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std::experi...llel::reduce(3) C++ Standard Libarystd::experi...llel::reduce(3) NAME std::experimental::parallel::reduce - std::experimental::parallel::re- duce 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 last, (4) (parallelism TS) 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> (6) (parallelism TS) T reduce(ExecutionPolicy&& policy, InputIt first, InputIt last, T init, BinaryOp binary_op); 1) same as reduce(first, last, typename std::iterator_traits<In- putIt>::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 in- validates 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 unspeci- fied order to the binary_op - result of dereferencing the input iterators, the re- sults 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 N in other words, the elements of the range may be grouped and re- arranged 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 paral- lel_execution_policy, the algorithm exits with an exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered. * if policy is some other type, the behavior is implemen- tation-defined * 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 op- erations are performed in the calling thread. * If policy is an instance of parallel_execution_policy, opera- tions may be performed in unspecified number of threads, indeterminately se- quenced with each other * If policy is an instance of parallel_vector_execution_policy, execution 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 <iostream> #include <chrono> #include <vector> #include <numeric> #include <experimental/execution_policy> #include <experimental/numeric> 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 a range of elements (function template) applies a function to a range of elements, storing results in a transform destination range (function template) transform_reduce applies a functor, then reduces out of order (parallelism TS) (function template) http://cppreference.com 2022.07.31 std::experi...llel::reduce(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Complexity | Exceptions | Notes | Example | Possible output: | See also
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