hpx::nth_element

Defined in header hpx/algorithm.hpp.

See Public API for a list of names and headers that are part of the public HPX API.

namespace hpx

Functions

template<typename RandomIt, typename Pred = hpx::parallel::detail::less>
void nth_element(RandomIt first, RandomIt nth, RandomIt last, Pred &&pred = Pred())

nth_element is a partial sorting algorithm that rearranges elements in [first, last) such that the element pointed at by nth is changed to whatever element would occur in that position if [first, last) were sorted and all of the elements before this new nth element are less than or equal to the elements after the new nth element. Executed according to the policy.

The comparison operations in the parallel nth_element algorithm invoked without an execution policy object execute in sequential order in the calling thread.

Note

Complexity: Linear in std::distance(first, last) on average. O(N) applications of the predicate, and O(N log N) swaps, where N = last - first.

Template Parameters

• RandomIt – The type of the source begin, nth, and end iterators used (deduced). This iterator type must meet the requirements of a random access iterator.

• Pred – Comparison function object which returns true if the first argument is less than the second. This defaults to std::less<>.

Parameters

• first – Refers to the beginning of the sequence of elements the algorithm will be applied to.

• nth – Refers to the iterator defining the sort partition point

• last – Refers to the end of the sequence of elements the algorithm will be applied to.

• pred – Specifies the comparison function object which returns true if the first argument is less than (i.e. is ordered before) the second. The signature of this comparison function should be equivalent to:

  bool cmp(const Type1 &a, const Type2 &b);
  

  The signature does not need to have const&, but the function must not modify the objects passed to it. The type must be such that an object of type randomIt can be dereferenced and then implicitly converted to Type. This defaults to std::less<>.

Returns

The nth_element algorithms returns nothing.

template<typename ExPolicy, typename RandomIt, typename Pred = hpx::parallel::detail::less>
void nth_element(ExPolicy &&policy, RandomIt first, RandomIt nth, RandomIt last, Pred &&pred = Pred())

nth_element is a partial sorting algorithm that rearranges elements in [first, last) such that the element pointed at by nth is changed to whatever element would occur in that position if [first, last) were sorted and all of the elements before this new nth element are less than or equal to the elements after the new nth element.

The comparison operations in the parallel nth_element invoked with an execution policy object of type sequenced_policy execute in sequential order in the calling thread.

The assignments in the parallel nth_element algorithm invoked with an execution policy object of type parallel_policy or parallel_task_policy are permitted to execute in an unordered fashion in unspecified threads, and indeterminately sequenced within each thread.

Note

Complexity: Linear in std::distance(first, last) on average. O(N) applications of the predicate, and O(N log N) swaps, where N = last - first.

Template Parameters

• ExPolicy – The type of the execution policy to use (deduced). It describes the manner in which the execution of the algorithm may be parallelized and the manner in which it executes the assignments.

• RandomIt – The type of the source begin, nth, and end iterators used (deduced). This iterator type must meet the requirements of a random access iterator.

• Pred – Comparison function object which returns true if the first argument is less than the second. This defaults to std::less<>.

Parameters

• policy – The execution policy to use for the scheduling of the iterations.

• first – Refers to the beginning of the sequence of elements the algorithm will be applied to.

• nth – Refers to the iterator defining the sort partition point

• last – Refers to the end of the sequence of elements the algorithm will be applied to.

• pred – Specifies the comparison function object which returns true if the first argument is less than (i.e. is ordered before) the second. The signature of this comparison function should be equivalent to:

  bool cmp(const Type1 &a, const Type2 &b);
  

  The signature does not need to have const&, but the function must not modify the objects passed to it. The type must be such that an object of type randomIt can be dereferenced and then implicitly converted to Type. This defaults to std::less<>.

Returns

The nth_element algorithms returns nothing.