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# std::transform_reduce

From cppreference.com
< cpp‎ | algorithm

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.

### [edit] 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 `InputIterator`.
- `ForwardIt` must meet the requirements of `ForwardIterator`.

### [edit] 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.

### [edit] 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`.

### [edit] 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 three 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.

### [edit] 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

### [edit] 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

### [edit] See also

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

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