std::is_partitioned(3)

NAME
std::is_partitioned - std::is_partitioned

Synopsis
   Defined in header <algorithm>
   template< class InputIt, class UnaryPredicate > (since C++11)
   bool is_partitioned( InputIt first, InputIt last, (until C++20)
   UnaryPredicate p );
   template< class InputIt, class UnaryPredicate >
   constexpr bool is_partitioned( InputIt first, InputIt last, (1) (since C++20)
   UnaryPredicate p );
   template< class ExecutionPolicy, class ForwardIt, class
   UnaryPredicate > (2) (since C++17)
   bool is_partitioned( ExecutionPolicy&& policy, ForwardIt
   first, ForwardIt last, UnaryPredicate p );
   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 does not participate
   in overload resolution unless
   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) is true.

Parameters
   first, last - the range of elements to check
   policy - the execution policy to use. See execution policy for details.
                 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
   p - 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
// 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
   partition divides a range of elements into two groups
                        (function template)
   partition_point locates the partition point of a partitioned range
   (C++11) (function template)
   ranges::is_partitioned determines if the range is partitioned by the given predicate
   (C++20) (niebloid)