[hist] Add tutorial for multidimensional histograms

tutorials/hist/histv7/hist001_RHist_basics.C

@@ -60,7 +60,7 @@ void hist001_RHist_basics()
    ROOT::Experimental::RRegularAxis axis(40, {0.0, 20.0});
 
    // Create a first histogram and fill with random values.
-   ROOT::Experimental::RHist<int> hist1({axis});
+   ROOT::Experimental::RHist<int> hist1(axis);
 
    // Create a normal distribution with mean 5.0 and stddev 2.0.
    std::mt19937 gen;
@@ -75,7 +75,7 @@ void hist001_RHist_basics()
    std::cout << "\n";
 
    // Create a second histogram and fill with random values of a different distribution.
-   ROOT::Experimental::RHist<int> hist2({axis});
+   ROOT::Experimental::RHist<int> hist2(axis);
    std::normal_distribution normal2(13.0, 4.0);
    for (std::size_t i = 0; i < 1500; i++) {
       hist2.Fill(normal2(gen));
@@ -85,7 +85,7 @@ void hist001_RHist_basics()
    std::cout << "\n";
 
    // Create a third, merged histogram from the two previous two.
-   ROOT::Experimental::RHist<int> hist3({axis});
+   ROOT::Experimental::RHist<int> hist3(axis);
    hist3.Add(hist1);
    hist3.Add(hist2);
    std::cout << "hist3 with expected entries = " << (hist1.GetNEntries() + hist2.GetNEntries()) << "\n";

tutorials/hist/histv7/hist002_RHist_weighted.C

@@ -62,8 +62,8 @@ void hist002_RHist_weighted()
    ROOT::Experimental::RRegularAxis axis(40, {0.0, 20.0});
 
    // Create two histograms, one of which will be filled with weighted entries.
-   ROOT::Experimental::RHist<double> hist1({axis});
-   ROOT::Experimental::RHist<double> hist2({axis});
+   ROOT::Experimental::RHist<double> hist1(axis);
+   ROOT::Experimental::RHist<double> hist2(axis);
 
    // Create a normal distribution with mean 10.0 and stddev 5.0.
    std::mt19937 gen;
@@ -86,7 +86,7 @@ void hist002_RHist_weighted()
 
    // Create and fill a third histogram with the special RBinWithError bin content type.
    // In addition to the sum of weights, it tracks the sum of weights squared to compute the bin errors.
-   ROOT::Experimental::RHist<ROOT::Experimental::RBinWithError> hist3({axis});
+   ROOT::Experimental::RHist<ROOT::Experimental::RBinWithError> hist3(axis);
    for (std::size_t i = 0; i < 25000; i++) {
       double value = normal(gen);
       double weight = 0.2 + 0.008 * value * value;

tutorials/hist/histv7/hist003_RHist_multi.C

@@ -0,0 +1,64 @@
+/// \file
+/// \ingroup tutorial_histv7
+///
+/// Multidimensional RHist with dynamic axis types.
+///
+/// \macro_code
+/// \macro_output
+///
+/// \date January 2026
+/// \author The ROOT Team
+
+#include <ROOT/RBinIndex.hxx>
+#include <ROOT/RHist.hxx>
+#include <ROOT/RRegularAxis.hxx>
+#include <ROOT/RVariableBinAxis.hxx>
+
+#include <cmath>
+#include <cstddef>
+#include <iostream>
+#include <random>
+#include <vector>
+
+void hist003_RHist_multi()
+{
+   // Define the bin edges and variable bin axis for the eta dimension.
+   std::vector<double> etaEdges = {-5.0, -3.0, -2.4, -1.5, 0.0, 1.5, 2.4, 3.0, 5.0};
+   ROOT::Experimental::RVariableBinAxis etaAxis(etaEdges);
+
+   // Define the regular axis for the phi dimension (in radians), disabling under- and overflow bins.
+   static constexpr double Pi = 3.14159;
+   ROOT::Experimental::RRegularAxis phiAxis(20, {-Pi / 2, Pi / 2}, false);
+
+   // Define the logarithmic axis for the energy (in GeV).
+   ROOT::Experimental::RVariableBinAxis energyAxis({0, 1, 10, 100, 1000});
+
+   // Create the multidimensional histogram.
+   ROOT::Experimental::RHist<int> hist(etaAxis, phiAxis, energyAxis);
+
+   // Print some information about the histogram.
+   std::cout << "Created a histogram with " << hist.GetNDimensions() << " dimensions and " << hist.GetTotalNBins()
+             << "\n";
+
+   // Generate angles and energies to fill the histogram.
+   std::mt19937 gen;
+   std::uniform_real_distribution dist(0.0, 1.0);
+   static constexpr double maxE = 1000;
+   for (std::size_t i = 0; i < 100000; i++) {
+      double delta = 2 * Pi * dist(gen);
+      double eta = -std::log(std::tan(delta / 2));
+      double phi = std::acos(2 * dist(gen) - 1) - Pi / 2;
+      double E = maxE * dist(gen);
+      hist.Fill(eta, phi, E);
+   }
+   std::cout << " ... filled with " << hist.GetNEntries() << " entries.\n\n";
+
+   // Get the content for specific bins:
+   std::cout << "eta in [1.5, 2.4), phi in [0, 0.157), E between 10 GeV and 100 GeV\n";
+   auto content = hist.GetBinContent(5, 10, 2);
+   std::cout << "  content = " << content << "\n";
+
+   std::cout << "eta in [-1.5, 0), phi in [-0.785, -0.628), E between 1 GeV and 10 GeV\n";
+   content = hist.GetBinContent(3, 5, 1);
+   std::cout << "  content = " << content << "\n";
+}

tutorials/hist/histv7/index.md

@@ -7,3 +7,4 @@ Examples demonstrating ROOT's histogram package.
 |---|---|
 | hist001_RHist_basics.C | Basics of RHist, including filling and adding them. |
 | hist002_RHist_weighted.C | Weighted filling of RHist and RBinWithError bin content type. |
+| hist003_RHist_multi.C | Multidimensional RHist with dynamic axis types. |