std::ranges::next_permutation, std::ranges::next_permutation_result
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std::ranges
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Defined in header
<algorithm>
Call signature
template< std::bidirectional_iterator I, std::sentinel_for <I> S,
(1)
(since C++20)
class Comp = ranges::less, class Proj = std::identity >
requires std::sortable <I, Comp, Proj>
constexpr next_permutation_result<I>
template< ranges::bidirectional_range R, class Comp = ranges::less,
(2)
(since C++20)
class Proj = std::identity >
requires std::sortable <ranges::iterator_t <R>, Comp, Proj>
constexpr next_permutation_result<ranges::borrowed_iterator_t <R>>
Helper type
template< class I >
using next_permutation_result = ranges::in_found_result <I>;
(3)
(since C++20)
using next_permutation_result = ranges::in_found_result <I>;
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
[edit] Parameters
first, last
-
the iterator-sentinel pair defining the range of elements to permute
r
-
the
range of elements to permute
comp
-
comparison FunctionObject which returns true if the first argument is less than the second
proj
-
projection to apply to the elements
[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 <I> S, class Comp = ranges::less, class Proj = std::identity > requires std::sortable <I, Comp, Proj> constexpr ranges::next_permutation_result<I> 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 (!std::invoke (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 <R>, Comp, Proj> constexpr ranges::next_permutation_result<ranges::borrowed_iterator_t <R>> 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
Run this code
#include <algorithm> #include <array> #include <compare> #include <functional> #include <iostream> #include <string> 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
(C++20)
(algorithm function object)[edit]
(C++20)
(algorithm function object)[edit]
generates the next greater lexicographic permutation of a range of elements
(function template) [edit]
(function template) [edit]
generates the next smaller lexicographic permutation of a range of elements
(function template) [edit]
(function template) [edit]