# std::partition_point

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
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; });.

### Example

Run this code

    #include <algorithm>
    #include <array>
    #include <iostream>
    #include <iterator>

    int main()
    {
        std::array<int, 9> 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);

        auto p = std::partition_point(v.begin(), v.end(), is_even);

        std::cout << "Before partition:\n    ";
        std::copy(v.begin(), p, std::ostream_iterator<int>(std::cout, " "));
        std::cout << "\nAfter partition:\n    ";
        std::copy(p, v.end(), std::ostream_iterator<int>(std::cout, " "));
    }

Output:

    Before partition:
        8 2 6 4
    After partition:
        5 3 7 1 9

### See also

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)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/partition_point&oldid=104480"