< 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 UnaryPredicate >
ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );
(until C++20)
template< class ForwardIt, class UnaryPredicate >

constexpr ForwardIt partition( ForwardIt first, ForwardIt last,

                               UnaryPredicate p );
(since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >

ForwardIt partition( ExecutionPolicy&& policy,

                     ForwardIt first, ForwardIt last, UnaryPredicate 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 the 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 does not participate in overload resolution unless

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 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 ValueSwappable and LegacyForwardIterator. However, the operation is more efficient if ForwardIt also satisfies the requirements of LegacyBidirectionalIterator.
UnaryPredicate must meet the requirements of Predicate.

[edit] Return value

Iterator to the first element of the second group.

[edit] Complexity

Given N = 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·log N) swaps and O(N) 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

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

template<class ForwardIt, class UnaryPredicate>
ForwardIt partition(ForwardIt first, ForwardIt last, UnaryPredicate 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 LegacyForwardIterator (the complexity
requirement for non-bidirectional iterators is weaker)

[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