Start of chapter 9 code

Author: doctorlove

Chapter9/main.cpp

@@ -1,4 +1,79 @@
-int main()
+#include <algorithm>
+#include <cassert>
+#include <execution>
+#include <iostream>
+#include <iterator>
+#include <map>
+#include <numeric>
+#include <ranges>
+#include <utility>
+#include <vector>
+
+// Listing 9.1 Make the first few triangle numbers
+std::vector<int> make_triangle_numbers(int count)
+{
+	std::vector<int> numbers(count);
+	std::iota(numbers.begin(), numbers.end(), 1);
+	std::partial_sum(numbers.begin(), numbers.end(), numbers.begin());
+	return numbers;
+}
+
+// Listing 9.2 and 9.3 Test our triangle numbers
+void check_properties()
+{
+	const int count = 20;
+	const auto triangle_numbers = make_triangle_numbers(count);
+	std::vector<int> diffs(count);
+	std::adjacent_difference(triangle_numbers.begin(), triangle_numbers.end(), diffs.begin());
+	std::vector<int> numbers(count);
+	std::iota(numbers.begin(), numbers.end(), 1);
+	assert(numbers == diffs);
+	std::adjacent_difference(diffs.begin(), diffs.end(), diffs.begin());
+	assert(std::all_of(diffs.begin(), diffs.end(), [](int x) { return x == 1; }));
+	// parallel request
+	assert(std::count(std::execution::par, diffs.begin(), diffs.end(), 1) == count);
+
+	// Check closed form n * (n+1)/2 // TODO an algo rather than an old skool for loop?
+	for (size_t i=0; i< triangle_numbers.size(); ++i)
+	{
+		const int n = i + 1;
+		assert(triangle_numbers[i] == n*(n+1)/2);
+	}
+}
+
+void demo_further_properties()
 {
+	const int count = 20;
+	const auto triangle_numbers = make_triangle_numbers(count);
+	std::vector<char> odd_or_even;
+	std::ranges::transform(triangle_numbers,
+		std::back_inserter(odd_or_even),
+		[](int i) { return i % 2 ? '.' : '*'; });
+	std::ranges::copy(odd_or_even, std::ostream_iterator<char>(std::cout, ""));
+	std::cout << '\n';
+
+	std::map<int, int> last_digits;
+	//for (int number: triangle_numbers)
+	//{
+	//	++last_digits[number % 10];
+	//}
+	std::generate_n(std::inserter(last_digits, last_digits.begin()), 
+		10, // how many digits 
+		[n=0, triangle_numbers]() mutable  // TODO can I do a const & in the capture... is trasnfrom clearer?
+		{ 
+			auto key_value = std::make_pair(n, std::count_if(triangle_numbers.begin(), triangle_numbers.end(), [n](int i) { return i % 10 == n; }));
+			++n;
+			return key_value;
+		}
+	);
+	for (const auto& [key, value] : last_digits)
+	{
+		std::cout << key << " : " << value << '\n';
+	}
+}
 
+int main()
+{
+	check_properties();
+	demo_further_properties();
 }