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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 ); |  | (since C++11)

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.

### [edit] Parameters

first, last | - | the range of elements to check
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`.
- `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] 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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