C++ STL partition用法

一：功能

      将区间内元素分成两组，满足判断条件的放到第一组，不满足条件的在第二组。返回第二组中的首元素位置

二：使用

#include <vector>
#include <algorithm>
#include <string>
#include <iostream>

struct ExamResult {
    std::string student_name;
    int score;
};

std::vector<ExamResult> get_results() {
    return {
        {"Student 1", 100}, {"Student 2", 90}, {"Student 3", 15}, {"Student 4", 40},
        {"Student 5", 65}, {"Student 6", 75}, {"Student 7", 85}, {"Student 8", 30}
    };
}

int main() {
    std::vector<ExamResult> results = get_results();

    auto pp = std::partition(results.begin(), results.end(),
        [threshold = 49](const auto& r) {
            return r.score >= threshold;
        });

    // process passing students
    for (auto it = results.begin(); it != pp; ++it) {
        std::cout << "[PASS] " << it->student_name << "\n";
    }
    std::cout << "\n";
    // process failed students
    for (auto it = pp; it != results.end(); ++it) {
        std::cout << "[FAIL] " << it->student_name << "\n";
    }
}

三：实现 

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

template<class ForwardIt, class UnaryPred>
ForwardIt my_partition(ForwardIt first, ForwardIt last, UnaryPred p)
{
    first = std::find_if_not(first, last, p);
    if (first == last)
        return first;
 
    for (auto i = std::next(first); i != last; ++i)
        if (p(*i))
        {
            std::iter_swap(i, first);
            ++first;
        }
 
    return first;
}
template<class ForwardIt>
void quicksort(ForwardIt first, ForwardIt last)
{
    if (first == last)
        return;
 
    auto pivot = *std::next(first, std::distance(first, last) / 2);
    auto middle1 = my_partition(first, last, [pivot](const auto& em)
    {
        return em < pivot;
    });
    auto middle2 = my_partition(middle1, last, [pivot](const auto& em)
    {
        return !(pivot < em);
    });
 
    quicksort(first, middle1);
    quicksort(middle2, last);
}
 
int main()
{
    std::vector<int> v{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    std::cout << "Original vector: ";
    for (int elem : v)
        std::cout << elem << ' ';
 
    auto it = std::partition(v.begin(), v.end(), [](int i) {return i % 2 == 0;});
 
    std::cout << "\nPartitioned vector: ";
    std::copy(std::begin(v), it, std::ostream_iterator<int>(std::cout, " "));
    std::cout << "* ";
    std::copy(it, std::end(v), std::ostream_iterator<int>(std::cout, " "));
 
    std::forward_list<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
    std::cout << "\nUnsorted list: ";
    for (int n : fl)
        std::cout << n << ' ';
 
    quicksort(std::begin(fl), std::end(fl));
    std::cout << "\nSorted using quicksort: ";
    for (int fi : fl)
        std::cout << fi << ' ';
    std::cout << '\n';
}