std::partition() algorithm
- od C++20
- od C++17
- do C++17
// (1)
template< class ForwardIt, class UnaryPredicate >
constexpr ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );
// (2)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
ForwardIt partition( ExecutionPolicy&& policy,
ForwardIt first, ForwardIt last, UnaryPredicate p );
// (1)
template< class ForwardIt, class UnaryPredicate >
ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );
// (2)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
ForwardIt partition( ExecutionPolicy&& policy,
ForwardIt first, ForwardIt last, UnaryPredicate p );
// (1)
template< class ForwardIt, class UnaryPredicate >
ForwardIt partition( ForwardIt first, ForwardIt last, UnaryPredicate p );
-
(1) Reorders the elements in the range [
first
;last
) in such a way that all elements for which the predicatep
returnstrue
precede the elements for which predicatep
returnsfalse
.uwagaRelative order of the elements is not preserved.
-
(2) Same as (1), but executed according to
policy
.Overload ResolutionThese overloads participate in overload resolution only if
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(do C++20)std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(od C++20) istrue
.
Parameters
first last | The range of elements to reorder. |
policy | The execution policy to use. See execution policy for details. |
p | Unary predicate which returns The expression |
Type requirements
ForwardIt | LegacyForwardIterator ValueSwappable |
Implementations may carry out the operation more efficiently if ForwardIt
satisfies LegacyBidirectionalIterator.
UnaryPredicate | Predicate |
Return value
Iterator to the first element of the second group.
Complexity
Given N
as std::distance(first, last)
:
(1) Exactly N
applications of p
.
At most N / 2
swaps, if ForwardIt
meets the requirements of LegacyForwardIterator,
N
swaps otherwise.
(2) O(N * log(N)) swaps and O(N) applications of p
.
Exceptions
The overloads with a template parameter named ExecutionPolicy
report errors as follows:
- If execution of a function invoked as part of the algorithm throws an exception and
ExecutionPolicy
is one of the standard policies,std::terminate
is called. For any otherExecutionPolicy
, the behavior is implementation-defined. - If the algorithm fails to allocate memory,
std::bad_alloc
is thrown.
Possible implementation
partition (1)
template<class ForwardIt, class UnaryPredicate>
ForwardIt partition(ForwardIt first, ForwardIt last, UnaryPredicate p)
{
first = std::find_if_not(first, last, p);
if (first == last)
return first;
for (auto i = std::next(first); i != last; ++i)
if (p(*i))
{
std::iter_swap(i, first);
++first;
}
return first;
}
Examples
#include <algorithm>
#include <forward_list>
#include <iostream>
#include <iterator>
#include <vector>
template<class ForwardIt>
void quicksort(ForwardIt first, ForwardIt last)
{
if (first == last)
return;
auto pivot = *std::next(first, std::distance(first, last) / 2);
auto middle1 = std::partition(first, last, [pivot](const auto& em)
{
return em < pivot;
});
auto 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: ";
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: ";
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: ";
for (int n : fl)
std::cout << n << ' ';
quicksort(std::begin(fl), std::end(fl));
std::cout << "\nSorted using quicksort: ";
for (int fi : fl)
std::cout << fi << ' ';
std::cout << '\n';
}
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 92
Hover to see the original license.