy RDataFrame data source to concatenate computation graphs.

View in nbviewer Open in SWAN This tutorial illustrates how to take advantage of a lazy data source creating a data frame from columns of one or multiple parent dataframe(s), delaying the creation of the columns to the actual usage of the daughter data frame. Dataset Reference: McCauley, T. (2014). Dimuon event information derived from the Run2010B public Mu dataset. CERN Open Data Portal. DOI: 10.7483/OPENDATA.CMS.CB8H.MFFA. From the ROOT website: https://root.cern.ch/files/tutorials/tdf014_CsvDataSource_MuRun2010B.csv

int df015_LazyDataSource()
{
using namespace ROOT::RDF;
// Let's first create a RDF that will read from the CSV file.
// See the tutorial relative to CSV data sources for more details!
auto fileNameUrl = "http://root.cern.ch/files/tutorials/df014_CsvDataSource_MuRun2010B.csv";
auto fileName = "df015_CsvDataSource_MuRun2010B.csv";
if(gSystem->AccessPathName(fileName))
TFile::Cp(fileNameUrl, fileName);
auto csv_rdf = MakeCsvDataFrame(fileName);
// Now we take out four columns: px and py of the first muon in the muon pair
std::string px1Name = "px1";
auto px1 = csv_rdf.Take<double>(px1Name);
std::string py1Name = "py1";
auto py1 = csv_rdf.Take<double>(py1Name);
// Now we create a new dataframe built on top of the columns above. Note that up to now, no event loop
// has been carried out!
auto df = MakeLazyDataFrame(std::make_pair(px1Name, px1), std::make_pair(py1Name, py1));
// We build a histogram of the transverse momentum the muons.
auto ptFormula = [](double px, double py) { return sqrt(px * px + py * py); };
auto pt_h = df.Define("pt", ptFormula, {"px1", "py1"})
.Histo1D<double>({"pt", "Muon p_{T};p_{T} [GeV/c];", 128, 0, 128}, "pt");
auto can = new TCanvas();
can->SetLogy();
pt_h->DrawCopy();
return 0;
}
Date
February 2018
Author
Danilo Piparo

Definition in file df015_LazyDataSource.C.

ROOT::RDF::MakeLazyDataFrame
RDataFrame MakeLazyDataFrame(std::pair< std::string, RResultPtr< std::vector< ColumnTypes >>> &&... colNameProxyPairs)
Factory method to create a Lazy RDataFrame.
Definition: RLazyDS.hxx:40
TFile::Cp
virtual Bool_t Cp(const char *dst, Bool_t progressbar=kTRUE, UInt_t buffersize=1000000)
Allows to copy this file to the dst URL.
Definition: TFile.cxx:4843
TSystem::AccessPathName
virtual Bool_t AccessPathName(const char *path, EAccessMode mode=kFileExists)
Returns FALSE if one can access a file using the specified access mode.
Definition: TSystem.cxx:1294
sqrt
double sqrt(double)
ROOT::RDF::MakeCsvDataFrame
RDataFrame MakeCsvDataFrame(std::string_view fileName, bool readHeaders=true, char delimiter=',', Long64_t linesChunkSize=-1LL)
Factory method to create a CSV RDataFrame.
Definition: RCsvDS.cxx:475
gSystem
R__EXTERN TSystem * gSystem
Definition: TSystem.h:559
ROOT::RDF
Definition: RArrowDS.hxx:20
TCanvas
Definition: TCanvas.h:23