C++ STL is_partitioned 用法和实现

一：功能

        检查一个序列是否已按条件分组，第二个参数指定了分组的条件

二：用法

#include <algorithm>
#include <iostream>

int main() {
    std::vector<int> data{2, 4, 6, 7, 9, 11};

    auto is_even = [](int v) { return v % 2 == 0; };
    bool test1 = std::ranges::is_partitioned(data, is_even);
    std::cout << std::boolalpha << "is_partitioned(is_even) == " << test1 << "\n";

    bool test2 = true;
    for (int i = 0; i < 16; ++i) {
        test2 = test2 && std::is_partitioned(data.begin(), data.end(),
            [&i](int v) { return v < i; });
    }
    std::cout << std::boolalpha << "is_sorted() == " << test2 << "\n";
}

三：实现 

#include <vector>
#include <algorithm>
#include <array>
#include <iostream>
 
template<class InputIt, class UnaryPred>
bool my_is_partitioned(InputIt first, InputIt last, UnaryPred p)
{
    for (; first != last; ++first)
        if (!p(*first))
            break;
    for (; first != last; ++first)
        if (p(*first))
            return false;
    return true;
}


int main()
{
    std::array<int, 9> v {1, 2, 3, 4, 5, 6, 7, 8, 9};
 
    auto is_even = [](int i) { return i % 2 == 0; };
    std::cout.setf(std::ios_base::boolalpha);
    std::cout << my_is_partitioned(v.begin(), v.end(), is_even) << ' ';
 
    std::partition(v.begin(), v.end(), is_even);
    std::cout << my_is_partitioned(v.begin(), v.end(), is_even) << ' ';
 
    std::reverse(v.begin(), v.end());
    std::cout << my_is_partitioned(v.cbegin(), v.cend(), is_even) << ' ';
    std::cout << my_is_partitioned(v.crbegin(), v.crend(), is_even) << '\n';
}