std::is_partitioned - cppreference.com (original) (raw)

Defined in header
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)

Checks whether [first, last) is partitioned by the predicate p: all elements satisfy p appear before all elements that do not.
Same as (1), but executed according to policy.

This overload participates in overload resolution only if all following conditions are satisfied:

[edit] 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.

[edit] Return value

true if the elements e of [first, last) are partitioned with respect to the expression p(e). false otherwise.

[edit] Complexity

At most std::distance(first, last) applications of p.

[edit] 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.

[edit] 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; }

[edit] Example

#include #include #include 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: