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std::partition_point

Defined in header <algorithm>

Defined in header `<algorithm>`
template< class ForwardIt, class UnaryPredicate > ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPredicate p );		(since C++11) (until C++20)
template< class ForwardIt, class UnaryPredicate > constexpr ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPredicate p );		(since C++20)

template< class ForwardIt, class UnaryPredicate >
ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPredicate p );

(since C++11)
(until C++20)

template< class ForwardIt, class UnaryPredicate >
constexpr ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPredicate p );

(since C++20)

Examines the partitioned (as if by std::partition) 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.

Parameters

first, last	-	the partitioned range of elements to examine
p	-	unary predicate which returns `true` for the elements found in the beginning of the range. The expression `p(v)` must be convertible to `bool` for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `ForwardIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of `VT&`is not allowed, nor is `VT` unless for `VT` a move is equivalent to a copy (since C++11).
Type requirements
-`ForwardIt` must meet the requirements of LegacyForwardIterator.
-`UnaryPredicate` must meet the requirements of Predicate.

Return value

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

Complexity

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

However, for non-LegacyRandomAccessIterators, the number of iterator increments is O(N).

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 [&](auto const& e) { return e < value; });.

Possible implementation

template<class ForwardIt, class UnaryPredicate>
constexpr //< since C++20
ForwardIt partition_point(ForwardIt first, ForwardIt last, UnaryPredicate 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());
}

Possible output:

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

findfind_iffind_if_not (C++11)	finds the first element satisfying specific criteria (function template)
is_sorted (C++11)	checks whether a range is sorted into ascending order (function template)
lower_bound	returns an iterator to the first element not less than the given value (function template)
ranges::partition_point (C++20)	locates the partition point of a partitioned range (niebloid)