std::experimental::parallel::reduce
| Defined in header <experimental/numeric>
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| template< class InputIt > typename std::iterator_traits<InputIt>::value_type reduce( |
(1) | (parallelism TS) |
| template< class ExecutionPolicy, class InputIterator > typename std::iterator_traits<InputIt>::value_type reduce( |
(2) | (parallelism TS) |
| template< class InputIt, class T > T reduce( InputIt first, InputIt last, T init ); |
(3) | (parallelism TS) |
| template< class ExecutionPolicy, class InputIt, class T > T reduce( ExecutionPolicy&& policy, InputIt first, InputIt last, T init ); |
(4) | (parallelism TS) |
| template< class InputIt, class T, class BinaryOp > T reduce( InputIt first, InputIt last, T init, BinaryOp binary_op ); |
(5) | (parallelism TS) |
| template< class ExecutionPolicy, class InputIt, class T, class BinaryOp > T reduce( ExecutionPolicy&& policy, |
(6) | (parallelism TS) |
[first, last), 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 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 |
| binary_op | - | binary FunctionObject that will be applied in unspecified order to the 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, a1, ..., aN) is defined as follows:
- if N=1, a1
- if N > 1, op(GSUM(op, b1, ..., bK), GSUM(op, bM, ..., bN)) where
- b1, ..., bN 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 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
policyisparallel_vector_execution_policy, std::terminate is called. - if
policyissequential_execution_policyorparallel_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 inexception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered. - if
policyis some other type, the behavior is implementation-defined.
- if
- If the algorithm fails to allocate memory (either for itself or to construct an
exception_listwhen handling a user exception), std::bad_alloc is thrown.
Notes
If the range is empty, init is returned, unmodified.
- If
policyis an instance ofsequential_execution_policy, all operations are performed in the calling thread. - If
policyis an instance ofparallel_execution_policy, operations may be performed in unspecified number of threads, indeterminately sequenced with each other. - If
policyis an instance ofparallel_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:
#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
| sums up or folds a range of elements (function template) | |
| applies a function to a range of elements, storing results in a destination range (function template) | |
| (parallelism TS) |
applies a functor, then reduces out of order (function template) |