Process collections in RDataFrame with the help of RVec.
This tutorial shows the potential of the VecOps approach for treating collections stored in datasets, a situation very common in HEP data analysis.
Author: Danilo Piparo (CERN)
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Tuesday, May 20, 2025 at 04:18 PM.
using namespace ROOT;
We re-create a set of points in a square. This is a technical detail, just to create a dataset to play with!
auto unifGen = [](double) { return gRandom->Uniform(-1.0, 1.0); };
auto vGen = [&](int len) {
RVecD v(len);
std::transform(v.begin(), v.end(), v.begin(), unifGen);
return v;
};
RDataFrame d(1024);
auto d0 = d.Define("len", []() { return (int)gRandom->Uniform(0, 16); })
.Define("x", vGen, {"len"})
.Define("y", vGen, {"len"});
Now we have in our hands d, a RDataFrame with two columns, x and y, which hold collections of coordinates. The sizes of these collections vary. Let's now define radii from the x and y coordinates. We'll do it treating the collections stored in the columns without looping on the individual elements.
auto d1 = d0.Define("r", "sqrt(x*x + y*y)");
Now we want to plot 2 quarters of a ring with radii .5 and 1. Note how the cuts are performed on RVecs, comparing them with integers and among themselves.
auto ring_h = d1.Define("rInFig", "r > .5 && r < 1 && x*y < 0")
.Define("yFig", "y[rInFig]")
.Define("xFig", "x[rInFig]")
.Histo2D({"fig", "Two quarters of a ring", 64, -1.1, 1.1, 64, -1.1, 1.1}, "xFig", "yFig");
auto cring = new TCanvas();
ring_h->DrawCopy("Colz");
return 0;
Draw all canvases
gROOT->GetListOfCanvases()->Draw()