< cpp‎ | algorithm
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
Sampling operations

Sorting and related operations
Partitioning operations

Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
Defined in header <algorithm>
template< class ForwardIt, class UnaryPred >
ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPred p );
(since C++11)
(constexpr since C++20)

Examines the partitioned range [firstlast) 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.

If the elements elem of [firstlast) are not partitioned with respect to the expression bool(p(elem)), the behavior is undefined.


[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.
UnaryPred must meet the requirements of Predicate.

[edit] Return value

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

[edit] Complexity

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

[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 [&](const auto& e) { return e < value; });.

[edit] Possible implementation

template<class ForwardIt, class UnaryPred>
constexpr //< since C++20
ForwardIt partition_point(ForwardIt first, ForwardIt last, UnaryPred p)
    for (auto length = std::distance(first, last); 0 < length; )
        auto half = length / 2;
        auto middle = std::next(first, half);
        if (p(*middle))
            first = std::next(middle);
            length -= (half + 1);
            length = half;
    return first;

[edit] 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

[edit] See also

finds the first element satisfying specific criteria
(function template) [edit]
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]
locates the partition point of a partitioned range