| Defined in header <algorithm> | ||
|---|---|---|
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) |
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.
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:
The signature does not need to have |
| 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.
Complexity
Exactly std::distance(first,last) applications of the predicate and at most std::distance(first,last) swaps. If ForwardIt meets the requirements of BidirectionalIterator at most std::distance(first,last)/2 swaps are done.
Possible implementation
template<class ForwardIt, class UnaryPredicate>
ForwardIt partition(ForwardIt first, ForwardIt last, UnaryPredicate p)
{
if (first == last) return first;
ForwardIt part(first++);
if (first == last) return p(*part) ? first : part;
while (first != last) {
if (p(*part))
++part;
else if (p(*first)) {
iter_swap(part, first);
++part;
} ++first;
} return part;
} |
Example
#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 92See also
| (C++11) | determines if the range is partitioned by the given predicate (function template) |
| divides elements into two groups while preserving their relative order (function template) | |
| std::experimental::parallel::partition
(parallelism TS) | parallelized version of std::partition (function template) |
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