std::ranges::prev_permutation, std::ranges::prev_permutation_result
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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| template< std::bidirectional_iterator I, std::sentinel_for<I> S, class Comp = ranges::less, class Proj = std::identity > |
(1) | (since C++20) |
| template< ranges::bidirectional_range R, class Comp = ranges::less, class Proj = std::identity > |
(2) | (since C++20) |
| Helper type |
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| template< class I > using prev_permutation_result = ranges::in_found_result<I>; |
(3) | (since C++20) |
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
ranges::sort(first, last, comp, proj); ranges::reverse(first, last);
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 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.
In practice, they may be implemented as function objects, or with special compiler extensions.
Contents |
Parameters
| first, last | - | the range of elements to "permute" |
| r | - | the range of elements to "permute" |
| comp | - | comparison function object which returns true if the first argument is less than the second |
| proj | - | projection to apply to the elements |
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>>.
Exceptions
Any exceptions thrown from iterator operations or the element swap.
Complexity
At most N / 2 swaps, where 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.
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.
Possible implementation
struct prev_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::prev_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}; 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 (!std::invoke(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<R>, Comp, Proj> constexpr ranges::prev_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 prev_permutation_fn prev_permutation {}; |
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{"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} }See also
| (C++20) |
generates the next greater lexicographic permutation of a range of elements (niebloid) |
| (C++20) |
determines if a sequence is a permutation of another sequence (niebloid) |
| generates the next greater lexicographic permutation of a range of elements (function template) | |
| generates the next smaller lexicographic permutation of a range of elements (function template) | |
| (C++11) |
determines if a sequence is a permutation of another sequence (function template) |
