std::transform_reduce

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

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:

#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:

accumulate	sums up a range of elements (function template)
transform	applies a function to a range of elements (function template)
reduce (C++17)	similar to `std::accumulate`, except out of order (function template)