std::ranges::partition(3)

NAME
std::ranges::partition - std::ranges::partition

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::permutable I, std::sentinel_for<I> S, class Proj =
   std::identity,
             std::indirect_unary_predicate<std::projected<I, Proj>> (1) (since C++20)
   Pred >
   constexpr ranges::subrange<I>
       partition( I first, S last, Pred pred, Proj proj = {} );
   template< ranges::forward_range R, class Proj = std::identity,
             std::indirect_unary_predicate<
                 std::projected<ranges::iterator_t<R>, Proj>> Pred > (2) (since C++20)
   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. Relative
   order of elements is not preserved.
   2) Same as (1), but uses r as the source 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, that is:
     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first, last - the range of elements to reorder
   r - the range of elements to reorder
   pred - predicate to apply to the projected elements
   proj - 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 = 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.

Possible implementation
   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;

Example
// Run this code
 #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

See also
   ranges::partition_copy copies a range dividing the elements into two groups
   (C++20) (niebloid)
   ranges::is_partitioned determines if the range is partitioned by the given
   (C++20) predicate
                        (niebloid)
   ranges::stable_partition divides elements into two groups while preserving their
   (C++20) relative order
                        (niebloid)
   partition divides a range of elements into two groups
                        (function template)