std::transform_reduce (3) - Linux Manuals

std::transform_reduce: std::transform_reduce

NAME

std::transform_reduce - std::transform_reduce

Synopsis

Defined in header <numeric>
template<class InputIt1, class InputIt2, class T> (1) (since C++17)
T transform_reduce(InputIt1 first1, InputIt1 last1, InputIt2 first2, T init);
template <class InputIt1, class InputIt2, class T, class BinaryOp1, class BinaryOp2>
T transform_reduce(InputIt1 first1, InputIt1 last1, InputIt2 first2, (2) (since C++17)
T init, BinaryOp1 binary_op1, BinaryOp2 binary_op2);
template<class InputIt, class T, class BinaryOp, class UnaryOp>
T transform_reduce(InputIt first, InputIt last, (3) (since C++17)
T init, BinaryOp binop, UnaryOp unary_op);
template<class ExecutionPolicy,
class ForwardIt1, class ForwardIt2, class T> (4) (since C++17)
T transform_reduce(ExecutionPolicy&& policy,
ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, T init);
template<class ExecutionPolicy,
class ForwardIt1, class ForwardIt2, class T, class BinaryOp1, class BinaryOp2>
T transform_reduce(ExecutionPolicy&& policy, (5) (since C++17)
ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2,
T init, BinaryOp1 binary_op1, BinaryOp2 binary_op2);
template<class ExecutionPolicy,
class ForwardIt, class T, class BinaryOp, class UnaryOp>
T transform_reduce(ExecutionPolicy&& policy, (6) (since C++17)
ForwardIt first, ForwardIt last,
T init, BinaryOp binary_op, UnaryOp unary_op);

1) Equivalent to transform_reduce(first1, last1, first2, init, std::plus<>(), std::multiplies<>());, effectively parallelized version of the default std::inner_product
2) Applies binary_op2 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 binary_op1
3) 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.
4-6) Same as (1-3), but executed according to policy. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true
The behavior is non-deterministic if binary_op/binary_op2 is not associative or not commutative.
The behavior is undefined if unary_op, binary_op, binary_op1, or binary_op2 modifies any element 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 generalized sum
policy - the execution policy to use. See execution_policy for details.
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.

Type requirements

-
T must meet the requirements of MoveConstructible in order to use overloads (3,6). and the result of the expressions binary_op(init, unary_op(*first)), binary_op(unary_op(*first), init), binary_op(init, init), and binary_op(unary_op(*first), unary_op(*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 binary_op1(init, binary_op2(*first1, *first2)), binary_op1(binary_op2(*first1, *first2), init), binary_op1(init, init), and binary_op1(binary_op2(*first1, *first2), binary_op2(*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 binary_op2(*first,*first2), binary_op2(*(first+1),*(first2+1)), ..., over binary_op1
3) Generalized sum of init and unary_op(*first), unary_op(*(first+1)), ... unary_op(*(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 results of unary_op or of binary_op1 may be grouped and arranged in arbitrary order.

Complexity

1,2,4,5) O(last1 - first1) applications each of binary_op1 and binary_op2.
3,6) O(last - first) applications each of unary_op and binary_op.

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::terminate 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), unary_op 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 <vector>
  #include <functional>
  #include <iostream>
  #include <numeric>
  #include <execution>

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

      double result = std::transform_reduce(
          std::execution::par,
          xvalues.begin(), xvalues.end(),
          yvalues.begin(), 0.0
      );
      std::cout << result << '\n';
  }

Output:

  10007