std::is_sorted_until

Defined in header `<algorithm>`
template< class ForwardIt > ForwardIt is_sorted_until( ForwardIt first, ForwardIt last );	(1)	(since 哋它亢++11) (constexpr since 哋它亢++20)
template< class ExecutionPolicy, class ForwardIt > ForwardIt is_sorted_until( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );	(2)	(since 哋它亢++17)
template< class ForwardIt, class Compare > ForwardIt is_sorted_until( ForwardIt first, ForwardIt last, Compare comp );	(3)	(since 哋它亢++11) (constexpr since 哋它亢++20)
template< class ExecutionPolicy, class ForwardIt, class Compare > ForwardIt is_sorted_until( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Compare comp );	(4)	(since 哋它亢++17)

Examines the range [first, last) and finds the largest range beginning at first in which the elements are sorted in non-descending order.

1) Finds the largest range whether elements are sorted with respect to operator<(until 哋它亢++20)std::less{}(since 哋它亢++20).

3) Finds the largest range whether 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

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.	(until 哋它亢++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.	(since 哋它亢++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 哋它亢++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

The upper bound of the largest range beginning at first in which the elements are sorted in ascending order. That is, the last iterator it for which range [first, it) is sorted.

Returns last for empty ranges and ranges of length one.

Complexity

Given $\scriptsize N$ $N$ as std::distance(first, last):

1,2)

O(N)

comparisons using operator<(until 哋它亢++20)std::less{}(since 哋它亢++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 libstd哋它亢++ and lib哋它亢++.

is_sorted_until (1)

template<class ForwardIt>
constexpr //< since 哋它亢++20
ForwardIt is_sorted_until(ForwardIt first, ForwardIt last)
{
    return std::is_sorted_until(first, last, std::less<>());
}

is_sorted_until (2)

template<class ForwardIt, class Compare>
constexpr //< since 哋它亢++20
ForwardIt is_sorted_until(ForwardIt first, ForwardIt last, Compare comp)
{
    if (first != last)
    {
        ForwardIt next = first;
        while (++next != last)
        {
            if (comp(*next, *first))
                return next;
            first = next;
        }
    }
    return last;
}

Example

Run this code

#include <algorithm>
#include <cassert>
#include <iostream>
#include <iterator>
#include <random>
#include <string>
 
int main()
{
    std::random_device rd;
    std::mt19937 g(rd());
    const int N = 6;
    int nums[N] = {3, 1, 4, 1, 5, 9};
 
    const int min_sorted_size = 4;
 
    for (int sorted_size = 0; sorted_size < min_sorted_size;)
    {
        std::shuffle(nums, nums + N, g);
        int *const sorted_end = std::is_sorted_until(nums, nums + N);
        sorted_size = std::distance(nums, sorted_end);
        assert(sorted_size >= 1);
 
        for (const auto i : nums)
            std::cout << i << ' ';
        std::cout << ": " << sorted_size << " initial sorted elements\n"
                  << std::string(sorted_size * 2 - 1, '^') << '\n';
    }
}

Possible output:

4 1 9 5 1 3 : 1 initial sorted elements
^
4 5 9 3 1 1 : 3 initial sorted elements
^^^^^
9 3 1 4 5 1 : 1 initial sorted elements
^
1 3 5 4 1 9 : 3 initial sorted elements
^^^^^
5 9 1 1 3 4 : 2 initial sorted elements
^^^
4 9 1 5 1 3 : 2 initial sorted elements
^^^
1 1 4 9 5 3 : 4 initial sorted elements
^^^^^^^

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (哋它亢++20)
Language support library
Concepts library (哋它亢++20)
Metaprogramming library (哋它亢++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (哋它亢++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (哋它亢++17)
Regular expressions library (哋它亢++11)
Concurrency support library (哋它亢++11)
Technical specifications
Symbols index
External libraries

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