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std::partition_point

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; });.

Example

#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

(C++11)
finds the first element satisfying specific criteria
(function template)
(C++11)
checks whether a range is sorted into ascending order
(function template)
returns an iterator to the first element not less than the given value
(function template)
(C++20)
locates the partition point of a partitioned range
(niebloid)

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