std::transform_reduce

 

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Technical Specifications

 

Algorithm library

Execution policies (C++17)
is_execution_policy	execution::seqexecution::parexecution::par_unseq	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policy
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	findfind_iffind_if_not (C++11) find_end find_first_of	search search_n lexicographical_compare lexicographical_compare_3way (C++20)
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17)	uninitialized_copy_n (C++11) uninitialized_move_n (C++17)	uninitialized_fill	uninitialized_fill_n
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17) compare_3way (C++20)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
C library
qsort	bsearch

 

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 InputIterator.
- ForwardIt must meet the requirements of ForwardIterator.

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 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.

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

See also

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)