C++でのfilter/map操作 (C++98～C++20, range-v3)

cpp_filter_map.cpp

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>

// after C++20
#if __cplusplus >= 202002L
#include <ranges>
#endif

// range-v3
#ifdef USE_RANGE_V3
#include <range/v3/all.hpp>
#endif

void print(const std::vector<double> &output)
{
  for (int i = 0; i < output.size(); ++i)
  {
    std::cout << output[i] << " ";
  }
  std::cout << std::endl;
}

int main()
{
  // basic
  {
    std::vector<int> input; /* {1, 2, 3, 4, 5, 6} */
    std::vector<int> tmp;
    std::vector<double> output; /* {4, 8, 12} */

    input.push_back(1);
    input.push_back(2);
    input.push_back(3);
    input.push_back(4);
    input.push_back(5);
    input.push_back(6);

    // filter
    for (int i = 0; i < input.size(); ++i)
    {
      if (input[i] % 2 == 0)
      {
        tmp.push_back(input[i]);
      }
    }

    // map
    for (int i = 0; i < tmp.size(); ++i)
    {
      output.push_back(tmp[i] * 2.0f);
    }

    print(output);
  }

  // after C++11
#if __cplusplus >= 201103L
  {
    std::vector<int> input = {1, 2, 3, 4, 5, 6};
    std::vector<double> output; /* {4, 8, 12} */

    auto even = [](int it) { return it % 2 == 0; };
    auto square = [](int it) { return it * 2.0f; };

    std::vector<int> tmp;

    // filter
    std::copy_if(std::begin(input), std::end(input), std::back_inserter(tmp), even);

    // map
    std::transform(std::begin(tmp), std::end(tmp), std::back_inserter(output), square);

    print(output);
  }
#endif

  // after C++14
#if __cplusplus >= 201402L
  {
    std::vector<int> input = {1, 2, 3, 4, 5, 6};
    std::vector<double> output; /* {4, 8, 12} */

    auto even = [](auto it) { return it % 2 == 0; };
    auto square = [](auto it) { return it * 2.0f; };

    std::vector<int> tmp;

    // filter
    std::copy_if(std::begin(input), std::end(input), std::back_inserter(tmp), even);

    // map
    std::transform(std::begin(tmp), std::end(tmp), std::back_inserter(output), square);

    print(output);
  }
#endif

// after C++20
#if __cplusplus >= 202002L
  // https://en.cppreference.com/w/cpp/ranges
  {
    std::vector<int> input = {1, 2, 3, 4, 5, 6};
    std::vector<double> output;

    auto even = [](auto it) { return it % 2 == 0; };
    auto square = [](auto it) { return it * 2.0f; };

    auto tmp = input | std::views::filter(even) | std::views::transform(square);
    std::ranges::copy(tmp, std::back_inserter(output));

    print(output);
  }
#endif

// range-v3
#ifdef USE_RANGE_V3
  // https://github.com/ericniebler/range-v3
  {
    std::vector<int> input = {1, 2, 3, 4, 5, 6};

    auto even = [](auto it) { return it % 2 == 0; };
    auto square = [](auto it) { return it * 2.0f; };

    auto output = input | ranges::views::filter(even) | ranges::views::transform(square) | ranges::to<std::vector<double>>;

    print(output);
  }
#endif

  return 0;
}