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