std::ranges::partition

Since C++20

Header: <algorithm>

  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.

# Declarations

Call signature

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 = {} );

(since C++20)

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 = {} );

(since C++20)

# Parameters

# 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.

# Example

#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';
}

# See also