# std::is_sorted_until

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< cpp‎ | algorithm

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

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

1) Elements are compared using `operator<`.
3) Elements are compared using the given binary comparison function `comp`.
2,4) Same as (1,3), but executed according to `policy`. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

### 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 C++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.

### 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`

### 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

First version
    template<class ForwardIt>
    ForwardIt is_sorted_until(ForwardIt first, ForwardIt last)
    {
        return is_sorted_until(first, last, std::less<>());
    }
Second version
    template <class ForwardIt, class Compare>
    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;
    }

### Notes

std::is_sorted and std::is_sorted_until both return true for empty ranges and ranges of length one.

### 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::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

### See also

is_sorted (C++11) | checks whether a range is sorted into ascending order (function template)

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