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std::ranges::is_permutation

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< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
(C++11)
(C++11)
(C++17)

 
Constrained algorithms
All names in this menu belong to namespace std::ranges
       
       
    
     
         
       
       
is_permutation
    
(C++23)
(C++23)  
(C++23)
(C++23)  
(C++23)            
 
Defined in header <algorithm>
Call signature
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
    is_permutation( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},

                    Proj1 proj1 = {}, Proj2 proj2 = {} );
(1) (since C++20)
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
    is_permutation( R1&& r1, R2&& r2, Pred pred = {},

                    Proj1 proj1 = {}, Proj2 proj2 = {} );
(2) (since C++20)
1) Returns true if there exists a permutation of the elements in range [first1last1) that makes the range equal to [first2last2) (after application of corresponding projections Proj1, Proj2, and using the binary predicate Pred as a comparator). Otherwise returns false.
2) Same as (1), but uses r1 as the first source range and r2 as the second source range, as if using ranges::begin (r1) as first1, ranges::end (r1) as last1, ranges::begin (r2) as first2, and ranges::end (r2) as last2.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

[edit] Parameters

first1, last1 - the iterator-sentinel pair defining the first range of elements
first2, last2 - the iterator-sentinel pair defining the second range of 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

[edit] Return value

true if the range [first1last1) is a permutation of the range [first2last2).

[edit] Complexity

At most \(\scriptsize \mathcal{O}(N^2)\)O(N2) applications of the predicate and each projection, or exactly \(\scriptsize N\)N if the sequences are already equal, where \(\scriptsize N\)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.

[edit] Notes

The permutation relation is an equivalence relation.

The ranges::is_permutation can be used in testing, e.g. to check the correctness of rearranging algorithms such as sorting, shuffling, partitioning. If p is an original sequence and q is a "mutated" sequence, then ranges::is_permutation(p, q) == true means that q consist of "the same" elements (maybe permuted) as p.

[edit] 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 {};

[edit] Example

Run this code
#include <algorithm>
#include <array>
#include <cmath>
#include <iostream>
#include <ranges>
 
auto& operator<<(auto& os, std::ranges::forward_range auto const& v)
{
 os << "{ ";
 for (const auto& 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: true

[edit] See also

generates the next greater lexicographic permutation of a range of elements
(algorithm function object)[edit]
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
(function template) [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]
specifies that a relation imposes an equivalence relation
(concept) [edit]
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