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# std::partition_point

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class ForwardIt, class UnaryPredicate > ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPredicate p ); | (1) | (since C++11)

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`.

### [edit] 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 signature of the predicate function should be equivalent to the following: bool pred(const Type &a); The signature does not need to have const &, but the function must not modify the objects passed to it. The type Type must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to Type. ​
Type requirements
- `ForwardIt` must meet the requirements of `ForwardIterator`.
- `UnaryPredicate` must meet the requirements of `Predicate`.

### [edit] Return value

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

### [edit] Complexity

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

### [edit] 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

### [edit] See also

is_sorted (C++11) | checks whether a range is sorted into ascending order (function template) [edit]

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