std::experi...sform_reduce(3)

💖 NAME 💖

okay so bestie, welcome to the std::experi...sform_reduce(3) page from the C++ Standard Libary manual (yes, that spelling is literally right there at the top — not me, I promise 💀).

std::experimental::parallel::transform_reduce — std::experimental::parallel::transform_reduce

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✨ Synopsis ✨

Defined in header <experimental/numeric>.

Overload (1) — (parallelism TS):

template< class InputIt, class UnaryOp, class T, class BinaryOp
>
T transform_reduce( InputIt first, InputIt last,
                    UnaryOp unary_op, T init, BinaryOp binary_op );

Overload (2) — (parallelism TS):

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

okay so what it does: it applies unary_op to each element in the range [first, last) and reduces the results — possibly permuted and aggregated in unspecified manner — along with the initial value init over binary_op. periodt.

The behavior is non-deterministic if binary_op is not associative or not commutative. no cap.

The behavior is undefined if unary_op or binary_op modifies any element or invalidates any iterator in [first, last). that's the danger zone, bestie 💀.

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🎀 Parameters 🎀

• first, last — the range of elements to apply the algorithm to
• init — the initial value of the generalized sum
• policy — the execution policy
• unary_op — unary FunctionObject that will be applied to each element of the input range. The return type must be acceptable as input to binary_op
• binary_op — binary FunctionObject that will be applied in unspecified order to the results of unary_op, the results of other binary_op and init

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💜 Type requirements 💜

• InputIt must meet the requirements of LegacyInputIterator.

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🌈 Return value 🌈

Generalized sum of init and unary_op(*first), unary_op(*(first + 1)), ... unary_op(*(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 results of unary_op may be grouped and arranged in arbitrary order. it's giving flexible-sum energy 💅.

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⚡ Complexity ⚡

O(last - first) applications each of unary_op and binary_op.

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💀 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.

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🦋 Notes 🦋

unary_op is not applied to init.

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).

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🍓 Example 🍓

transform_reduce can be used to parallelize std::inner_product:

// Run this code

#include <boost/iterator/zip_iterator.hpp>
#include <boost/tuple.hpp>
#include <experimental/execution_policy>
#include <experimental/numeric>
#include <functional>
#include <iostream>
#include <iterator>
#include <vector>

int main()
{
    std::vector<double> xvalues(10007, 1.0), yvalues(10007, 1.0);

    double result = std::experimental::parallel::transform_reduce(
        std::experimental::parallel::par,
        boost::iterators::make_zip_iterator(
            boost::make_tuple(std::begin(xvalues),
       std::begin(yvalues))),
        boost::iterators::make_zip_iterator(
            boost::make_tuple(std::end(xvalues), std::end(yvalues))),
        [](auto r) { return boost::get<0>(r) * boost::get<1>(r); }
        0.0,
        std::plus<>()
    );
    std::cout << result << '\n';
}

Output:

10007

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👑 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)
• reduce — similar to std::accumulate, except out of order (parallelism TS) (function template)

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okay that's the whole page bestie 🫶 source/version: http://cppreference.com, date: 2024.06.10, page: std::experi...sform_reduce(3). mwah 💋