Collect examples for chapter 6

chapter_06/solutions/normal_distribution.cc

+#include <iostream>
+#include <map>
+#include <random>
+
+auto main() -> int
+{
+    double mn{0.}, var{1.};
+    std::cout << "Mean : ";
+    std::cin >> mn;
+    std::cout << "Variance: ";
+    std::cin >> var;
+
+    std::mt19937_64 engine{ std::random_device{}() };
+    // Above: First create an object of the type std::random_device.
+    // random_device objects can be called like functions.
+    // They give you non-deterministic random numbers, but
+    // not necessarily of the best mathematical quality.
+    // We create the object, and immediately call it:
+    // which is why we have the () after the empty initialiser
+    // braces {}. This gives us a random unsigned long
+    // integer, which we then use to seed the Mersenne Twister
+    // engine.
+    std::normal_distribution<> dist{mn, std::sqrt(var) };
+    auto generator = [&]{ return dist(engine); };
+    std::map<int, unsigned> H;
+    for (unsigned i = 0; i < 5000000; ++i)
+        H[static_cast<int>(std::floor(generator()))]++;
+    for (auto& i : H)
+        std::cout << i.first << " " << i.second << "\n";
+}

chapter_06/solutions/weighted_die.cc

+#include <array>
+#include <iostream>
+#include <random>
+
+auto main() -> int
+{
+    std::array freq{ 0.3, 0.2, 0.2, 0.1, 0.1, 0.1 };
+    std::mt19937_64 engine{ std::random_device{}() };
+    std::discrete_distribution dist{freq.begin(), freq.end()};
+    auto gen = [&]{ return dist(engine); };
+    for (unsigned long i = 0; i < 10'000'000; ++i)
+        freq[gen()]++;
+    for (unsigned f : freq)
+        std::cout << f << std::endl;
+}