std::is_sorted_until

template< class ForwardIt >
ForwardIt is_sorted_until( ForwardIt first, ForwardIt last );

template< class ForwardIt >
constexpr ForwardIt is_sorted_until( ForwardIt first, ForwardIt last );

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

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

template< class ForwardIt, class Compare >
constexpr ForwardIt is_sorted_until( ForwardIt first, ForwardIt last,
                                     Compare comp );

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

Examines the range [first, last) and finds the largest range beginning at first in which the elements 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.

Parameters

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

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++.

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

template<class ForwardIt, class Compare>
constexpr //< since C++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

#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 (auto i : nums)
            std::cout << i << ' ';
        std::cout << " : " << sorted_size << " initial sorted elements\n"
                  << std::string(sorted_size * 2 - 1, '^') << '\n';
    }
}

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
^^^^^^^

See also