< cpp‎ | algorithm
Revision as of 09:54, 9 June 2013 by P12 (Talk | contribs)

Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations

Numeric operations
C library
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.



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.

Return value

Iterator to the first element of the second group.


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.

Possible implementation

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


#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

See also

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