# std::partition_point

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< cpp‎ | 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)

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

Run this code

    #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

### See also

find find_if find_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)

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