std::partition

From cppreference.com
 
 
 
Defined in header <algorithm>
template< class BidirectionalIterator, class UnaryPredicate >

BidirectionalIterator partition( BidirectionalIterator first, BidirectionalIterator last,
                                 UnaryPredicate p );
template< class ForwardIterator, class UnaryPredicate >
ForwardIterator partition( ForwardIterator first, ForwardIterator last,

                                 UnaryPredicate p );
(until C++11)

(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 ForwardIterator can be dereferenced and then implicitly converted to Type. ​

[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)/2 swaps if ForwardIterator meets the requirements of BidirectionalIterator, otherwise at most last-first swaps.

[edit] Possible implementation

[edit] Example

#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] 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)