std::experimental::parallel::reduce - cppreference.com (original) (raw)

1. Same as reduce(first, last, typename std::iterator_traits<InputIt>::value_type{}).

2. Same as reduce(first, last, init, std::plus<>()).

3. 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 invalidates any iterator in [first, last).

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Exceptions
• 5 Notes
• 6 Example
• 7 See also

[edit] Parameters

[edit] 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.

[edit] Complexity

O(last - first) applications of binary_op.

[edit] 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_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 implementation-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.

[edit] Notes

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

• If policy is an instance of sequential_execution_policy, all operations 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 sequenced 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).

[edit] Example

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

Possible output:

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

[edit] See also

| | sums up or folds a range of elements (function template) [edit] | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | applies a function to a range of elements, storing results in a destination range (function template) [edit] | | | applies a functor, then reduces out of order (function template) [edit] |