NAME

std::reduce - std::reduce

Synopsis

Defined in header <numeric>
template< class InputIt > (since
C++17)
typename std::iterator_traits<InputIt>::value_type (until
C++20)
reduce( InputIt first, InputIt last );
template< class InputIt >

constexpr typename (since
std::iterator_traits<InputIt>::value_type C++20)

reduce( InputIt first, InputIt last );
template< class ExecutionPolicy, class ForwardIt >

typename std::iterator_traits<ForwardIt>::value_type (2) (since
reduce( ExecutionPolicy&& policy, C++17)

ForwardIt first, ForwardIt last );
(since
template< class InputIt, class T > C++17)
T reduce( InputIt first, InputIt last, T init ); (until
C++20)
template< class InputIt, class T > (1) (since
constexpr T reduce( InputIt first, InputIt last, T C++20)
init );
template< class ExecutionPolicy, class ForwardIt,
class T >
(4) (since
T reduce( ExecutionPolicy&& policy, C++17)

ForwardIt first, ForwardIt last, T init );
template< class InputIt, class T, class BinaryOp > (3) (since
T reduce( InputIt first, InputIt last, T init, C++17)
BinaryOp binary_op ); (until
C++20)
template< class InputIt, class T, class BinaryOp > (since
constexpr T reduce( InputIt first, InputIt last, T C++20)
init, BinaryOp binary_op ); (5)
template< class ExecutionPolicy, class ForwardIt,
class T, class BinaryOp >
(since
T reduce( ExecutionPolicy&& policy, (6) C++17)

ForwardIt first, ForwardIt last, T init, BinaryOp
binary_op );

1) same as reduce(first, last, typename std::iterator_traits<InputIt>::value_type{})
3) same as reduce(first, last, init, std::plus<>())
5) Reduces the range [first; last), possibly permuted and aggregated in unspecified
manner, along with the initial value init over binary_op.
2,4,6) Same as (1,3,5), but executed according to policy. These overloads do not
participate in overload resolution unless
std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20) is true.

The behavior is non-deterministic if binary_op is not associative or not
commutative.

The behavior is undefined if binary_op modifies any element or invalidates any
iterator in [first; last], including the end iterator.

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.
binary FunctionObject that will be applied in unspecified order
binary_op - to the result of dereferencing the input iterators, the results
of other binary_op and init.

Type requirements

-
InputIt must meet the requirements of LegacyInputIterator.
-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
T must meet the requirements of MoveConstructible. and binary_op(init, *first),
binary_op(*first, init), binary_op(init, init), and binary_op(*first, *first) must
be convertible to T.

Return value

Generalized sum of init and *first, *(first+1), ... *(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, reduce behaves like std::accumulate except the elements of the range
may be grouped and rearranged in arbitrary order

Complexity

O(last - first) applications of 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

If the range is empty, init is returned, unmodified

Example

side-by-side comparison between reduce and std::accumulate:

// Run this code

#include <chrono>
#include <execution>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <utility>
#include <vector>

int main()
{
auto eval = [](auto fun) {
const auto t1 = std::chrono::high_resolution_clock::now();
const auto [name, result] = fun();
const auto t2 = std::chrono::high_resolution_clock::now();
const std::chrono::duration<double, std::milli> ms = t2 - t1;
std::cout << std::fixed << std::setprecision(1) << name << " result "
<< result << " took " << ms.count() << " ms\n";
};
{
const std::vector<double> v(100'000'007, 0.1);

eval([&v]{ return std::pair{"std::accumulate (double)",
std::accumulate(v.cbegin(), v.cend(), 0.0)}; } );
eval([&v]{ return std::pair{"std::reduce (seq, double)",
std::reduce(std::execution::seq, v.cbegin(), v.cend())}; } );
eval([&v]{ return std::pair{"std::reduce (par, double)",
std::reduce(std::execution::par, v.cbegin(), v.cend())}; } );
}{
const std::vector<long> v(100'000'007, 1);

eval([&v]{ return std::pair{"std::accumulate (long)",
std::accumulate(v.cbegin(), v.cend(), 0)}; } );
eval([&v]{ return std::pair{"std::reduce (seq, long)",
std::reduce(std::execution::seq, v.cbegin(), v.cend())}; } );
eval([&v]{ return std::pair{"std::reduce (par, long)",
std::reduce(std::execution::par, v.cbegin(), v.cend())}; } );
}
}

Possible output:

std::accumulate (double) result 10000000.7 took 163.6 ms
std::reduce (seq, double) result 10000000.7 took 162.9 ms
std::reduce (par, double) result 10000000.7 took 97.5 ms
std::accumulate (long) result 100000007 took 62.3 ms
std::reduce (seq, long) result 100000007 took 64.3 ms
std::reduce (par, long) result 100000007 took 49.0 ms