c-resources/CPlusPlus20ForProgrammers-master/examples/ch06/fig06_13.cpp

// fig06_13.cpp
// Functional-style programming with C++20 ranges and views.
#include <array>
#include <iostream>
#include <numeric>
#include <ranges>

int main() {
   // lambda to display results of range operations
   auto showValues{
      [](auto& values, const std::string& message) {
         std::cout <<  message << "\n";

         for (const auto& value : values) {
            std::cout << value << " ";
         }

         std::cout << '\n';
      }
   };

   auto values1{std::views::iota(1, 11)}; // generate integers 1-10
   showValues(values1, "Generate integers 1-10");

   // filter each value in values1, keeping only the even integers
   auto values2{values1 |
      std::views::filter([](const auto& x) {return x % 2 == 0;})};
   showValues(values2, "Filtering even integers");

   // map each value in values2 to its square
   auto values3{
      values2 | std::views::transform([](const auto& x) {return x * x;})};
   showValues(values3, "Mapping even integers to squares");

   // combine filter and transform to get squares of the even integers
   auto values4{
      values1 | std::views::filter([](const auto& x) {return x % 2 == 0;})
              | std::views::transform([](const auto& x) {return x * x; })};
   showValues(values4, "Squares of even integers");

   // total the squares of the even integers 
   std::cout << "Sum squares of even integers 2-10: \n",
      std::accumulate(std::begin(values4), std::end(values4), 0);

   // process a container's elements
   constexpr std::array numbers{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
   auto values5{
      numbers | std::views::filter([](const auto& x) {return x % 2 == 0;})
              | std::views::transform([](const auto& x) {return x * x; })};
   showValues(values5, "Squares of even integers in array numbers");
}


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