< cpp‎ | algorithm
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations

Numeric operations
C library
Defined in header <algorithm>
template< class InputIt, class UnaryPredicate >
bool is_partitioned( InputIt first, InputIt last, UnaryPredicate p );
(1) (since C++11)
template< class ExecutionPolicy, class InputIt, class UnaryPredicate >
bool is_partitioned( ExecutionPolicy&& policy, InputIt first, InputIt last, UnaryPredicate p );
(2) (since C++17)
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>> is true


[edit] Parameters

first, last - the range of elements to check
policy - the execution policy to use. See execution policy for details.
p - unary predicate which returns ​true for the elements expected to be found in the beginning of the range.

The signature of the predicate function should be equivalent to the following:

 bool pred(const Type &a);

The signature does not need to have const &, but the function must not modify the objects passed to it.
The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type. ​

Type requirements
InputIt must meet the requirements of InputIterator.
UnaryPredicate must meet the requirements of Predicate.

[edit] Return value

true if the range [first, last) is empty or is partitioned by p. 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, std::terminate is called.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

template< class InputIt, class UnaryPredicate >
bool is_partitioned(InputIt first, InputIt last, UnaryPredicate p)
    for (; first != last; ++first)
        if (!p(*first))
    for (; first != last; ++first)
        if (p(*first))
            return false;
    return true;

[edit] Example

#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 << 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.begin(), v.end(), is_even);


false true false

[edit] See also

divides a range of elements into two groups
(function template) [edit]
locates the partition point of a partitioned range
(function template) [edit]
parallelized version of std::is_partitioned
(function template) [edit]