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std::experi...llel::reduce(3)  C++ Standard Libary std::experi...llel::reduce(3)

NAME
     std::experimental::parallel::reduce - std::experimental::parallel::reduce

Synopsis
	Defined in header <experimental/numeric>
	template< class InputIt >

	typename   std::iterator_traits<InputIt>::value_type   reduce(	     (1)
     (parallelism TS)

	    InputIt first, InputIt last );
	template< class ExecutionPolicy, class InputIterator >

	typename  std::iterator_traits<InputIt>::value_type   reduce(	     (2)
     (parallelism TS)

	    ExecutionPolicy&& policy, InputIt first, InputIt last );
	template<  class  InputIt,  class  T  > 			     (3)
     (parallelism TS)
	T reduce( InputIt first, InputIt last, T init );
	template< class ExecutionPolicy, class InputIt, class T >
	T reduce( ExecutionPolicy&&  policy,  InputIt  first,  InputIt	     (4)
     (parallelism TS)
	last, T init );
	template< class InputIt, class T, class BinaryOp >
	T  reduce(  InputIt  first,  InputIt  last, T init, BinaryOp	     (5)
     (parallelism TS)
	binary_op );
	template< class ExecutionPolicy, class InputIt, class T, class
	BinaryOp >

	T  reduce(  ExecutionPolicy&&  policy,				     (6)
     (parallelism TS)

		  InputIt first, InputIt last, T init, BinaryOp
	binary_op );

	1) Same as reduce(first, last, typename
	std::iterator_traits<InputIt>::value_type{}).
	3) Same as reduce(first, last, init, std::plus<>()).
	5)  Reduces the range [first, last), possibly permuted and aggregated in
     unspecified
	manner, along with the initial value init over binary_op.
	2,4,6) Same as (1,3,5), but executed according to policy.

	The behavior is non-deterministic if binary_op is not associative or not
	commutative.

	The behavior is undefined if binary_op modifies any element  or  invali-
     dates any
	iterator in [first, last).

Parameters
	first, last - the range of elements to apply the algorithm to
	init	    - the initial value of the generalized sum
	policy	    - the execution policy
		      binary  FunctionObject that will be applied in unspecified
     order to the
	binary_op   - result of dereferencing the input iterators,  the  results
     of other
		      binary_op and init

Type requirements
	-
	InputIt must meet the requirements of LegacyInputIterator.

Return value
	Generalized  sum  of init and *first, *(first + 1), ... *(last - 1) over
     binary_op,

	where generalized sum GSUM(op, a
	1, ..., a
	N) is defined as follows:

	  * if N=1, a
	    1
	  * if N > 1, op(GSUM(op, b
	    1, ..., b
	    K), GSUM(op, b
	    M, ..., b
	    N)) where

	  * b
	    1, ..., b
	    N may be any permutation of a1, ..., aN and
	  * 1 < K+1 = M ax N

	in other words, the elements of the range may be grouped and  rearranged
     in arbitrary
	order.

Complexity
	O(last - first) applications of binary_op.

Exceptions
	  *  If  execution of a function invoked as part of the algorithm throws
     an exception,

	  * if policy  is  parallel_vector_execution_policy,  std::terminate  is
     called.
	  *  if policy is sequential_execution_policy or parallel_execution_pol-
     icy, the
	    algorithm exits with an exception_list containing all  uncaught  ex-
     ceptions. If
	    there  was only one uncaught exception, the algorithm may rethrow it
     without
	    wrapping in exception_list. It is unspecified how much work the  al-
     gorithm will
	    perform before returning after the first exception was encountered.
	  *  if  policy  is  some other type, the behavior is implementation-de-
     fined.
	  * If the algorithm fails to allocate memory (either for itself  or  to
     construct an
	    exception_list  when  handling  a user exception), std::bad_alloc is
     thrown.

Notes
	If the range is empty, init is returned, unmodified.

	  * If policy is an instance of sequential_execution_policy, all  opera-
     tions are
	    performed in the calling thread.
	  *  If  policy  is an instance of parallel_execution_policy, operations
     may be
	    performed in unspecified  number  of  threads,  indeterminately  se-
     quenced with each
	    other.
	  * If policy is an instance of parallel_vector_execution_policy, execu-
     tion may be
	    both  parallelized	and vectorized: function body boundaries are not
     respected and
	    user code may be overlapped and combined  in  arbitrary  manner  (in
     particular,
	    this  implies that a user-provided Callable must not acquire a mutex
     to access a
	    shared resource).

Example
	reduce is the out-of-order version of std::accumulate:

     // Run this code

      #include <chrono>
      #include <experimental/execution_policy>
      #include <experimental/numeric>
      #include <iostream>
      #include <numeric>
      #include <vector>

      int main()
      {
	  std::vector<double> v(10'000'007, 0.5);

	  {
	      auto t1 = std::chrono::high_resolution_clock::now();
	      double result = std::accumulate(v.begin(), v.end(), 0.0);
	      auto t2 = std::chrono::high_resolution_clock::now();
	      std::chrono::duration<double, std::milli> ms = t2 - t1;
	      std::cout << std::fixed << "std::accumulate result " << result
			<< " took " << ms.count() << " ms\n";
	  }

	  {
	      auto t1 = std::chrono::high_resolution_clock::now();
	      double result = std::experimental::parallel::reduce(
				  std::experimental::parallel::par,
				  v.begin(), v.end());
	      auto t2 = std::chrono::high_resolution_clock::now();
	      std::chrono::duration<double, std::milli> ms = t2 - t1;
	      std::cout << "parallel::reduce result "
			<< result << " took " << ms.count() << " ms\n";
	  }
      }

Possible output:
      std::accumulate result 5000003.50000 took 12.7365 ms
      parallel::reduce result 5000003.50000 took 5.06423 ms

See also
	accumulate	 sums up or folds a range of elements
			 (function template)
			 applies a function to a range of elements, storing  re-
     sults in a
	transform	 destination range
			 (function template)
	transform_reduce applies a functor, then reduces out of order
	(parallelism TS) (function template)

http://cppreference.com 	   2024.06.10	   std::experi...llel::reduce(3)
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