# std::ranges::is_sorted

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
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 bool is_sorted( I first, S last, Comp comp = {}, Proj proj = {} ); | (1) | (since C++20)
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 bool is_sorted( R&& r, Comp comp = {}, Proj proj = {} ); | (2) | (since C++20)

Checks if the elements in range `[`first`,` last`)` 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.

1) Elements are compared using the given binary comparison function comp.
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are 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.

In practice, they may be implemented as function objects, or with special compiler extensions.

### Parameters

first, last | - | iterator-sentinel defining the range to check if it is sorted
r | - | the range to check if it is sorted
comp | - | comparison function to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

true if the elements in the range are sorted according to `comp`.

### Complexity

Linear in the distance between first and last.

### Possible implementation

    struct is_sorted_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 bool operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
        {
            return ranges::is_sorted_until(first, last, comp, proj) == last;
        }

        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 bool 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_fn is_sorted;

### Notes

`ranges::is_sorted` returns true for empty ranges and ranges of length one.

### Example

Run this code

    #include <algorithm>
    #include <array>
    #include <functional>
    #include <iostream>
    #include <iterator>

    int main()
    {
        namespace ranges = std::ranges;

        std::array digits {3, 1, 4, 1, 5};

        ranges::copy(digits, std::ostream_iterator<int>(std::cout, " "));
        ranges::is_sorted(digits)
            ? std::cout << ": sorted\n"
            : std::cout << ": not sorted\n";

        ranges::sort(digits);

        ranges::copy(digits, std::ostream_iterator<int>(std::cout, " "));
        ranges::is_sorted(ranges::begin(digits), ranges::end(digits))
            ? std::cout << ": sorted\n"
            : std::cout << ": not sorted\n";

        ranges::reverse(digits);

        ranges::copy(digits, std::ostream_iterator<int>(std::cout, " "));
        ranges::is_sorted(digits, ranges::greater {})
            ? std::cout << ": sorted (with 'greater')\n"
            : std::cout << ": not sorted\n";
    }

Output:

    3 1 4 1 5 : not sorted
    1 1 3 4 5 : sorted
    5 4 3 1 1 : sorted (with 'greater')

### See also

ranges::is_sorted_until (C++20) | finds the largest sorted subrange (niebloid)
is_sorted (C++11) | checks whether a range is sorted into ascending order (function template)

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