# 27 Algorithms library [algorithms]

## 27.6 Non-modifying sequence operations [alg.nonmodifying]

### 27.6.14 Is permutation [alg.is.permutation]

```template<class ForwardIterator1, class ForwardIterator2> constexpr bool is_permutation(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> constexpr bool is_permutation(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, BinaryPredicate pred); template<class ForwardIterator1, class ForwardIterator2> constexpr bool is_permutation(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2); template<class ForwardIterator1, class ForwardIterator2, class BinaryPredicate> constexpr bool is_permutation(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2, ForwardIterator2 last2, BinaryPredicate pred); ```
Let last2 be first2 + (last1 - first1) for the overloads with no parameter named last2, and let pred be equal_to{} for the overloads with no parameter pred.
Mandates: ForwardIterator1 and ForwardIterator2 have the same value type.
Preconditions: The comparison function is an equivalence relation.
Returns: If last1 - first1 != last2 - first2, return false.
Otherwise return true if there exists a permutation of the elements in the range [first2, last2), beginning with ForwardIterator2 begin, such that equal(first1, last1, begin, pred) returns true; otherwise, returns false.
Complexity: No applications of the corresponding predicate if ForwardIterator1 and ForwardIterator2 meet the requirements of random access iterators and last1 - first1 != last2 - first2.
Otherwise, exactly last1 - first1 applications of the corresponding predicate if equal(first1, last1, first2, last2, pred) would return true; otherwise, at worst , where N has the value last1 - first1.
```template<forward_iterator I1, sentinel_for<I1> S1, forward_iterator I2, sentinel_for<I2> S2, class Proj1 = identity, class Proj2 = identity, indirect_equivalence_relation<projected<I1, Proj1>, projected<I2, Proj2>> Pred = ranges::equal_to> constexpr bool ranges::is_permutation(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<forward_range R1, forward_range R2, class Proj1 = identity, class Proj2 = identity, indirect_equivalence_relation<projected<iterator_t<R1>, Proj1>, projected<iterator_t<R2>, Proj2>> Pred = ranges::equal_to> constexpr bool ranges::is_permutation(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); ```
Returns: If last1 - first1 != last2 - first2, return false.
Otherwise return true if there exists a permutation of the elements in the range [first2, last2), bounded by [pfirst, plast), such that ranges​::​equal(first1, last1, pfirst, plast, pred, proj1, proj2) returns true; otherwise, returns false.
Complexity: No applications of the corresponding predicate and projections if:
Otherwise, exactly last1 - first1 applications of the corresponding predicate and projections if ranges​::​equal(​first1, last1, first2, last2, pred, proj1, proj2) would return true; otherwise, at worst , where N has the value last1 - first1.