std::ranges:: is_sorted_until

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, std:: invoke ( comp, std:: invoke ( proj, * ( it + n ) ) , std:: invoke ( proj, * it ) ) evaluates to false .

The function-like entities described on this page are algorithm function objects (informally known as niebloids ), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup .
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

Return value

The upper bound of the largest range beginning at first in which the elements are sorted in non-descending 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

struct is_sorted_until_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
             std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = ranges::less>
    constexpr I operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
    {
        if (first == last)
            return first;
 
        for (auto next = first; ++next != last; first = next)
            if (std::invoke(comp, std::invoke(proj, *next), std::invoke(proj, *first)))
                return next;
 
        return first;
    }
 
    template<ranges::forward_range R, class Proj = std::identity,
             std::indirect_strict_weak_order<
                 std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less>
    constexpr ranges::borrowed_iterator_t<R>
        operator()(R&& r, Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::ref(comp), std::ref(proj));
    }
};
 
inline constexpr is_sorted_until_fn is_sorted_until;

Notes

ranges::is_sorted_until returns an iterator equal to last for empty ranges and ranges of length one.

Example

#include <array>
#include <algorithm>
#include <iostream>
#include <iterator>
#include <random>
 
int main()
{
    std::random_device rd;
    std::mt19937 g {rd()};
    std::array nums {3, 1, 4, 1, 5, 9};
 
    constexpr int min_sorted_size = 4;
    int sorted_size = 0;
    do
    {
        std::ranges::shuffle(nums, g);
        const auto sorted_end = std::ranges::is_sorted_until(nums);
        sorted_size = std::ranges::distance(nums.begin(), sorted_end);
 
        std::ranges::copy(nums, std::ostream_iterator<int>(std::cout, " "));
        std::cout << " : " << sorted_size << " leading sorted element(s)\n";
    }
    while (sorted_size < min_sorted_size);
}

4 1 9 5 1 3  : 1 leading sorted element(s)
4 5 9 3 1 1  : 3 leading sorted element(s)
9 3 1 4 5 1  : 1 leading sorted element(s)
1 3 5 4 1 9  : 3 leading sorted element(s)
5 9 1 1 3 4  : 2 leading sorted element(s)
4 9 1 5 1 3  : 2 leading sorted element(s)
1 1 4 9 5 3  : 4 leading sorted element(s)

See also