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std::transform_reduce(3) C++ Standard Libary std::transform_reduce(3) NAME std::transform_reduce - std::transform_reduce Synopsis Defined in header <numeric> template< class InputIt1, class InputIt2, class T > T transform_reduce( InputIt1 (since C++17) first1, InputIt1 last1, (until C++20) InputIt2 first2, T init ); template< class InputIt1, class InputIt2, class T > constexpr T transform_reduce( InputIt1 (since C++20) first1, InputIt1 last1, InputIt2 first2, T init ); template< class InputIt1, class InputIt2, class T, class BinaryReductionOp, class BinaryTransformOp > (since C++17) T transform_reduce( InputIt1 (until C++20) first1, InputIt1 last1, InputIt2 first2, T init, BinaryReductionOp reduce, BinaryTransformOp transform ); template< class InputIt1, class InputIt2, class T, class BinaryReductionOp, class BinaryTransformOp > constexpr (since C++20) T transform_reduce( InputIt1 first1, InputIt1 last1, InputIt2 first2, T init, BinaryReductionOp reduce, BinaryTransformOp transform ); template< class InputIt, class T, class BinaryReductionOp, class UnaryTransformOp > (since C++17) T transform_reduce( InputIt first, (until C++20) InputIt last, T init, (1) BinaryReductionOp reduce, UnaryTransformOp transform ); template< class InputIt, class T, class BinaryReductionOp, class UnaryTransformOp > constexpr T transform_reduce( InputIt first, (2) (since C++20) InputIt last, T init, BinaryReductionOp reduce, UnaryTransformOp transform ); template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class T > T transform_reduce( ExecutionPolicy&& policy, (4) (since C++17) ForwardIt1 first1, ForwardIt1 last1, (3) ForwardIt2 first2, T init ); template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class T, class BinaryReductionOp, class BinaryTransformOp > T transform_reduce( ExecutionPolicy&& policy, (5) (since C++17) ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, T init, BinaryReductionOp reduce, BinaryTransformOp transform ); template< class ExecutionPolicy, class ForwardIt, class T, class BinaryReductionOp, class UnaryTransformOp > T transform_reduce( (6) (since C++17) ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, T init, BinaryReductionOp reduce, UnaryTransformOp transform ); 1) Equivalent to std::transform_reduce(first1, last1, first2, init, std::plus<>(), std::multiplies<>());, effectively parallelized version of the de- fault std::inner_product 2) Applies transform to each pair of elements from the ranges [first; last) and the range starting at first2 and reduces the results (possibly permuted and aggregated in unspecified manner) along with the initial value init over reduce 3) Applies transform 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 reduce. 4-6) Same as (1-3), but executed according to policy. These over- loads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> (since C++20) is true. The behavior is non-deterministic if reduce is not associative or not commutative. The behavior is undefined if reduce, or transform modifies any ele- ment or invalidates any iterator in the input ranges, including their end iterators. Parameters first, last - the range of elements to apply the algorithm to init - the initial value of the general- ized sum policy - the execution policy to use. See execution policy for details. binary FunctionObject that will be applied in reduce - unspecified order to the results of transform, the results of other reduce and init. unary or binary FunctionObject that will be transform - applied to each element of the in- put range(s). The return type must be acceptable as input to reduce Type requirements - T must meet the requirements of MoveConstructible in order to use overloads (3,6). and the result of the expressions reduce(init, transform(*first)), reduce(transform(*first), init), reduce(init, init), and re- duce(transform(*first), transform(*first)) must be convertible to T - T must meet the requirements of MoveConstructible in order to use overloads (2,5). and the result of the expressions reduce(init, transform(*first1, *first2)), reduce(transform(*first1, *first2), init), reduce(init, init), and reduce(transform(*first1, *first2), transform(*first1, *first2)) must be convertible to T - InputIt must meet the requirements of LegacyInputIterator. - ForwardIt must meet the requirements of LegacyForwardIterator. Return value 2) Generalized sum of init and transform(*first,*first2), transform(*(first+1),*(first2+1)), ..., over reduce 3) Generalized sum of init and transform(*first), trans- form(*(first+1)), ... transform(*(last-1)) over reduce, 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 results of transform or of reduce may be grouped and arranged in arbitrary order. Complexity 1,2,4,5) O(last1 - first1) applications each of reduce and trans- form. 3,6) O(last - first) applications each of transform and reduce. Exceptions The overloads with a template parameter named ExecutionPolicy report errors as follows: * If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::termi- nate is called. For any other ExecutionPolicy, the behavior is implementation- defined. * If the algorithm fails to allocate memory, std::bad_alloc is thrown. Notes In the unary-binary overload (3,6), transform is not applied to init If first == last or first1 == last1, init is returned, unmodified Example transform_reduce can be used to parallelize std::inner_product: // Run this code #include <algorithm> #include <execution> #include <iostream> #include <iterator> #include <locale> #include <numeric> #include <vector> // to parallelize non-associate accumulative operation, you'd better choose // transform_reduce instead of reduce; e.g., a + b * b != b + a * a void print_sum_squared(long const num) { std::cout.imbue(std::locale{"en_US.UTF8"}); std::cout << "num = " << num << '\n'; // create an immutable vector filled with pattern: 1,2,3,4, 1,2,3,4 ... const std::vector<long> v { [n = num * 4] { std::vector<long> v; v.reserve(n); std::generate_n(std::back_inserter(v), n, [i=0]() mutable { return 1 + i++ % 4; }); return v; }()}; auto squared_sum = [](auto sum, auto val) { return sum + val * val; }; auto sum1 = std::accumulate(v.cbegin(), v.cend(), 0L, squared_sum); std::cout << "accumulate(): " << sum1 << '\n'; auto sum2 = std::reduce(std::execution::par, v.cbegin(), v.cend(), 0L, squared_sum); std::cout << "reduce(): " << sum2 << '\n'; auto sum3 = std::transform_reduce(std::execution::par, v.cbegin(), v.cend(), 0L, std::plus{}, [](auto val) { return val * val; }); std::cout << "transform_reduce(): " << sum3 << '\n'; std::cout << '\n'; } int main() { print_sum_squared(1); print_sum_squared(1'000); print_sum_squared(1'000'000); } Possible output: num = 1 accumulate(): 30 reduce(): 30 transform_reduce(): 30 num = 1,000 accumulate(): 30,000 reduce(): -7,025,681,278,312,630,348 transform_reduce(): 30,000 num = 1,000,000 accumulate(): 30,000,000 reduce(): -5,314,886,882,370,003,032 transform_reduce(): 30,000,000 See also accumulate sums up a range of elements (function template) applies a function to a range of elements, storing re- sults in a transform destination range (function template) reduce similar to std::accumulate, except out of order (C++17) (function template) http://cppreference.com 2022.07.31 std::transform_reduce(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Complexity | Exceptions | Notes | Example | Possible output: | See also
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