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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, ...
    Execution policies (C++17)
    Non-modifying sequence operations
    Batch operations
    (C++17)
    Search operations
    (C++11)                (C++11)(C++11)

    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
    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
    C library
    Numeric operations
    Operations on uninitialized memory
     
    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 all following conditions are satisfied:
    See More

    std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

    		(until C++20)

    std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

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