The Ranges Library (2) — C++20

比较std与std::ranges算法

比较一下std::sort和std::ranges::sort

• std::sort

template< class RandomIt >
constexpr void sort( RandomIt first, RandomIt last );template< class ExecutionPolicy, class RandomIt >
void sort( ExecutionPolicy&& policy,RandomIt first, RandomIt last );template< class RandomIt, class Compare >
constexpr void sort( RandomIt first, RandomIt last, Compare comp );template< class ExecutionPolicy, class RandomIt, class Compare >
void sort( ExecutionPolicy&& policy,RandomIt first, RandomIt last, Compare comp );

• std::ranges::sort

template <std::random_access_iterator I, std::sentinel_for<I> S,class Comp = ranges::less, class Proj = std::identity>
requires std::sortable<I, Comp, Proj>
constexpr I sort(I first, S last, Comp comp = {}, Proj proj = {});template <ranges::random_access_range R, class Comp = ranges::less, class Proj = std::identity>
requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
constexpr ranges::borrowed_iterator_t<R> sort(R&& r, Comp comp = {}, Proj proj = {});

你观察一下第二个重载,你会发现, 他需一个支持ranges::random_access_range的容器

	std::forward_list	std::list	std::deque	std::array	std::vector
std::ranges::random_access_range			✅	✅	✅

一个谓词 Comp(返回bool的可调用对象),一个std::identity,Comp默认是less,Prog是std::identity :就是参数本身,本质是使用完美转发, 按照Prog将一个集合映射到子集

#include <iostream>
#include <ranges>
#include <vector>
#include <algorithm>using namespace std;struct PhoneBookEntry
{std::string name;int number;
};void printPhoneBook(const std::vector<PhoneBookEntry>& phoneBook)
{for (const auto& entry : phoneBook) std::cout << "(" << entry.name << ", " << entry.number << ")";std::cout << "\n\n";
}int main()
{std::cout << '\n';std::vector<PhoneBookEntry> phoneBook{{"Brown", 111}, {"Smith", 444},{"Grimm", 666}, {"Butcher", 222},{"Taylor", 555}, {"Wilson", 333}};std::ranges::sort(phoneBook, {}, &PhoneBookEntry::name); // 按姓名升序printPhoneBook(phoneBook);std::ranges::sort(phoneBook, std::ranges::greater(),&PhoneBookEntry::name); //按姓名降序printPhoneBook(phoneBook);std::ranges::sort(phoneBook, {}, &PhoneBookEntry::number); // 按号码升序printPhoneBook(phoneBook);std::ranges::sort(phoneBook, std::ranges::greater(),&PhoneBookEntry::number); // 按号码降序printPhoneBook(phoneBook);
}

如果你有更加复杂的要求,可以使用lambda函数这样的可调用对象

#include <iostream>
#include <ranges>
#include <vector>
#include <algorithm>
#include <string>
using namespace std;struct PhoneBookEntry
{std::string name;int number;
};void printPhoneBook(const std::vector<PhoneBookEntry>& phoneBook)
{for (const auto& entry : phoneBook) std::cout << "(" << entry.name << ", " << entry.number << ")";std::cout << "\n\n";
}int main()
{std::cout << '\n';std::vector<PhoneBookEntry> phoneBook{{"Brown", 111}, {"Smith", 444},{"Grimm", 666}, {"Butcher", 222},{"Taylor", 555}, {"Wilson", 333}};std::ranges::sort(phoneBook, {}, [](auto p) { return p.name; });printPhoneBook(phoneBook);std::ranges::sort(phoneBook, [](auto p, auto p2){return std::to_string(p.number) + p.name <std::to_string(p2.number) + p2.name;});printPhoneBook(phoneBook);
}

直接查看key和value

可以直接查看map中的 key 和 value

#include <iostream>
#include <ranges>
#include <vector>
#include <algorithm>
#include <string>
#include <unordered_map>using namespace std;int main()
{std::unordered_map<std::string, int> m{{"李宁", 12}, {"张三", 34}, {"张杰", 23}};auto names = std::views::keys(m);cout << "keys: ";for (auto name : names){cout << name << " ";}cout << endl;auto ages = std::views::values(m);cout << "values: ";for (auto age : ages){cout << age << " ";}cout << " ";
}

但我们还有更加灵活的方式,使用lambda函数来完成一些更复杂的逻辑

Function Composition

上一篇文章中有提及, 但不是很复杂, 我们实现一些更复杂的逻辑

#include <iostream>
#include <ranges>
#include <vector>
#include <algorithm>
#include <string>
#include <unordered_map>using namespace std;int main()
{std::unordered_map<std::string, int> m{{"李宁", 12}, {"张三", 34}, {"张杰", 23}, {"西施", 18}, {"修勾", 4}};auto names = std::views::keys(m);cout << "keys: ";for (auto name : names){cout << name << " ";}cout << endl;auto ages = std::views::values(m);cout << "values: ";for (auto age : ages){cout << age << " ";}cout << " " << endl;std::cout << "张 开头的姓名: ";auto fistZ = [](const std::string& name) { return name.find("张", 0) == 0; };for (const auto& name : std::views::keys(m) | std::views::filter(fistZ)){cout << name << " ";}cout << endl;
}

| 是一个语法糖, C(R)等价于 R | C

auto rev1 = std::views::reverse(std::views::keys(m));
auto rev2 = std::views::keys(m) | std::views::reverse;
auto rev3 = freqWord | std::views::keys | std::views::reverse;

Lazy Evaluation 惰性执行

上一篇文章已经介绍了

自定义View

#include <ranges>
#include <vector>
#include <iostream>template <class T, class A>
class VectorView : public std::ranges::view_interface<VectorView<T, A>>
{
public:VectorView() = default;VectorView(const std::vector<T, A>& vec) :m_begin(vec.cbegin()), m_end(vec.cend()){}auto begin() const { return m_begin; }auto end() const { return m_end; }
private:typename std::vector<T, A>::const_iterator m_begin{}, m_end{};
};int main()
{std::vector<int> v = {1, 4, 9, 16};VectorView view_over_v{v};// We can iterate with begin() and end().for (int n : view_over_v){std::cout << n << ' ';}std::cout << '\n';// We get operator[] for free when inheriting from view_interface// since we satisfy the random_access_range concept.for (std::ptrdiff_t i = 0; i < view_over_v.size(); i++){std::cout << "v[" << i << "] = " << view_over_v[i] << '\n';}
}

perator[] for free when inheriting from view_interface
// since we satisfy the random_access_range concept.
for (std::ptrdiff_t i = 0; i < view_over_v.size(); i++)
{
std::cout << “v[” << i << "] = " << view_over_v[i] << ‘\n’;
}
}

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