std::is_sorted_until

Defined in header <algorithm>

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

(1)

(since C++11)

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

(2)

(since C++11)

Examines the range [first, last) and finds the largest range beginning at first in which the elements are sorted in ascending order. The first version of the function uses operator< to compare the elements, the second uses the given comparison function comp.

Parameters

first, last	-	the range of elements to examine
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);` The signature does not need to have `const &`, but the function object must not modify the objects passed to it. 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 `ForwardIterator`.

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.

Complexity

linear in the distance between first and last.

Possible implementation

First version

template<class ForwardIt>
ForwardIt is_sorted_until(ForwardIt first, ForwardIt last)
{
    if (first != last) {
        ForwardIt next = first;
        while (++next != last) {
            if (*next < *first)
                return next;
            first = next;
        }
    }
    return last;
}

Second version

template <class ForwardIt, class Compare>
ForwardIt is_sorted_until(ForwardIt first, ForwardIt last, Compare comp) 
{
    using namespace std::placeholders;
    ForwardIt it = std::adjacent_find(first, last, std::bind(comp, _2, _1));
    return it == last ? last : std::next(it);
}

Example

#include <iostream>
#include <algorithm>
#include <iterator>
#include <random>
  
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;
    int sorted_size = 0;
    do {
        std::shuffle(nums, nums + N, g);
        int *sorted_end = std::is_sorted_until(nums, nums + N);
        sorted_size = std::distance(nums, sorted_end);
  
        for (auto i : nums) std::cout << i << ' ';
        std::cout << " : " << sorted_size << " initial sorted elements\n";
    } while (sorted_size < min_sorted_size);
}

Possible output:

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

is_sorted (C++11)	checks whether a range is sorted into ascending order (function template)
std::experimental::parallel::is_sorted_until (parallelism TS)	parallelized version of `std::is_sorted_until` (function template)