std::ranges::stable_partition() algorithm
- od C++20
- since C++26
- Simplified
- Detailed
// (1)
ranges::subrange<I>
stable_partition( I first, S last, Pred pred, Proj proj = {} );
// (2)
ranges::borrowed_subrange_t<R>
stable_partition( R&& r, Pred pred, Proj proj = {} );
The type of arguments are generic and have the following constraints:
I
-std::bidirectional_iterator
S
-std::sentinel_for<I>
R
-std::ranges::bidirectional_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:
- (1) -
std::permutable<I>
- (2) -
std::permutable<ranges::iterator_t<R>>
// (1)
template<
std::bidirectional_iterator I,
std::sentinel_for<I> S,
class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred
>
requires std::permutable<I>
ranges::subrange<I>
stable_partition( I first, S last, Pred pred, Proj proj = {} );
// (2)
template<
ranges::bidirectional_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>>
ranges::borrowed_subrange_t<R>
stable_partition( R&& r, Pred pred, Proj proj = {} );
- Simplified
- Detailed
// (1)
constexpr ranges::subrange<I>
stable_partition( I first, S last, Pred pred, Proj proj = {} );
// (2)
constexpr ranges::borrowed_subrange_t<R>
stable_partition( R&& r, Pred pred, Proj proj = {} );
The type of arguments are generic and have the following constraints:
I
-std::bidirectional_iterator
S
-std::sentinel_for<I>
R
-std::ranges::bidirectional_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:
- (1) -
std::permutable<I>
- (2) -
std::permutable<ranges::iterator_t<R>>
// (1)
template<
std::bidirectional_iterator I,
std::sentinel_for<I> S,
class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred
>
requires std::permutable<I>
constexpr ranges::subrange<I>
stable_partition( I first, S last, Pred pred, Proj proj = {} );
// (2)
template<
ranges::bidirectional_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>
stable_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
.The algorithms is stable, the relative order of elements is 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
An object equal to
{
pivot,
last
}
Where pivot is an iterator to the first element of the second group.
- (2) Same as (1) if
r
is an lvalue or of aborrowed_range
type. Otherwise returnsstd::ranges::dangling
.
Complexity
Given N
ranges::distance(first, last)
:
The complexity is at worst N * log(N) swaps, and only O(N) swaps in case an extra memory buffer is used.
Exactly N
applications of the predicate pred
and projection proj
.
Exceptions
(none)
Possible implementation
stable_partition(1) and stable_partition(2)
struct stable_partition_fn
{
template<std::bidirectional_iterator I, std::sentinel_for<I> S,
class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
requires std::permutable<I>
constexpr ranges::subrange<I>
operator()(I first, S last, Pred pred, Proj proj = {}) const
{
first = ranges::find_if_not(first, last, pred, proj);
I mid = first;
while (mid != last)
{
mid = ranges::find_if(mid, last, pred, proj);
if (mid == last)
break;
I last2 = ranges::find_if_not(mid, last, pred, proj);
ranges::rotate(first, mid, last2);
first = ranges::next(first, ranges::distance(mid, last2));
mid = last2;
}
return {std::move(first), std::move(mid)};
}
template<ranges::bidirectional_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::move(pred), std::move(proj));
}
};
inline constexpr stable_partition_fn stable_partition {};
Notes
This function attempts to allocate a temporary buffer. If the allocation fails, the less efficient algorithm is chosen.
Examples
#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
namespace rng = std::ranges;
template<std::permutable I, std::sentinel_for<I> S>
constexpr void stable_sort(I first, S last)
{
if (first == last)
return;
auto pivot = *rng::next(first, rng::distance(first, last) / 2, last);
auto left = [pivot](const auto& em) { return em < pivot; };
auto tail1 = rng::stable_partition(first, last, left);
auto right = [pivot](const auto& em) { return !(pivot < em); };
auto tail2 = rng::stable_partition(tail1, right);
stable_sort(first, tail1.begin());
stable_sort(tail2.begin(), tail2.end());
}
void print(const auto rem, auto first, auto last, bool end = true)
{
std::cout << rem;
for (; first != last; ++first)
std::cout << *first << ' ';
std::cout << (end ? "\n" : "");
}
int main()
{
const auto original = {9, 6, 5, 2, 3, 1, 7, 8};
std::vector<int> vi {};
auto even = [](int x) { return 0 == (x % 2); };
print("Original vector:\t", original.begin(), original.end(), "\n");
vi = original;
const auto ret1 = rng::stable_partition(vi, even);
print("Stable partitioned:\t", vi.begin(), ret1.begin(), 0);
print("│ ", ret1.begin(), ret1.end());
vi = original;
const auto ret2 = rng::partition(vi, even);
print("Partitioned:\t\t", vi.begin(), ret2.begin(), 0);
print("│ ", ret2.begin(), ret2.end());
vi = {16, 30, 44, 30, 15, 24, 10, 18, 12, 35};
print("Unsorted vector: ", vi.begin(), vi.end());
stable_sort(rng::begin(vi), rng::end(vi));
print("Sorted vector: ", vi.begin(), vi.end());
}
Original vector: 9 6 5 2 3 1 7 8
Stable partitioned: 6 2 8 │ 9 5 3 1 7
Partitioned: 8 6 2 │ 5 3 1 7 9
Unsorted vector: 16 30 44 30 15 24 10 18 12 35
Sorted vector: 10 12 15 16 18 24 30 30 35 44
Hover to see the original license.