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std::partition_point

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
partition_point
(C++11)
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)
Permutations
Numeric operations
C library
 
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.

Contents

[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 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.

[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-LegacyRandomAccessIterators, the number of iterator increments is O(N).

[edit] 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; });.

[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, " "));
}

Output:

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

[edit] See also

(C++11)
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]