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

Sorts the elements in the range [first, last) in non-descending order. The order of equivalent elements is stable, i.e. guaranteed to be preserved.

A sequence is sorted with respect to a comparator comp if for any iterator it pointing to the sequence and any non-negative integer n such that it + n is a valid iterator pointing to an element of the sequence, std::invoke(comp, std::invoke(proj, *(it + n)), std::invoke(proj, *it) evaluates to false.

1. Elements are compared using the given binary comparison function comp.

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 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 Notes
• 5 Possible implementation
• 6 Example
• 7 See also

[edit] Parameters

[edit] Return value

An iterator equal to last.

[edit] Complexity

\(\scriptsize N\cdot\log{(N)}\)N·log(N) comparisons, if extra memory is available; where \(\scriptsize N\)N is ranges::distance(first, last). \(\scriptsize N\cdot\log^2{(N)}\)N·log²(N) comparisons otherwise. Twice as many projections as the number of comparisons in both cases.

[edit] Notes

[edit] Possible implementation

This implementation only shows the slower algorithm used when no additional memory is available. See also implementation in MSVC STL and libstdc++.

struct stable_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 //< since C++26 I operator()(I first, S last, Comp comp = {}, Proj proj = {}) const { auto count = ranges::distance(first, last); auto mid = first + count / 2; auto last_it = first + count;   if (count <= 1) return last_it;   (*this)(first, mid, std::ref(comp), std::ref(proj)); (*this)(mid, last_it, std::ref(comp), std::ref(proj));   ranges::inplace_merge(first, mid, last_it);   return last_it; }   template<ranges::random_access_range R, class Comp = ranges::less, class Proj = std::identity> requires std::sortable<ranges::iterator_t, Comp, Proj> constexpr //< since C++26 ranges::borrowed_iterator_t operator()(R&& r, Comp comp = {}, Proj proj = {}) const { return (*this)(ranges::begin(r), ranges::end(r), std::move(comp), std::move(proj)); } };   inline constexpr stable_sort_fn stable_sort{};

[edit] Example

#include #include #include #include #include   void print(const auto& seq) { for (const auto& elem : seq) std::cout << elem << ' '; std::cout << '\n'; }   struct Particle { std::string name; double mass; // MeV friend std::ostream& operator<<(std::ostream& os, const Particle& p) { return os << '\n' << std::left << std::setw(8) << p.name << " : " << p.mass; } };   int main() { std::array s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};   // sort using the default operator< std::ranges::stable_sort(s); print(s);   // sort using a standard library compare function object std::ranges::stable_sort(s, std::ranges::greater()); print(s);   // sort using a custom function object struct { bool operator()(int a, int b) const { return a < b; } } customLess; std::ranges::stable_sort(s.begin(), s.end(), customLess); print(s);   // sort using a lambda expression std::ranges::stable_sort(s, [](int a, int b) { return a > b; }); print(s);   // sort with projection Particle particles[] { {"Electron", 0.511}, {"Muon", 105.66}, {"Tau", 1776.86}, {"Positron", 0.511}, {"Proton", 938.27}, {"Neutron", 939.57} }; print(particles); std::ranges::stable_sort(particles, {}, &Particle::name); //< sorts by name print(particles); std::ranges::stable_sort(particles, {}, &Particle::mass); //< sorts by mass print(particles); }

Output:

0 1 2 3 4 5 6 7 8 9 9 8 7 6 5 4 3 2 1 0 0 1 2 3 4 5 6 7 8 9 9 8 7 6 5 4 3 2 1 0   Electron : 0.511 Muon  : 105.66 Tau  : 1776.86 Positron : 0.511 Proton  : 938.27 Neutron  : 939.57   Electron : 0.511 Muon  : 105.66 Neutron  : 939.57 Positron : 0.511 Proton  : 938.27 Tau  : 1776.86   Electron : 0.511 Positron : 0.511 Muon  : 105.66 Proton  : 938.27 Neutron  : 939.57 Tau  : 1776.86

[edit] See also