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.

Contents

[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

#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

#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

determines if the range is partitioned by the given predicate
(function template)
divides elements into two groups while preserving their relative order
(function template)