std::experimental::parallel::reduce

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Technical specifications

Filesystem library (filesystem TS)

Library fundamentals (library fundamentals TS)

Library fundamentals 2 (library fundamentals 2 TS)

Extensions for parallelism (parallelism TS)

Concepts (concepts TS)

Extensions for concurrency (concurrency TS)

 

Extensions for parallelism

Execution policies

parallel::execution_policy

parallel::sequential_execution_policyparallel::parallel_execution_policyparallel::parallel_vector_execution_policy

parallel::seqparallel::parparallel::par_vec

Parallel algorithms

Parallel exceptions

parallel::exception_list

Parallelized version of existing algorithms

New algorithms

parallel::for_eachparallel::for_each_n

parallel::reduce

parallel::exclusive_scan

parallel::inclusive_scan

parallel::transform_reduce

parallel::transform_exclusive_scan

parallel::transform_inclusive_scan

 

Defined in header <experimental/numeric>
template<class InputIt> typename std::iterator_traits<InputIt>::value_type reduce(     InputIt first, InputIt last);	(1)
template<class ExecutionPolicy, class InputIterator> typename std::iterator_traits<InputIt>::value_type reduce(     ExecutionPolicy&& exec, InputIt first, InputIt last);	(2)
template<class InputIt, class T> T reduce(InputIt first, InputIt last, T init);	(3)
template<class ExecutionPolicy, class InputIt, class T> T reduce(ExecutionPolicy&& exec, InputIt first, InputIt last, T init);	(4)
template<class InputIt, class T, class BinaryOp> T reduce(InputIt first, InputIt last, T init, BinaryOp binary_op);	(5)
template<class ExecutionPolicy, class InputIt, class T, class BinaryOp> T reduce(ExecutionPolicy&& exec,          InputIt first, InputIt last, T init, BinaryOp binary_op);	(6)

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 exec

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 Callable 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 InputIterator.

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 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_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.

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)

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)
transform	applies a function to a range of elements (function template)
transform_reduce (parallelism TS)	applies a functor, then reduces out of order (function template)