< cpp‎ | algorithm
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations

Numeric operations
C library
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

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

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

[edit] Example

#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, " "));


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

[edit] See also

checks whether a range is sorted into ascending order
(function template) [edit]
returns an iterator to the first element not less than the given value
(function template) [edit]