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    std::is_partitioned

    From cppreference.com
     
    C++
     
    Algorithm library
    Constrained algorithms and algorithms on ranges (C++20)
    Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
    Non-modifying sequence operations    
    Batch operations
    (C++17)
    Search operations
    Modifying sequence operations
    Copy operations
    (C++11)
    (C++11)
    (C++11)
    Swap operations
    Transformation operations
    Generation operations
    Removing operations
    Order-changing operations
    (until C++17)(C++11)
    (C++20)(C++20)
    Sampling operations
    (C++17)

    Sorting and related operations
    Partitioning operations
    (C++11)    
    (C++11)

    Sorting operations
    Binary search operations
    (on partitioned ranges)
    Set operations (on sorted ranges)
    Merge operations (on sorted ranges)
    Heap operations
    Minimum/maximum operations
    (C++11)
    (C++17)
    Lexicographical comparison operations
    (C++20)
    Permutation operations


     
    Defined in header <algorithm> 
    template< class InputIt, class UnaryPred >
bool is_partitioned( InputIt first, InputIt last, UnaryPred p );
    		(1) (since C++11)
    (constexpr since C++20) 
    template< class ExecutionPolicy, class ForwardIt, class UnaryPred >
bool is_partitioned( ExecutionPolicy&& policy,
                     ForwardIt first, ForwardIt last, UnaryPred p );
    		(2) (since C++17)
    1) Checks whether [first, last) is partitioned by the predicate p: all elements satisfy p appear before all elements that do not.
    2) Same as (1), but executed according to policy.
    This overload participates in overload resolution only if the value of the following expression is true:

    std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>

    		(until C++20)

    std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>

    		(since C++20)

    Parameters

    first, last - the pair of iterators defining the range of elements to examine
    policy - the execution policy to use
    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 UnaryPred's parameter type.
    -
    UnaryPred must meet the requirements of Predicate.

    Return value

    true if the elements e of [first, last) are partitioned with respect to the expression p(e). 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 UnaryPred>
bool is_partitioned(InputIt first, InputIt last, UnaryPred 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

    		 divides a range of elements into two groups
    (function template & algorithm function object)[edit]
    (C++20)
    (C++11)
    		 locates the partition point of a partitioned range
    (function template & algorithm function object)[edit]
    (C++20)
    (C++20)
    		 determines if the range is partitioned by the given predicate
    (algorithm function object)[edit]