Update chapter_08 examples

chapter_08/examples/eratostenes_sieve.cc

+#include <chrono>
+#include <ranges>
+#include <algorithm>
+#include <iostream>
+#include <vector>
+#include <span>
+#include <cxxopts.hpp>
+
+namespace sr = std::ranges;
+
+template <class T>
+void assign_each_nth(std::span<T> R, size_t n, T v)
+{
+    for (auto i = 0UL; i < R.size(); i += n)
+        R[i] = v;
+}
+
+auto main(int argc, char* argv[]) -> int
+{
+    std::cout << "Erastothene Sieve...\n";
+    using namespace std;
+    using namespace std::chrono;
+    auto N = 100UL;
+    vector<int> candidate(N + 1, 1);
+    candidate[0] = candidate[1] = 0;
+    auto proc = std::span(candidate);
+    auto t0 = steady_clock::now();
+    for (size_t i = 2; i <= (N / 2); ++i) {
+        if (candidate[i]) {
+            assign_each_nth(proc.subspan(2 * i), i, 0);
+        }
+    }
+    auto t1 = steady_clock::now();
+    std::cout << "Calculating primes up to " << N << " took "
+              << duration<double>(t1 - t0)<< "\n";
+
+    std::cout << "Write primes ? (y/n): ";
+    char ch;
+    std::cin >> ch;
+    if (ch == 'y')
+        for (size_t i = 0; i < N; ++i)
+            if (candidate[i])
+                std::cout << i << "\n";
+    std::cout << "Prime count = "
+              << sr::count(candidate, 1) << "\n";
+}

chapter_08/examples/hello_m.cc

-import <iostream>;
+import <print>;
 
 auto main() -> int
 {
-    std::cout << "Hello, world!\n";
+    std::print("Hello, world!\n");
 }