< 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
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations

Numeric operations
Operations on uninitialized storage
C library
Defined in header <algorithm>
template< class BidirIt, class UnaryPredicate >
BidirIt partition( BidirIt first, BidirIt last, UnaryPredicate p );
(until C++11)
template< class ForwardIt, class UnaryPredicate >
ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );
(since C++11)
(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 [first, last) 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
BidirIt must meet the requirements of LegacyBidirectionalIterator.
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 the predicate. 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 the predicate.

[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 <iostream>
#include <iterator>
#include <vector>
#include <forward_list>
template <class ForwardIt>
 void quicksort(ForwardIt first, ForwardIt last)
    if(first == last) return;
    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:\n    ";
    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:\n    ";
    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:\n    ";
    for(int n : fl) std::cout << n << ' ';
    std::cout << '\n';  
    quicksort(std::begin(fl), std::end(fl));
    std::cout << "Sorted using quicksort:\n    ";
    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] 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
(niebloid) [edit]