std::experi...llel::reduce(3)

✨ std::experimental::parallel::reduce ✨

okay so bestie, welcome to the C++ Standard Libary manual page for std::experi...llel::reduce(3) 🫶 this is the parallelism TS era, she’s giving parallel algorithm energy, and she’s literally the out-of-order version of std::accumulate. let’s get into it 💅

🦋 NAME 🦋

std::experimental::parallel::reduce - std::experimental::parallel::reduce

(yes, the same name twice — she’s an icon, she contains multitudes ✨)

🌸 Synopsis 🌸

Defined in header <experimental/numeric>

Overload (1) — (parallelism TS)

template< class InputIt >
typename std::iterator_traits<InputIt>::value_type reduce(
    InputIt first, InputIt last );

Overload (2) — (parallelism TS)

template< class ExecutionPolicy, class InputIterator >
typename std::iterator_traits<InputIt>::value_type reduce(
    ExecutionPolicy&& policy, InputIt first, InputIt last );

Overload (3) — (parallelism TS)

template< class InputIt, class T >
T reduce( InputIt first, InputIt last, T init );

Overload (4) — (parallelism TS)

template< class ExecutionPolicy, class InputIt, class T >
T reduce( ExecutionPolicy&& policy, InputIt first, InputIt last, T init );

Overload (5) — (parallelism TS)

template< class InputIt, class T, class BinaryOp >
T reduce( InputIt first, InputIt last, T init, BinaryOp binary_op );

Overload (6) — (parallelism TS)

template< class ExecutionPolicy, class InputIt, class T, class BinaryOp >
T reduce( ExecutionPolicy&& policy,
          InputIt first, InputIt last, T init, BinaryOp binary_op );

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 policy.

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

🚨 bestie, this is the serious one: the behavior is undefined if binary_op modifies any element or invalidates any iterator in [first, last). do not do that. 💀

💖 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 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 <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 👑

• accumulate — sums up or folds a range of elements (function template)
• transform — applies a function to a range of elements, storing results in a destination range (function template)
• transform_reduce — applies a functor, then reduces out of order (parallelism TS) (function template)

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and that’s the whole page, bestie 🫶 source/version: http://cppreference.com; date: 2024.06.10; manual: std::experi...llel::reduce(3). periodt. 💖✨