std::ranges::partition_point

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< cpp‎ | algorithm‎ | ranges
 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
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
Sorting operations
(C++11)
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
(C++11)
Minimum/maximum operations
(C++11)
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
ranges::partition_point
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
 
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::indirect_unary_predicate<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 [first, last) 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.

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

Return value

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

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).

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); });.

Example

Run this code
#include <algorithm>
#include <array>
#include <iostream>
#include <iterator>
 
int main()
{
    std::array v = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
 
    namespace ranges = std::ranges;
    auto is_even = [](int i){ return i % 2 == 0; };
    ranges::partition(v, is_even);
 
    auto p = ranges::partition_point(v.begin(), v.end(), is_even);
 
    std::cout << "Before partition:\n    ";
    ranges::copy(v.begin(), p, std::ostream_iterator<int>(std::cout, " "));
    std::cout << "\nAfter partition:\n    ";
    ranges::copy(p, v.end(), std::ostream_iterator<int>(std::cout, " "));
}

Output: 

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

See also

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