# 24 Algorithms library [algorithms]

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

### 24.5.12 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); ```
Requires: ForwardIterator1 and ForwardIterator2 shall have the same value type.
The comparison function shall be an equivalence relation.
Remarks: If last2 was not given in the argument list, it denotes first2 + (last1 - first1) below.
Returns: If last1 - first1 != last2 - first2, return false.
Otherwise return true if there exists a permutation of the elements in the range [first2, first2 + (last1 - first1)), beginning with ForwardIterator2 begin, such that equal(first1, last1, begin) returns true or 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) would return true if pred was not given in the argument list or equal(first1, last1, first2, last2, pred) would return true if pred was given in the argument list; otherwise, at worst , where N has the value last1 - first1.
```namespace ranges { template<ForwardIterator I1, Sentinel<I1> S1, ForwardIterator I2, Sentinel<I2> S2, class Pred = ranges::equal_to<>, class Proj1 = identity, class Proj2 = identity> requires IndirectlyComparable<I1, I2, Pred, Proj1, Proj2> constexpr bool is_permutation(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}); template<ForwardRange R1, ForwardRange R2, class Pred = ranges::equal_to<>, class Proj1 = identity, class Proj2 = identity> requires IndirectlyComparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2> constexpr bool 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:
• S1 and I1 model SizedSentinel,
• S2 and I2 model SizedSentinel, and
• last1 - first1 != last2 - first2.
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.