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# std::partition

From cppreference.com
< 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)
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

### [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 signature of the predicate function should be equivalent to the following: bool pred(const Type &a); The signature does not need to have const &, but the function must not modify the objects passed to it. The type Type must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to Type. ​
Type requirements
- `BidirIt` must meet the requirements of `BidirectionalIterator`.
- `ForwardIt` must meet the requirements of `ValueSwappable` and `ForwardIterator`. However, the operation is more efficient if `ForwardIt` also satisfies the requirements of `BidirectionalIterator`
- `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 `BidirectionalIterator`, 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 three 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

    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;
    }

### [edit] 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

### [edit] See also

is_partitioned (C++11) | determines if the range is partitioned by the given predicate (function template) [edit]
stable_partition | divides elements into two groups while preserving their relative order (function template) [edit]

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