< 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 ForwardIt, class UnaryPred >
ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPred p );
(1) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred >

ForwardIt partition( ExecutionPolicy&& policy,

                     ForwardIt first, ForwardIt last, UnaryPred p );
(2) (since C++17)
1) Reorders the elements in the range [firstlast) in such a way that all elements for which the predicate p returns true precede all elements for which predicate p returns false. Relative order of the elements is not preserved.
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)

If the type of *first is not Swappable(until C++11)ForwardIt is not ValueSwappable(since C++11), the behavior is undefined.


[edit] Parameters

first, last - the range of elements to reorder
policy - the execution policy to use. See execution policy for details.
p - unary predicate which returns ​true if the element should be ordered before other elements.

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 ForwardIt, 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
ForwardIt must meet the requirements of LegacyForwardIterator.
UnaryPred must meet the requirements of Predicate.

[edit] Return value

Iterator to the first element of the second group.

[edit] Complexity

Given N as std::distance(first, last):

1) Exactly N applications of p.
At most N/2 swaps if ForwardIt meets the requirements of LegacyBidirectionalIterator, and at most N swaps otherwise.
2) O(N) applications of p.
O(N·log(N)) swaps.

[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

Implements overload (1) preserving C++11 compatibility.

template<class ForwardIt, class UnaryPred>
ForwardIt partition(ForwardIt first, ForwardIt last, UnaryPred p)
    first = std::find_if_not(first, last, p);
    if (first == last)
        return first;
    for (auto i = std::next(first); i != last; ++i)
        if (p(*i))
            std::iter_swap(i, first);
    return first;

[edit] Example

#include <algorithm>
#include <forward_list>
#include <iostream>
#include <iterator>
#include <vector>
template<class ForwardIt>
void quicksort(ForwardIt first, ForwardIt last)
    if (first == last)
    auto pivot = *std::next(first, std::distance(first, last) / 2);
    auto middle1 = std::partition(first, last, [pivot](const auto& em)
        return em < pivot;
    auto middle2 = std::partition(middle1, last, [pivot](const auto& em)
        return !(pivot < em);
    quicksort(first, middle1);
    quicksort(middle2, last);
int main()
    std::vector<int> v{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
    std::cout << "Original vector: ";
    for (int elem : v)
        std::cout << elem << ' ';
    auto it = std::partition(v.begin(), v.end(), [](int i) {return i % 2 == 0;});
    std::cout << "\nPartitioned vector: ";
    std::copy(std::begin(v), it, std::ostream_iterator<int>(std::cout, " "));
    std::cout << "* ";
    std::copy(it, std::end(v), std::ostream_iterator<int>(std::cout, " "));
    std::forward_list<int> fl {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};
    std::cout << "\nUnsorted list: ";
    for (int n : fl)
        std::cout << n << ' ';
    quicksort(std::begin(fl), std::end(fl));
    std::cout << "\nSorted using quicksort: ";
    for (int fi : fl)
        std::cout << fi << ' ';
    std::cout << '\n';

Possible output:

Original vector: 0 1 2 3 4 5 6 7 8 9 
Partitioned vector: 0 8 2 6 4 * 5 3 7 1 9 
Unsorted list: 1 30 -4 3 5 -4 1 6 -8 2 -5 64 1 92 
Sorted using quicksort: -8 -5 -4 -4 1 1 1 2 3 5 6 30 64 92

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 498 C++98 std::partition required first and
last to be LegacyBidirectionalIterator
only required to be
LWG 2150 C++98 std::partition was only required to place one element
satisfying p before one element not satisfying p
corrected the

[edit] See also

determines if the range is partitioned by the given predicate
(function template) [edit]
divides elements into two groups while preserving their relative order
(function template) [edit]
divides a range of elements into two groups