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.

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`.

[edit] 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.

[edit] Complexity

linear in the distance between first and last

[edit] 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);
}

[edit] Example

Run this code

#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::random_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:

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

[edit] See also

is_sorted

(C++11)

checks whether a range is sorted into ascending order
(function template)

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)