# std::is_partitioned

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class InputIt, class UnaryPredicate > bool is_partitioned( InputIt first, InputIt last, UnaryPredicate p ); |  | (since C++11) (until C++20)
template< class InputIt, class UnaryPredicate > constexpr bool is_partitioned( InputIt first, InputIt last, UnaryPredicate p ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate > bool is_partitioned( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPredicate p ); | (2) | (since C++17)

1) Returns true if all elements in the range `[`first`,` last`)` that satisfy the predicate p appear before all elements that don't. Also returns true if `[`first`,` last`)` is empty.
2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### Parameters

first, last | - | the range of elements to check
policy | - | the execution policy to use. See execution policy for details.
p | - | unary predicate which returns ​true for the elements expected to be found in the beginning of the range. The expression p(v) must be convertible to bool for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of VT&is not allowed, nor is VT unless for `VT` a move is equivalent to a copy (since C++11). ​
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator. and its value type must be convertible to UnaryPredicate's argument type.
- `UnaryPredicate` must meet the requirements of Predicate.

### Return value

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

### Complexity

At most std::distance(first, last) applications of p.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports 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

    template<class InputIt, class UnaryPredicate>
    bool is_partitioned(InputIt first, InputIt last, UnaryPredicate p)
    {
        for (; first != last; ++first)
            if (!p(*first))
                break;
        for (; first != last; ++first)
            if (p(*first))
                return false;
        return true;
    }

### Example

Run this code

    #include <algorithm>
    #include <array>
    #include <iostream>

    int main()
    {
        std::array<int, 9> v {1, 2, 3, 4, 5, 6, 7, 8, 9};

        auto is_even = [](int i) { return i % 2 == 0; };
        std::cout.setf(std::ios_base::boolalpha);
        std::cout << std::is_partitioned(v.begin(), v.end(), is_even) << ' ';

        std::partition(v.begin(), v.end(), is_even);
        std::cout << std::is_partitioned(v.begin(), v.end(), is_even) << ' ';

        std::reverse(v.begin(), v.end());
        std::cout << std::is_partitioned(v.cbegin(), v.cend(), is_even) << ' ';
        std::cout << std::is_partitioned(v.crbegin(), v.crend(), is_even) << '\n';
    }

Output:

    false true false true

### See also

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

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