< cpp‎ | algorithm‎ | ranges
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
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations (on sorted ranges)
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutation operations
Numeric operations
Fold operations
Operations on uninitialized storage
Return types
Defined in header <algorithm>
Call signature
template< std::forward_iterator I, std::sentinel_for<I> S,

          class Proj = std::identity,
          std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
constexpr I

    partition_point( I first, S last, Pred pred, Proj proj = {} );
(1) (since C++20)
template< ranges::forward_range R,

          class Proj = std::identity,
              std::projected<ranges::iterator_t<R>, Proj>> Pred >
constexpr ranges::borrowed_iterator_t<R>

    partition_point( R&& r, Pred pred, Proj proj = {} );
(2) (since C++20)

Examines the partitioned (as if by ranges::partition) range [firstlast) or r and locates the end of the first partition, that is, the projected element that does not satisfy pred or last if all projected elements satisfy pred.

The function-like entities described on this page are niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.


[edit] Parameters

first, last - iterator-sentinel defining the partially-ordered range to examine
r - the partially-ordered range to examine
pred - predicate to apply to the projected elements
proj - projection to apply to the elements

[edit] Return value

The iterator past the end of the first partition within [firstlast) or the iterator equal to last if all projected elements satisfy pred.

[edit] Complexity

Given N = ranges::distance(first, last), performs O(log N) applications of the predicate pred and projection proj.

However, if sentinels don't model std::sized_sentinel_for<I>, the number of iterator increments is O(N).

[edit] Notes

This algorithm is a more general form of ranges::lower_bound, which can be expressed in terms of ranges::partition_point with the predicate [&](auto const& e) { return std::invoke(pred, e, value); });.

[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::ranges::partition(v, is_even);
    print_seq("After partitioning, v: ", v.cbegin(), v.cend());
    const auto pp = std::ranges::partition_point(v, is_even);
    const auto i = std::ranges::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: 2 4 6 8 5 3 7 1 9
Partition point is at 4; v[4] = 5
First partition (all even elements): 2 4 6 8
Second partition (all odd elements): 5 3 7 1 9

[edit] See also

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