std::ranges::partition() algorithm
- od C++20
- Simplified
- Detailed
// (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>>
- (1) -
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>>
// (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 projectionproj
of all elements for which the predicatepred
returnstrue
precede the projectionproj
of elements for which predicatepred
returnsfalse
.uwagaRelative order of elements is not preserved.
-
(2) Same as (1), but uses
r
as the range, as if usingranges::begin(r)
asfirst
andranges::end(r)
aslast
.
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
ifr
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
#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';
}
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
Hover to see the original license.