std::ranges::stable_partition
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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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 = {} );
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(1) | (since C++20) (constexpr since C++26) |
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 = {} );
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(2) | (since C++20) (constexpr since C++26) |
template< /*execution-policy*/ Ep,
std::random_access_iterator I, std::sized_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( Ep&& policy, I first, S last,
Pred pred, Proj proj = {} );
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(3) | (since C++26) |
template< /*execution-policy*/ Ep,
/*sized-random-access-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( Ep&& policy, R&& r, Pred pred, Proj proj = {} );
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(4) | (since C++26) |
For the definition of /*execution-policy*/, see this page; for the definition of /*sized-random-access-range*/, see this page.
1,2) Partitions the elements
e in the target range [first, last) or r with respect to the expression bool(std::invoke(pred, std::invoke(proj, e))): all elements (projected by proj) satisfy pred appear before all elements that do not. The relative order of the elements in both groups is preserved.3,4) Same as (1,2), but executed according to
policy.The function-like entities described on this page are algorithm function objects (informally known as 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.
Parameters
| first, last | - | the iterator-sentinel pair defining the target range |
| r | - | the target range |
| pred | - | the predicate to be applied to the (projected) elements |
| proj | - | the projection to be applied to the elements |
| policy | - | the execution policy to use |
Return value
A subrange from the partition point to the end of the target range. All elements outside the subrange satisfy p, while all elements in the subrange do not.
Complexity
Given N as ranges::distance(first, last) or ranges::distance(r):
1,2) At most N⋅log2(N) swaps (or only 𝓞(N) swaps if there is enough extra memory), and exactly N applications of
pred and proj.3,4) 𝓞(N·log(N)) swaps, and 𝓞(N) applications of
pred and proj.Exceptions
3,4) During the execution process:
- If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
- If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).
Notes
This function attempts to allocate a temporary buffer. If the allocation fails, the less efficient algorithm is chosen.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_constexpr_algorithms |
202306L |
(C++26) | constexpr stable sorting
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Possible implementation
This implementation does not use extra memory buffer and as such can be less efficient. See also the implementation in MSVC STL and libstdc++.
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::next(ranges::begin(r), ranges::end(r)),
std::move(pred), std::move(proj));
}
};
inline constexpr stable_partition_fn stable_partition{};
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Example
Run this code
#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: ", original.begin(), original.end(), "\n");
vi = original;
const auto ret1 = rng::stable_partition(vi, even);
print("Stable partitioned: ", vi.begin(), ret1.begin(), 0);
print("│ ", ret1.begin(), ret1.end());
vi = original;
const auto ret2 = rng::partition(vi, even);
print("Partitioned: ", 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());
}
Possible output:
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