std::is_partitioned

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.

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`.

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.

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

#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

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)
std::experimental::parallel::is_partitioned (parallelism TS)	parallelized version of `std::is_partitioned` (function template)