std::ranges::next_permutation, std::ranges::next_permutation_result - cppreference.com (original) (raw)

1. Transforms the range [first, last) into the next permutation, where the set of all permutations is ordered lexicographically with respect to binary comparison function object comp and projection function object proj. Returns {last, true} if such a "next permutation" exists; otherwise transforms the range into the lexicographically first permutation as if by ranges::sort(first, last, comp, proj), and returns {last, false}.

2. Same as (1), but uses r as the source 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 Exceptions
• 4 Complexity
• 5 Notes
• 6 Possible implementation
• 7 Example
• 8 See also

[edit] Parameters

[edit] Return value

1. ranges::next_permutation_result<I>{last, true} if the new permutation is lexicographically greater than the old one. ranges::next_permutation_result<I>{last, false} if the last permutation was reached and the range was reset to the first permutation.

2. Same as (1) except that the return type is ranges::next_permutation_result<ranges::borrowed_iterator_t<R>>.

[edit] Exceptions

Any exceptions thrown from iterator operations or the element swap.

[edit] Complexity

At most \(\scriptsize N/2\)N / 2 swaps, where \(\scriptsize N\)N is ranges::distance(first, last) in case (1) or ranges::distance(r) in case (2). Averaged over the entire sequence of permutations, typical implementations use about 3 comparisons and 1.5 swaps per call.

[edit] Notes

Implementations (e.g. MSVC STL) may enable vectorization when the iterator type models contiguous_iterator and swapping its value type calls neither non-trivial special member function nor ADL-found swap.

[edit] Possible implementation

struct next_permutation_fn { template<std::bidirectional_iterator I, std::sentinel_for S, class Comp = ranges::less, class Proj = std::identity> requires std::sortable<I, Comp, Proj> constexpr ranges::next_permutation_result operator()(I first, S last, Comp comp = {}, Proj proj = {}) const { // check that the sequence has at least two elements if (first == last) return {std::move(first), false}; I i_last{ranges::next(first, last)}; I i{i_last}; if (first == --i) return {std::move(i_last), false}; // main "permutating" loop for (;;) { I i1{i}; if (std::invoke(comp, std::invoke(proj, *--i), std::invoke(proj, *i1))) { I j{i_last}; while ((comp, std::invoke(proj, *i), std::invoke(proj, *--j))) {} std::iter_swap(i, j); std::reverse(i1, i_last); return {std::move(i_last), true}; } // permutation "space" is exhausted if (i == first) { std::reverse(first, i_last); return {std::move(i_last), false}; } } }   template<ranges::bidirectional_range R, class Comp = ranges::less, class Proj = std::identity> requires std::sortable<ranges::iterator_t, Comp, Proj> constexpr ranges::next_permutation_result<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 next_permutation_fn next_permutation {};

[edit] Example

#include #include #include #include #include #include   struct S { char c; int i; auto operator<=>(const S&) const = default; friend std::ostream& operator<<(std::ostream& os, const S& s) { return os << "{'" << s.c << "', " << s.i << "}"; } };   auto print = [](auto const& v, char term = ' ') { std::cout << "{ "; for (const auto& e : v) std::cout << e << ' '; std::cout << '}' << term; };   int main() { std::cout << "Generate all permutations (iterators case):\n"; std::string s{"abc"}; do { print(s); } while (std::ranges::next_permutation(s.begin(), s.end()).found);   std::cout << "\n" "Generate all permutations (range case):\n"; std::array a{'a', 'b', 'c'}; do { print(a); } while (std::ranges::next_permutation(a).found);   std::cout << "\n" "Generate all permutations using comparator:\n"; using namespace std::literals; std::array z{"█"s, "▄"s, "▁"s}; do { print(z); } while (std::ranges::next_permutation(z, std::greater()).found);   std::cout << "\n" "Generate all permutations using projection:\n"; std::array<S, 3> r{S{'A',3}, S{'B',2}, S{'C',1}}; do { print(r, '\n'); } while (std::ranges::next_permutation(r, {}, &S::c).found); }

Output:

Generate all permutations (iterators case): { a b c } { a c b } { b a c } { b c a } { c a b } { c b a } Generate all permutations (range case): { a b c } { a c b } { b a c } { b c a } { c a b } { c b a } Generate all permutations using comparator: { █ ▄ ▁ } { █ ▁ ▄ } { ▄ █ ▁ } { ▄ ▁ █ } { ▁ █ ▄ } { ▁ ▄ █ } Generate all permutations using projection: { {'A', 3} {'B', 2} {'C', 1} } { {'A', 3} {'C', 1} {'B', 2} } { {'B', 2} {'A', 3} {'C', 1} } { {'B', 2} {'C', 1} {'A', 3} } { {'C', 1} {'A', 3} {'B', 2} } { {'C', 1} {'B', 2} {'A', 3} }

[edit] See also

| | generates the next smaller lexicographic permutation of a range of elements(algorithm function object)[edit] | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | determines if a sequence is a permutation of another sequence(algorithm function object)[edit] | | | generates the next greater lexicographic permutation of a range of elements (function template) [edit] | | | generates the next smaller lexicographic permutation of a range of elements (function template) [edit] | | | determines if a sequence is a permutation of another sequence (function template) [edit] |