Add simple dataset creation directory

course/intermediate/io-simple-dataset/README.md

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+This directory contains examples of basic reading and writing APIs of the ROOT data formats: TTree and RNTuple
+
+Compile and run a single file with:
+
+```
+$: g++ -o <example_basename>.out <example_basename>.cpp $(root-config --cflags --glibs)
+$: ./<example_basename>.out
+```
+
+The `example` programs can also be run with `root` directly as:
+
+```
+$: root -l -b -q <example_basename>.cpp
+```

course/intermediate/io-simple-dataset/example_01_ttree.cpp

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+#include <TFile.h>
+#include <TTree.h>
+
+#include <iostream>
+#include <memory>
+#include <random>
+
+void write(std::string_view dataset_name, std::string_view file_name,
+           unsigned n_entries) {
+  std::unique_ptr<TFile> file{TFile::Open(file_name.data(), "RECREATE")};
+  auto tree = std::make_unique<TTree>(dataset_name.data(), dataset_name.data());
+
+  std::mt19937 rng{std::random_device{}()};
+
+  std::normal_distribution<double> gaus{15, 5};
+  std::uniform_int_distribution<std::size_t> n_particles{0, 5};
+
+  // Writing API: TTree::Branch. Creates a new top-level branch with a certain
+  // name, taking values from the object pointed to by the address passed as
+  // second parameter. The pointer can be typed: TTree will infer from the type
+  // the information to properly store the data to disk.
+  unsigned event_id{0};
+  std::vector<float> muon_pt{};
+  tree->Branch("event_id", &event_id);
+  tree->Branch("muon_pt", &muon_pt);
+
+  for (auto i = 0; i < n_entries; i++) {
+    event_id = i;
+    auto n = n_particles(rng);
+    muon_pt.clear();
+    for (auto j = 0; j < n; j++) {
+      muon_pt.push_back(gaus(rng));
+    }
+
+    // Writing API: once all values are properly set for the current
+    // event, TTree::Fill ensures the data is written to the TTree in-memory
+    tree->Fill();
+  }
+
+  // Must write all data to disk before leaving the function
+  file->Write();
+}
+
+void read(std::string_view dataset_name, std::string_view file_name,
+          unsigned n_entries) {
+  std::unique_ptr<TFile> file{TFile::Open(file_name.data())};
+  // No unique_ptr here: TFile retains ownership of the object by default
+  auto *tree = file->Get<TTree>(dataset_name.data());
+
+  // Reading API: TTree::SetBranchAddress. The on-disk data will be read into
+  // the memory location pointed at by the second parameter.
+  // POD type: pass by pointer to value
+  unsigned event_id{0};
+  // Class type: pass by pointer to pointer
+  std::vector<float> *muon_pt{nullptr};
+  tree->SetBranchAddress("event_id", &event_id);
+  tree->SetBranchAddress("muon_pt", &muon_pt);
+
+  for (auto i = 0; i < n_entries; i++) {
+    // Reading API: TTree::GetEntry. Moves a cursor to the entry index
+    // parameter, reading data into the addresses previously set
+    tree->GetEntry(i);
+
+    std::cout << "event_id=" << event_id << ", muon_pt={";
+    for (const auto &v : *muon_pt)
+      std::cout << v << ",";
+    std::cout << "}\n";
+  }
+}
+
+void example_01_ttree() {
+  constexpr auto file_name{"example_01_ttree.root"};
+  constexpr auto dataset_name{"dataset"};
+  constexpr auto n_entries{10};
+
+  write(dataset_name, file_name, n_entries);
+  read(dataset_name, file_name, n_entries);
+}
+
+int main() {
+  example_01_ttree();
+  return 0;
+}