# std::partition

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
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 `[`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)

### 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.

### Return value

Iterator to the first element of the second group.

### 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.

### 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.

### 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);
                ++first;
            }

        return first;
    }

### Example

Run this code

    #include <algorithm>
    #include <forward_list>
    #include <iostream>
    #include <iterator>
    #include <vector>

    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: ";
        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

### 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)

### See also

is_partitioned (C++11) | determines if the range is partitioned by the given predicate (function template)
stable_partition | divides elements into two groups while preserving their relative order (function template)
ranges::partition (C++20) | divides a range of elements into two groups (niebloid)

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