# std::partition

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< cpp‎ | algorithm

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

### 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.

### Return value

Iterator to the first element of the second group.

### 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.

### 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

    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 (ForwardIt 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 <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);
        ForwardIt middle1 = std::partition(first, last,
                             [pivot](const auto& em){ return em < pivot; });
        ForwardIt 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';
    }

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

### 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)

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