std::ranges::partition() algorithm

since C++20

Simplified

// (1)
constexpr ranges::subrange<I>
    partition( I first, S last, Pred pred, Proj proj = {} );

// (2)
constexpr ranges::borrowed_subrange_t<R>
    partition( R&& r, Pred pred, Proj proj = {} );

The type of arguments are generic and have the following constraints:

• I - std::permutable
• S - std::sentinel_for<I>
• R - std::ranges::forward_range
• Pred:
  • (1) - std::indirect_unary_predicate<std::projected<I, Proj>>
  • (2) - std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>>
• Proj - (none)

The Proj template argument has the following default type std::identity for all overloads.

Additionally, each overload has the following constraints:

• (2) - std::permutable<ranges::iterator_t<R>>

Detailed

// (1)
template<
  std::permutable I,
  std::sentinel_for<I> S,
  class Proj = std::identity,
  std::indirect_unary_predicate<std::projected<I, Proj>> Pred
>
constexpr ranges::subrange<I>
    partition( I first, S last, Pred pred, Proj proj = {} );

// (2)
template<
  ranges::forward_range R,
  class Proj = std::identity,
  std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred
>
  requires std::permutable<ranges::iterator_t<R>>
constexpr ranges::borrowed_subrange_t<R>
    partition( R&& r, Pred pred, Proj proj = {} );

• (1) Reorders the elements in the range [first; last) in such a way that the projection proj of all elements for which the predicate pred returns true precede the projection proj of elements for which predicate pred returns false.

  caution

  Relative order of elements is not preserved.

• (2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids.

Parameters

first last	The range of elements to reorder.
r	The range of elements to reorder.
pred	The predicate to apply to the projected elements.
proj	The projection to apply to the elements.

Return value

A subrange starting with an iterator to the first element of the second group and finishing with an iterator equal to last.

• (2) Returns std::ranges::dangling if r is an rvalue of non-borrowed_range type.

Complexity

Given N as ranges::distance(first, last):

Exactly N applications of the predicate and projection.

At most N / 2 swaps if I models ranges::bidirectional_iterator, and at most N swaps otherwise.

Exceptions

(none)

Possible implementation

partition(1) and partition(2)

struct partition_fn
{
    template<std::permutable I, std::sentinel_for<I> S, class Proj = std::identity,
             std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
    constexpr ranges::subrange<I>
        operator()(I first, S last, Pred pred, Proj proj = {}) const
    {
        first = ranges::find_if_not(first, last, std::ref(pred), std::ref(proj));
        if (first == last)
            return {first, first};

        for (auto i = ranges::next(first); i != last; ++i)
        {
            if (std::invoke(pred, std::invoke(proj, *i)))
            {
                ranges::iter_swap(i, first);
                ++first;
            }
        }
        return {std::move(first), std::move(last)};
    }

    template<ranges::forward_range R, class Proj = std::identity,
             std::indirect_unary_predicate<
                 std::projected<ranges::iterator_t<R>, Proj>> Pred>
    requires std::permutable<ranges::iterator_t<R>>
    constexpr ranges::borrowed_subrange_t<R>
        operator()(R&& r, Pred pred, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r),
                       std::ref(pred), std::ref(proj));
    }
};

inline constexpr partition_fn partition;

Examples

Main.cpp

#include <algorithm>
#include <forward_list>
#include <functional>
#include <iostream>
#include <iterator>
#include <ranges>
#include <vector>

namespace ranges = std::ranges;

template<class I, std::sentinel_for<I> S, class Cmp = ranges::less>
requires std::sortable<I, Cmp>
void quicksort(I first, S last, Cmp cmp = Cmp {})
{
    using reference = std::iter_reference_t<I>;

    if (first == last)
        return;

    auto size = ranges::distance(first, last);
    auto pivot = ranges::next(first, size - 1);
    ranges::iter_swap(pivot, ranges::next(first, size / 2));

    auto tail = ranges::partition(first, pivot, [=](reference em)
    {
        return std::invoke(cmp, em, *pivot); // em < pivot
    });

    ranges::iter_swap(pivot, tail.begin());
    quicksort(first, tail.begin(), std::ref(cmp));
    quicksort(ranges::next(tail.begin()), last, std::ref(cmp));
}

int main()
{
    std::ostream_iterator<int> cout {std::cout, " "};

    std::vector<int> v {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    std::cout << "Original vector:  \t";
    ranges::copy(v, cout);

    auto tail = ranges::partition(v, [](int i) { return i % 2 == 0; });

    std::cout << "\nPartitioned vector: \t";
    ranges::copy(ranges::begin(v), ranges::begin(tail), cout);
    std::cout << "│ ";
    ranges::copy(tail, cout);

    std::forward_list<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
    std::cout << "\nUnsorted list: \t\t";
    ranges::copy(fl, cout);

    quicksort(ranges::begin(fl), ranges::end(fl), ranges::greater {});
    std::cout << "\nQuick-sorted list: \t";
    ranges::copy(fl, cout);

    std::cout << '\n';
}

Possible Output

Original vector:        0 1 2 3 4 5 6 7 8 9
Partitioned vector:     0 8 2 6 4 │ 5 3 7 1 9
Unsorted list:          1 30 -4 3 5 -4 1 6 -8 2 -5 64 1 92
Quick-sorted list:      92 64 30 6 5 3 2 1 1 1 -4 -4 -5 -8