std::ranges::partition_point

Since C++20

Header: <algorithm>

Examines the partitioned (as if by ranges::partition) range [first,last) or r and locates the end of the first partition, that is, the projected element that does not satisfy pred or last if all projected elements satisfy pred.

# Declarations

Call signature

template< std::forward_iterator I, std::sentinel_for<I> S,
class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
constexpr I
partition_point( I first, S last, Pred pred, Proj proj = {} );

(since C++20)

template< ranges::forward_range R,
class Proj = std::identity,
std::indirect_unary_predicate<
std::projected<ranges::iterator_t<R>, Proj>> Pred >
constexpr ranges::borrowed_iterator_t<R>
partition_point( R&& r, Pred pred, Proj proj = {} );

(since C++20)

# Parameters

# Return value

The iterator past the end of the first partition within [first,last) or the iterator equal to last if all projected elements satisfy pred.

# Notes

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

# 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::ranges::partition(v, is_even);
    print_seq("After partitioning, v: ", v.cbegin(), v.cend());
 
    const auto pp = std::ranges::partition_point(v, is_even);
    const auto i = std::ranges::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());
}

# See also