std::ranges::is_permutation
| Defined in header <algorithm>
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| Call signature |
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| template< std::forward_iterator I1, std::sentinel_for<I1> S1, std::forward_iterator I2, std::sentinel_for<I2> S2, |
(1) | (since C++20) |
| template< ranges::forward_range R1, ranges::forward_range R2, class Proj1 = std::identity, class Proj2 = std::identity, |
(2) | (since C++20) |
[first1, last1) that makes the range equal to [first2, last2) (after application of corresponding projections Proj1, Proj2, and using the binary predicate Pred as a comparator). Otherwise returns false.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.
Parameters
| first1, last1 | - | the first range of the elements |
| first2, last2 | - | the second range of the elements |
| r1 | - | the first range of the elements |
| r2 | - | the second range of the elements |
| pred | - | predicate to apply to the projected elements |
| proj1 | - | projection to apply to the elements in the first range |
| proj2 | - | projection to apply to the elements in the second range |
Return value
true if the range [first1, last1) is a permutation of the range [first2, last2).
Complexity
At most O(N2) applications of the predicate and each projection, or exactly N if the sequences are already equal, where N is ranges::distance(first1, last1). However if ranges::distance(first1, last1) != ranges::distance(first2, last2), no applications of the predicate and projections are made.
Notes
The permutation relation is an equivalence relation.
The ranges::is_permutation can be used in testing, namely to check the correctness of rearranging algorithms (e.g. sorting, shuffling, partitioning). If x is an original range and y is a permuted range then ranges::is_permutation(x, y) == true means that y consist of "the same" elements, maybe staying at other positions.
Possible implementation
struct is_permutation_fn { template<std::forward_iterator I1, std::sentinel_for<I1> S1, std::forward_iterator I2, std::sentinel_for<I2> S2, class Proj1 = std::identity, class Proj2 = std::identity, std::indirect_equivalence_relation<std::projected<I1, Proj1>, std::projected<I2, Proj2>> Pred = ranges::equal_to> constexpr bool operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const { // skip common prefix auto ret = std::ranges::mismatch(first1, last1, first2, last2, std::ref(pred), std::ref(proj1), std::ref(proj2)); first1 = ret.in1, first2 = ret.in2; // iterate over the rest, counting how many times each element // from [first1, last1) appears in [first2, last2) for (auto i {first1}; i != last1; ++i) { const auto i_proj {std::invoke(proj1, *i)}; auto i_cmp = [&]<typename T>(T&& t) { return std::invoke(pred, i_proj, std::forward<T>(t)); }; if (i != ranges::find_if(first1, i, i_cmp, proj1)) continue; // this *i has been checked if (const auto m {ranges::count_if(first2, last2, i_cmp, proj2)}; m == 0 or m != ranges::count_if(i, last1, i_cmp, proj1)) return false; } return true; } template<ranges::forward_range R1, ranges::forward_range R2, class Proj1 = std::identity, class Proj2 = std::identity, std::indirect_equivalence_relation<std::projected<ranges::iterator_t<R1>, Proj1>, std::projected<ranges::iterator_t<R2>, Proj2>> Pred = ranges::equal_to> constexpr bool operator()(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const { return (*this)(ranges::begin(r1), ranges::end(r1), ranges::begin(r2), ranges::end(r2), std::move(pred), std::move(proj1), std::move(proj2)); } }; inline constexpr is_permutation_fn is_permutation {}; |
Example
#include <algorithm> #include <array> #include <cmath> #include <iostream> #include <ranges> auto& operator<<(auto& os, std::ranges::forward_range auto const& v) { os << "{ "; for (auto const& e : v) os << e << ' '; return os << "}"; } int main() { static constexpr auto r1 = {1, 2, 3, 4, 5}; static constexpr auto r2 = {3, 5, 4, 1, 2}; static constexpr auto r3 = {3, 5, 4, 1, 1}; static_assert( std::ranges::is_permutation(r1, r1) && std::ranges::is_permutation(r1, r2) && std::ranges::is_permutation(r2, r1) && std::ranges::is_permutation(r1.begin(), r1.end(), r2.begin(), r2.end())); std::cout << std::boolalpha << "is_permutation( " << r1 << ", " << r2 << " ): " << std::ranges::is_permutation(r1, r2) << '\n' << "is_permutation( " << r1 << ", " << r3 << " ): " << std::ranges::is_permutation(r1, r3) << '\n' << "is_permutation with custom predicate and projections: " << std::ranges::is_permutation( std::array {-14, -11, -13, -15, -12}, // 1st range std::array {'F', 'E', 'C', 'B', 'D'}, // 2nd range [](int x, int y) { return abs(x) == abs(y); }, // predicate [](int x) { return x + 10; }, // projection for 1st range [](char y) { return int(y - 'A'); }) // projection for 2nd range << '\n'; }
Output:
is_permutation( { 1 2 3 4 5 }, { 3 5 4 1 2 } ): true
is_permutation( { 1 2 3 4 5 }, { 3 5 4 1 1 } ): false
is_permutation with custom predicate and projections: trueSee also
| (C++20) |
generates the next greater lexicographic permutation of a range of elements (niebloid) |
| (C++20) |
generates the next smaller lexicographic permutation of a range of elements (niebloid) |
| (C++11) |
determines if a sequence is a permutation of another sequence (function template) |
| 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++20) |
specifies that a relation imposes an equivalence relation (concept) |