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# std::partition

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |

template< class BidirIt, class UnaryPredicate > BidirectionalIterator 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)

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.

### [edit] Parameters

first, last | - | the range of elements to reorder
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

Exactly last-first applications of the predicate and at most last-first swaps. If `ForwardIt` meets the requirements of `BidirectionalIterator` at most (last-first)/2 swaps are done.

### [edit] Possible implementation

    template<class BidirIt, class UnaryPredicate>
    BidirIt partition(BidirIt first, BidirIt last, UnaryPredicate p)
    {
        while (1) {
            while ((first != last) && p(*first)) {
                ++first;
            }
            if (first == last--) break;
            while ((first != last) && !p(*last)) {
                --last;
            }
            if (first == last) break;
            std::iter_swap(first++, last);
        }
        return first;
    }

### [edit] Example 1

Run this code

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

    bool is_even(int i) { return i % 2 == 0; }

    int main()
    {
        std::vector<int> v;
        for (int i = 0; i < 10; ++i) v.push_back(i);

        std::cout << "Original vector:\n    ";
        std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));

        // Partition the vector
        std::vector<int>::iterator p =
            std::partition(v.begin(), v.end(), std::ptr_fun(is_even));

        std::cout << "\nPartitioned vector:\n    ";
        std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
        std::cout << "\nBefore partition:\n    ";
        std::copy(v.begin(), p,       std::ostream_iterator<int>(std::cout, " "));
        std::cout << "\nAfter partition:\n    ";
        std::copy(p,         v.end(), std::ostream_iterator<int>(std::cout, " "));
    }

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
    Before partition:
        0 8 2 6 4
    After partition:
        5 3 7 1 9

### [edit] Example 2

Run this code

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

    template <class ForwardIt>
     ForwardIt choose_pivot(ForwardIt first, ForwardIt last)
     {
       return std::next(first, std::distance(first,last)/2);
     }

    template <class ForwardIt>
     void quicksort(ForwardIt first, ForwardIt last)
     {
        using T = typename std::iterator_traits<ForwardIt>::value_type;

        if(first == last) return;

        T pivot = *choose_pivot(first,last);

        ForwardIt middle1 = std::partition(first, last,
                             [pivot](const T& em){ return em < pivot; });
        ForwardIt middle2 = std::partition(middle1, last,
                             [pivot](const T& em){ return !(pivot < em); });

        quicksort(first, middle1);
        quicksort(middle2, last);
     }

    int main()
    {
       std::forward_list<int> fl = {1, 30, -4, 3, 5, -4, 1, 6, -8, 2, -5, 64, 1, 92};

       quicksort(std::begin(fl), std::end(fl));

       for(int fi : fl) std::cout << fi << ' ';
       std::cout << '\n';
    }

Output:

    -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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