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# std::is_partitioned

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt, class UnaryPredicate > bool is_partitioned( InputIt first, InputIt last, UnaryPredicate p ); | (1) | (since C++11)
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 only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

### [edit] 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 signature of the predicate function should be equivalent to the following: bool pred(const Type &a); The signature does not need to have const &, but the function must not modify the objects passed to it. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type. ​
Type requirements
- `InputIt` must meet the requirements of `InputIterator`.
- `ForwardIt` must meet the requirements of `ForwardIterator`. and its value type must be convertible to UnaryPredicate's argument type
- `UnaryPredicate` must meet the requirements of `Predicate`.

### [edit] Return value

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

### [edit] Complexity

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

### [edit] 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 three 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.

### [edit] 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;
    }

### [edit] 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.begin(), v.end(), is_even);
    }

Output:

    false true false

### [edit] See also

partition | divides a range of elements into two groups (function template) [edit]
partition_point (C++11) | locates the partition point of a partitioned range (function template) [edit]

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