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std::is_partitioned

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 only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

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

#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)