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std::experi...sform_reduce(3)  C++ Standard Libary std::experi...sform_reduce(3)

NAME
     std::experimental::parallel::transform_reduce  -  std::experimental::paral-
     lel::transform_reduce

Synopsis
	Defined in header <experimental/numeric>
	template< class InputIt, class UnaryOp, class T, class BinaryOp
	>

	T transform_reduce(  InputIt  first,  InputIt  last,		     (1)
     (parallelism TS)

			    UnaryOp unary_op, T init, BinaryOp
	binary_op );
	template< class ExecutionPolicy,

		  class InputIt, class UnaryOp, class T, class BinaryOp
	>
	T   transform_reduce(  ExecutionPolicy&&  policy,		     (2)
     (parallelism TS)
			    InputIt first, InputIt last,

			    UnaryOp unary_op, T init, BinaryOp
	binary_op );

	Applies unary_op to each element in the range [first, last) and  reduces
     the results
	(possibly  permuted and aggregated in unspecified manner) along with the
     initial
	value init over binary_op.

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

	The behavior is undefined if unary_op or binary_op modifies any  element
     or
	invalidates 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
	unary_op     - unary FunctionObject that will be applied to each element
     of the input
		      range. The return type must be acceptable as input to  bi-
     nary_op
	binary_op    - binary FunctionObject that will be applied in unspecified
     order to the
		      results of unary_op, the results of  other  binary_op  and
     init

Type requirements
	-
	InputIt must meet the requirements of LegacyInputIterator.

Return value
	Generalized  sum  of  init and unary_op(*first), unary_op(*(first + 1)),
     ...
	unary_op(*(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 results of unary_op may be grouped and  arranged  in
     arbitrary
	order.

Complexity
	O(last - first) applications each of unary_op and 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
	unary_op is not applied to init.

	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
	transform_reduce can be used to parallelize std::inner_product:

     // Run this code

      #include <boost/iterator/zip_iterator.hpp>
      #include <boost/tuple.hpp>
      #include <experimental/execution_policy>
      #include <experimental/numeric>
      #include <functional>
      #include <iostream>
      #include <iterator>
      #include <vector>

      int main()
      {
	  std::vector<double> xvalues(10007, 1.0), yvalues(10007, 1.0);

	  double result = std::experimental::parallel::transform_reduce(
	      std::experimental::parallel::par,
	      boost::iterators::make_zip_iterator(
		  boost::make_tuple(std::begin(xvalues), std::begin(yvalues))),
	      boost::iterators::make_zip_iterator(
		  boost::make_tuple(std::end(xvalues), std::end(yvalues))),
	      [](auto r) { return boost::get<0>(r) * boost::get<1>(r); }
	      0.0,
	      std::plus<>()
	  );
	  std::cout << result << '\n';
      }

Output:
      10007

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)
	reduce		 similar to std::accumulate, except out of order
	(parallelism TS) (function template)

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