< cpp‎ | algorithm
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
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
Sampling operations

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
Lexicographical comparison operations
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 [firstlast) 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

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)


[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 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 [firstlast) 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))
    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.cbegin(), v.cend(), is_even) << ' ';
    std::cout << std::is_partitioned(v.crbegin(), v.crend(), is_even) << '\n';


false true false true

[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]
determines if the range is partitioned by the given predicate