std::ranges::partial_sort - cppreference.com (original) (raw)

1. Rearranges elements such that the range [first, middle) contains the sorted middle - first smallest elements in the range [first, last).

The order of equal elements is not guaranteed to be preserved. The order of the remaining elements in the range [middle, last) is unspecified.

The elements are compared using the given binary comparison function comp and projected using proj function object.

1. 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 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.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Possible implementation
• 5 Example
• 6 See also

[edit] Parameters

[edit] Return value

An iterator equal to last.

[edit] Complexity

\(\scriptsize \mathcal{O}(N\cdot\log{(M)})\)𝓞(N·log(M)) comparisons and twice as many projections, where \(\scriptsize N\)N is ranges::distance(first, last), \(\scriptsize M\)M is ranges::distance(first, middle).

[edit] Possible implementation

struct partial_sort_fn { template<std::random_access_iterator I, std::sentinel_for S, class Comp = ranges::less, class Proj = std::identity> requires std::sortable<I, Comp, Proj> constexpr I operator()(I first, I middle, S last, Comp comp = {}, Proj proj = {}) const { if (first == middle) return ranges::next(first, last); ranges::make_heap(first, middle, comp, proj); auto it {middle}; for (; it != last; ++it) { if (std::invoke(comp, std::invoke(proj, *it), std::invoke(proj, *first))) { ranges::pop_heap(first, middle, comp, proj); ranges::iter_swap(middle - 1, it); ranges::push_heap(first, middle, comp, proj); } } ranges::sort_heap(first, middle, comp, proj); return it; }   template<ranges::random_access_range R, class Comp = ranges::less, class Proj = std::identity> requires std::sortable<ranges::iterator_t, Comp, Proj> constexpr ranges::borrowed_iterator_t operator()(R&& r, ranges::iterator_t middle, Comp comp = {}, Proj proj = {}) const { return (*this)(ranges::begin(r), std::move(middle), ranges::end(r), std::move(comp), std::move(proj)); } };   inline constexpr partial_sort_fn partial_sort {};

[edit] Example

#include #include #include #include #include   void print(const auto& v) { for (const char e : v) std::cout << e << ' '; std::cout << '\n'; }   void underscore(int n) { while (n-- > 0) std::cout << "^ "; std::cout << '\n'; }   int main() { static_assert('A' < 'a'); std::vector v {'x', 'P', 'y', 'C', 'z', 'w', 'P', 'o'}; print(v); const int m {3}; std::ranges::partial_sort(v, v.begin() + m); print(v), underscore(m);   static_assert('1' < 'a'); std::string s {"3a1b41c5"}; print(s); std::ranges::partial_sort(s.begin(), s.begin() + m, s.end(), std::greater {}); print(s), underscore(m); }

Output:

x P y C z w P o C P P y z x w o ^ ^ ^ 3 a 1 b 4 1 c 5 c b a 1 3 1 4 5 ^ ^ ^

[edit] See also