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std::is_sorted - cppreference.com

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Defined in header <algorithm>

template< class ForwardIt > bool is_sorted( ForwardIt first, ForwardIt last );

 (1) (since C++11)
(constexpr since C++20)

template< class ExecutionPolicy, class ForwardIt > bool is_sorted( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );

 (2) (since C++17)

template< class ForwardIt, class Compare > bool is_sorted( ForwardIt first, ForwardIt last, Compare comp );

 (3) (since C++11)
(constexpr since C++20)

template< class ExecutionPolicy, class ForwardIt, class Compare > bool is_sorted( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Compare comp );

 (4) (since C++17)

Checks if the elements in range [first, last) are sorted in non-descending order.

1) Checks if the elements are sorted with respect to operator<(until C++20)std::less{}(since C++20).

3) Checks if the elements are sorted with respect to comp.

2,4) Same as (1,3), but executed according to policy.

These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)

Parameters

first, last - the pair of iterators defining the range of elements to examine
policy - the execution policy to use
comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second.

The signature of the comparison function should be equivalent to the following:

bool cmp(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 an object of type ForwardIt can be dereferenced and then implicitly converted to both of them. ​

Type requirements
-ForwardIt must meet the requirements of LegacyForwardIterator.
-Compare must meet the requirements of Compare.

Return value

true if the elements in the range are sorted in non-descending order, false otherwise.

Complexity

Given N as std::distance(first, last):

1,2) O(N) comparisons using operator<(until C++20)std::less{}(since C++20).

3,4) O(N) applications of the comparator comp.

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

  • If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

See also the implementations in libstdc++ and libc++.

is_sorted (1) 
template<class ForwardIt>
bool is_sorted(ForwardIt first, ForwardIt last)
{
    return std::is_sorted_until(first, last) == last;
}
is_sorted (3) 
template<class ForwardIt, class Compare>
bool is_sorted(ForwardIt first, ForwardIt last, Compare comp)
{
    return std::is_sorted_until(first, last, comp) == last;
}

Notes

std::is_sorted returns true for empty ranges and ranges of length one.

Example

#include <algorithm>
#include <cassert>
#include <functional>
#include <iterator>
#include <vector>

int main()
{
    std::vector<int> v;
    assert(std::is_sorted(v.cbegin(), v.cend()) && "an empty range is always sorted");
    v.push_back(42);
    assert(std::is_sorted(v.cbegin(), v.cend()) && "a range of size 1 is always sorted");
    
    int data[] = {3, 1, 4, 1, 5};
    assert(not std::is_sorted(std::begin(data), std::end(data)));
    
    std::sort(std::begin(data), std::end(data));
    assert(std::is_sorted(std::begin(data), std::end(data)));
    assert(not std::is_sorted(std::begin(data), std::end(data), std::greater<>{}));
}

See also