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>> Pred > (1) (since C++20)
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 may not be specified when calling any of them.
* None of them is visible to argument-dependent lookup.
* When one of them is found by normal unqualified lookup for the name to the left
of the function-call operator, it inhibits argument-dependent lookup.

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};
}

auto begin = 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));

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