# std::ranges::is_partitioned

From cppreference.com
< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::input_iterator I, std::sentinel_for <I> S, class Proj = std::identity, std::indirect_unary_predicate <std:: projected <I, Proj>> Pred > constexpr bool is_partitioned( I first, S last, Pred pred, Proj proj = {} ); | (1) | (since C++20)
template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate < std:: projected < ranges::iterator_t <R>, Proj>> Pred > constexpr bool is_partitioned( R&& r, Pred pred, Proj proj = {} ); | (2) | (since C++20)

1) Returns true if all elements in the range `[`first`,` last`)` that satisfy the predicate pred after projection appear before all elements that don't. Also returns true if `[`first`,` last`)` is empty.
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 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

first, last | - | iterator-sentinel pair denoting the range of elements to examine
r | - | the range of elements to examine
pred | - | predicate to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

true if the range `[`first`,` last`)` is empty or is partitioned by pred, false otherwise.

### Complexity

At most ranges::distance (first, last) applications of pred and proj.

### Possible implementation

    struct is_partitioned_fn
    {
        template<std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
                 std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
        constexpr bool operator()(I first, S last, Pred pred, Proj proj = {}) const
        {
            for (; first != last; ++first)
                if (!std::invoke(pred, std::invoke(proj, *first)))
                    break;

            for (; first != last; ++first)
                if (std::invoke(pred, std::invoke(proj, *first)))
                    return false;

            return true;
        }

        template<ranges::input_range R, class Proj = std::identity,
                 std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred>
        constexpr bool operator()(R&& r, Pred pred, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj));
        }
    };

    inline constexpr auto is_partitioned = is_partitioned_fn();

### Example

Run this code

    #include <algorithm>
    #include <array>
    #include <iostream>
    #include <numeric>
    #include <utility>

    int main()
    {
        std::array<int, 9> v;

        auto print = [&v](bool o)
        {
            for (int x : v)
                std::cout << x << ' ';
            std::cout << (o ? "=> " : "=> not ") << "partitioned\n";
        };

        auto is_even = [](int i) { return i % 2 == 0; };

        std::iota(v.begin(), v.end(), 1); // or std::ranges::iota(v, 1);
        print(std::ranges::is_partitioned(v, is_even));

        std::ranges::partition(v, is_even);
        print(std::ranges::is_partitioned(std::as_const(v), is_even));

        std::ranges::reverse(v);
        print(std::ranges::is_partitioned(v.cbegin(), v.cend(), is_even));
        print(std::ranges::is_partitioned(v.crbegin(), v.crend(), is_even));
    }

Output:

    1 2 3 4 5 6 7 8 9 => not partitioned
    2 4 6 8 5 3 7 1 9 => partitioned
    9 1 7 3 5 8 6 4 2 => not partitioned
    9 1 7 3 5 8 6 4 2 => partitioned

### See also

ranges::partition (C++20) | divides a range of elements into two groups (algorithm function object)
ranges::partition_point (C++20) | locates the partition point of a partitioned range (algorithm function object)
is_partitioned (C++11) | determines if the range is partitioned by the given predicate (function template)

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