Function template reduce

hpx::parallel::v1::reduce

Synopsis

// In header: <hpx/parallel/detail/reduce.hpp>


template<typename ExPolicy, typename InIter> 
  unspecified reduce(ExPolicy && policy, InIter first, InIter last);

Description

Returns GENERALIZED_SUM(+, T(), *first, ..., *(first + (last - first) - 1)).

	Note
	Complexity: O(last - first) applications of the operator+().

The reduce operations in the parallel reduce algorithm invoked with an execution policy object of type sequential_execution_policy execute in sequential order in the calling thread.

The reduce operations in the parallel copy_if algorithm invoked with an execution policy object of type parallel_execution_policy or task_execution_policy are permitted to execute in an unordered fashion in unspecified threads, and indeterminately sequenced within each thread.

Note

The type of the initial value (and the result type) T is determined from the value_type of the used InIter.

GENERALIZED_SUM(+, a1, ..., aN) is defined as follows:

a1 when N is 1
op(GENERALIZED_SUM(+, b1, ..., bK), GENERALIZED_SUM(+, bM, ..., bN)), where:
- b1, ..., bN may be any permutation of a1, ..., aN and
- 1 < K+1 = M <= N.

The difference between reduce and accumulate is that the behavior of reduce may be non-deterministic for non-associative or non-commutative binary predicate.

Parameters:

`first`	Refers to the beginning of the sequence of elements the algorithm will be applied to.
`last`	Refers to the end of the sequence of elements the algorithm will be applied to.
`policy`	The execution policy to use for the scheduling of the iterations.

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.
`InIter`	The type of the source iterators used (deduced). This iterator type must meet the requirements of an input iterator.

Returns:

The reduce algorithm returns a hpx::future<T> if the execution policy is of type task_execution_policy and returns T otherwise (where T is the the value_type of InIter). The reduce algorithm returns the result of the generalized sum (applying operator+()) over the elements given by the input range [first, last).