std::experimental::parallel::transform_reduce
<experimental/numeric>
T transform_reduce( InputIt first, InputIt last,
class InputIt, class UnaryOp, class T, class BinaryOp >
T transform_reduce( ExecutionPolicy&& policy,
InputIt first, InputIt last,
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.
The behavior is non-deterministic if binary_op is not associative or not commutative.
The behavior is undefined if unary_op or binary_op modifies any element or invalidates any iterator in [
first,
last)
.
[edit] Parameters
InputIt
must meet the requirements of LegacyInputIterator.
[edit] 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.
[edit] Complexity
O(last - first) applications each of unary_op and binary_op.
[edit] Exceptions
- If execution of a function invoked as part of the algorithm throws an exception,
- if
policy
isparallel_vector_execution_policy
, std::terminate is called. - if
policy
issequential_execution_policy
orparallel_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 inexception_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
- 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
unary_op is not applied to init.
If the range is empty, init is returned, unmodified.
- If
policy
is an instance ofsequential_execution_policy
, all operations are performed in the calling thread. - If
policy
is an instance ofparallel_execution_policy
, operations may be performed in unspecified number of threads, indeterminately sequenced with each other. - If
policy
is an instance ofparallel_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
transform_reduce can be used to parallelize std::inner_product :
#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
[edit] See also
(function template) [edit]