std::ranges::stable_partition

Defined in header `<algorithm>`
Call signature
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 = {} );	(1)	(since C++20)
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 = {} );	(2)	(since C++20)

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. The algorithms is stable, i.e. the relative order of elements is preserved.

2) Same as (1), but uses r as the 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

1) 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 a borrowed_range type. Otherwise returns std::ranges::dangling.

Complexity

Given N = ranges::distance(first, last), the complexity is at worst $\scriptsize N\cdot\log{(N)}$ $N\cdotlog(N)$ swaps, and only $\scriptsize \mathcal{O}(N)$ $𝓞(N)$ swaps in case an extra memory buffer is used. Exactly $\scriptsize N$ $N$ applications of the predicate pred and projection proj.

Notes

This function attempts to allocate a temporary buffer. If the allocation fails, the less efficient algorithm is chosen.

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>
    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>>
    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{};

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:\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());
}

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

Compiler support
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Technical specifications
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External libraries

ranges::partition (C++20)	divides a range of elements into two groups (niebloid)
ranges::partition_copy (C++20)	copies a range dividing the elements into two groups (niebloid)
ranges::is_partitioned (C++20)	determines if the range is partitioned by the given predicate (niebloid)
stable_partition	divides elements into two groups while preserving their relative order (function template)