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std::is_sorted

Defined in header `<algorithm>`
	(1)
template< class ForwardIt > bool is_sorted( ForwardIt first, ForwardIt last );		(since C++11) (until C++20)
template< class ForwardIt > constexpr bool is_sorted( ForwardIt first, ForwardIt last );		(since C++20)
template< class ExecutionPolicy, class ForwardIt > bool is_sorted( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );	(2)	(since C++17)
	(3)
template< class ForwardIt, class Compare > bool is_sorted( ForwardIt first, ForwardIt last, Compare comp );		(since C++11) (until C++20)
template< class ForwardIt, class Compare > constexpr bool is_sorted( ForwardIt first, ForwardIt last, Compare comp );		(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.

A sequence is sorted with respect to a comparator comp if for any iterator it pointing to the sequence and any non-negative integer n such that it + n is a valid iterator pointing to an element of the sequence, comp(*(it + n), *it) evaluates to false.

1) Elements are compared using operator<.

3) Elements are compared using the given binary comparison function comp.

2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless

`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 range of elements to examine.
policy	-	the execution policy to use. See execution policy for details.
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.

Return value

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

Complexity

Linear in the distance between first and last.

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; }

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 <iostream>
#include <iterator>
 
int main()
{
    int digits[] = {3, 1, 4, 1, 5};
 
    for (auto i : digits) std::cout << i << ' ';
    std::cout << ": is_sorted: " << std::boolalpha
              << std::is_sorted(std::begin(digits), std::end(digits)) << '\n';
 
    std::sort(std::begin(digits), std::end(digits));
 
    for (auto i : digits) std::cout << i << ' ';
    std::cout << ": is_sorted: "
              << std::is_sorted(std::begin(digits), std::end(digits)) << '\n';
}

Output:

3 1 4 1 5 : is_sorted: false
1 1 3 4 5 : is_sorted: true

is_sorted_until (C++11)	finds the largest sorted subrange (function template)
ranges::is_sorted (C++20)	checks whether a range is sorted into ascending order (niebloid)