std:: partition_point

Examines the partitioned range [ first , last ) and locates the end of the first partition, that is, the first element that does not satisfy p or last if all elements satisfy p .

If the elements elem of [ first , last ) are not partitioned with respect to the expression bool ( p ( elem ) ) , the behavior is undefined.

Parameters

Return value

The iterator past the end of the first partition within [ first , last ) or last if all elements satisfy p .

Complexity

Given \(\scriptsize N\) N as std:: distance ( first, last ) , performs \(\scriptsize O(log(N))\) O(log(N)) applications of the predicate p .

Notes

This algorithm is a more general form of std::lower_bound , which can be expressed in terms of std::partition_point with the predicate [ & ] ( const auto & e ) { return e < value ; } ) ; .

Possible implementation

template<class ForwardIt, class UnaryPred>
constexpr //< since C++20
ForwardIt partition_point(ForwardIt first, ForwardIt last, UnaryPred p)
{
    for (auto length = std::distance(first, last); 0 < length; )
    {
        auto half = length / 2;
        auto middle = std::next(first, half);
        if (p(*middle))
        {
            first = std::next(middle);
            length -= (half + 1);
        }
        else
            length = half;
    }
 
    return first;
}

Example

#include <algorithm>
#include <array>
#include <iostream>
#include <iterator>
 
auto print_seq = [](auto rem, auto first, auto last)
{
    for (std::cout << rem; first != last; std::cout << *first++ << ' ') {}
    std::cout << '\n';
};
 
int main()
{
    std::array v{1, 2, 3, 4, 5, 6, 7, 8, 9};
 
    auto is_even = [](int i) { return i % 2 == 0; };
 
    std::partition(v.begin(), v.end(), is_even);
    print_seq("After partitioning, v: ", v.cbegin(), v.cend());
 
    const auto pp = std::partition_point(v.cbegin(), v.cend(), is_even);
    const auto i = std::distance(v.cbegin(), pp);
    std::cout << "Partition point is at " << i << "; v[" << i << "] = " << *pp << '\n';
 
    print_seq("First partition (all even elements): ", v.cbegin(), pp);
    print_seq("Second partition (all odd elements): ", pp, v.cend());
}

After partitioning, v: 8 2 6 4 5 3 7 1 9
Partition point is at 4; v[4] = 5
First partition (all even elements): 8 2 6 4
Second partition (all odd elements): 5 3 7 1 9

See also