std::ranges::prev_permutation, std::ranges::prev_permutation_result - cppreference.com (original) (raw)

1. Transforms the range [first, last) into the previous 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 "previous" permutation exists. Otherwise,
• {last, false}, and transforms the range into the (lexicographically) last permutation, as if by

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

2. Same as (1) except that the return type is ranges::prev_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 prev_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::prev_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}; auto i{first}; ++i; if (i == last) return {std::move(i), false}; auto i_last{ranges::next(first, last)}; i = i_last; --i; // main "permutating" loop for (;;) { auto i1{i}; --i; if (std::invoke(comp, std::invoke(proj, *i1), std::invoke(proj, *i))) { auto j{i_last}; while ((comp, std::invoke(proj, *--j), std::invoke(proj, *i))) ; ranges::iter_swap(i, j); ranges::reverse(i1, last); return {std::move(i_last), true}; } // permutation "space" is exhausted if (i == first) { ranges::reverse(first, 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::prev_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 prev_permutation_fn prev_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{"cba"}; do print(s); while (std::ranges::prev_permutation(s.begin(), s.end()).found);   std::cout << "\nGenerate all permutations (range case):\n"; std::array a{'c', 'b', 'a'}; do print(a); while (std::ranges::prev_permutation(a).found);   std::cout << "\nGenerate all permutations using comparator:\n"; using namespace std::literals; std::array z{"▁"s, "▄"s, "█"s}; do print(z); while (std::ranges::prev_permutation(z, std::greater()).found);   std::cout << "\nGenerate all permutations using projection:\n"; std::array<S, 3> r{S{'C',1}, S{'B',2}, S{'A',3}}; do print(r, '\n'); while (std::ranges::prev_permutation(r, {}, &S::c).found); }

Output:

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

[edit] See also

| | generates the next greater 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] |