std::is_permutation

 

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Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
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is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Defined in header <algorithm>
template< class ForwardIt1, class ForwardIt2 > bool is_permutation( ForwardIt1 first1, ForwardIt1 last1,                      ForwardIt2 first2 );	(1)	(since C++11) (constexpr since C++20)
template< class ForwardIt1, class ForwardIt2,           class BinaryPredicate > bool is_permutation( ForwardIt1 first1, ForwardIt1 last1,                      ForwardIt2 first2, BinaryPredicate p );	(2)	(since C++11) (constexpr since C++20)
template< class ForwardIt1, class ForwardIt2 > bool is_permutation( ForwardIt1 first1, ForwardIt1 last1,                      ForwardIt2 first2, ForwardIt2 last2 );	(3)	(since C++14) (constexpr since C++20)
template< class ForwardIt1, class ForwardIt2,           class BinaryPredicate > bool is_permutation( ForwardIt1 first1, ForwardIt1 last1,                      ForwardIt2 first2, ForwardIt2 last2,                      BinaryPredicate p );	(4)	(since C++14) (constexpr since C++20)

Checks whether [first1, last1) is a permutation of a range starting from first2:

• For overloads (1,2), the second range has std::distance(first1, last1) elements.
• For overloads (3,4), the second range is [first2, last2).

1,3) Elements are compared using operator==.

2,4) Elements are compared using the given binary predicate p.

If ForwardIt1 and ForwardIt2 have different value types, the program is ill-formed.

If the comparison function is not an equivalence relation, the behavior is undefined.

Parameters

first1, last1	-	the range of elements to compare
first2, last2	-	the second range to compare
p	-	binary predicate which returns ​true if the elements should be treated as equal. The signature of the predicate function should be equivalent to the following:  bool pred(const Type1 &a, const Type2 &b); While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)). The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. ​
Type requirements
- ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.

Return value

true if the range [first1, last1) is a permutation of the range [first2, last2), false otherwise.

Complexity

Given \(\scriptsize N\)N as std::distance(first1, last1):

1) Exactly \(\scriptsize N\)N comparisons using operator== if the two ranges are equal, otherwise \(\scriptsize O(N^2)\)O(N2) comparisons in the worst case.

2) Exactly \(\scriptsize N\)N applications of the predicate p if the two ranges are equal, otherwise \(\scriptsize O(N^2)\)O(N2) applications in the worst case.

3,4) If ForwardIt1 and ForwardIt2 are both LegacyRandomAccessIterator, and last1 - first1 != last2 - first2 is true, no comparison will be made.

Otherwise:

3) Exactly \(\scriptsize N\)N comparisons using operator== if the two ranges are equal, otherwise \(\scriptsize O(N^2)\)O(N2) comparisons in the worst case.

4) Exactly \(\scriptsize N\)N applications of the predicate p if the two ranges are equal, otherwise \(\scriptsize O(N^2)\)O(N2) applications in the worst case.

Possible implementation

template<class ForwardIt1, class ForwardIt2> bool is_permutation(ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first) { // skip common prefix std::tie(first, d_first) = std::mismatch(first, last, d_first);   // iterate over the rest, counting how many times each element // from [first, last) appears in [d_first, d_last) if (first != last) { ForwardIt2 d_last = std::next(d_first, std::distance(first, last)); for (ForwardIt1 i = first; i != last; ++i) { if (i != std::find(first, i, *i)) continue; // this *i has been checked   auto m = std::count(d_first, d_last, *i); if (m == 0 || std::count(i, last, *i) != m) return false; } } return true; }

Note

The std::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 std::is_permutation(x, y) == true means that y consist of "the same" elements, maybe staying at other positions.

Example

Run this code

#include <algorithm>
#include <iostream>
 
template<typename Os, typename V>
Os& operator<<(Os& os, const V& v)
{
    os << "{ ";
    for (const auto& e : v)
        os << e << ' ';
    return os << '}';
}
 
int main()
{
    static constexpr auto v1 = {1, 2, 3, 4, 5};
    static constexpr auto v2 = {3, 5, 4, 1, 2};
    static constexpr auto v3 = {3, 5, 4, 1, 1};
 
    std::cout << v2 << " is a permutation of " << v1 << ": " << std::boolalpha
              << std::is_permutation(v1.begin(), v1.end(), v2.begin()) << '\n'
              << v3 << " is a permutation of " << v1 << ": "
              << std::is_permutation(v1.begin(), v1.end(), v3.begin()) << '\n';
}

Output:

{ 3 5 4 1 2 } is a permutation of { 1 2 3 4 5 }: true
{ 3 5 4 1 1 } is a permutation of { 1 2 3 4 5 }: false

See also

next_permutation	generates the next greater lexicographic permutation of a range of elements (function template)
prev_permutation	generates the next smaller lexicographic permutation of a range of elements (function template)
equivalence_relation (C++20)	specifies that a relation imposes an equivalence relation (concept)
ranges::is_permutation (C++20)	determines if a sequence is a permutation of another sequence (niebloid)