using namespace ROOT;

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"});

auto d1 = d0.Define("r", "sqrt(x*x + y*y)");

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;

gROOT->GetListOfCanvases()->Draw()