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std::reduce

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
reduce
(C++17)
C library
 
Defined in header <numeric>
template<class InputIt>

typename std::iterator_traits<InputIt>::value_type reduce(

    InputIt first, InputIt last);
(1) (since C++17)
template<class ExecutionPolicy, class InputIt>

typename std::iterator_traits<InputIt>::value_type reduce(
    ExecutionPolicy&& policy,

    InputIt first, InputIt last);
(2) (since C++17)
template<class InputIt, class T>
T reduce(InputIt first, InputIt last, T init);
(3) (since C++17)
template<class ExecutionPolicy, class InputIt, class T>

T reduce(ExecutionPolicy&& policy,

         InputIt first, InputIt last, T init);
(4) (since C++17)
template<class InputIt, class T, class BinaryOp>
T reduce(InputIt first, InputIt last, T init, BinaryOp binary_op);
(5) (since C++17)
template<class ExecutionPolicy, class InputIt, class T, class BinaryOp>

T reduce(ExecutionPolicy&& policy,

         InputIt first, InputIt last, T init, BinaryOp binary_op);
(6) (since C++17)
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>> 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).

Contents

[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.
binary_op - binary FunctionObject that will be applied in unspecified order to the result of dereferencing the input iterators, the results of other binary_op and init.
Type requirements
-
InputIt must meet the requirements of InputIterator.

[edit] 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

[edit] Complexity

O(last - first) applications of 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

If the range is empty, init is returned, unmodified

[edit] Example

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

#include <iostream>
#include <chrono>
#include <vector>
#include <numeric>
#include <execution>
 
int main()
{
    std::vector<double> v(10'000'007, 0.5);
 
    {
        auto t1 = std::chrono::high_resolution_clock::now();
        double result = std::accumulate(v.begin(), v.end(), 0.0);
        auto t2 = std::chrono::high_resolution_clock::now();
        std::chrono::duration<double, std::milli> ms = t2 - t1;
        std::cout << std::fixed << "std::accumulate result " << result
                  << " took " << ms.count() << " ms\n";
    }
 
    {
        auto t1 = std::chrono::high_resolution_clock::now();
        double result = std::reduce(std::execution::par, v.begin(), v.end());
        auto t2 = std::chrono::high_resolution_clock::now();
        std::chrono::duration<double, std::milli> ms = t2 - t1;
        std::cout << "std::reduce result "
                  << result << " took " << ms.count() << " ms\n";
    }
}

Possible output:

std::accumulate result 5000003.50000 took 12.7365 ms
std::reduce result 5000003.50000 took 5.06423 ms

[edit] See also

sums up a range of elements
(function template) [edit]
applies a function to a range of elements
(function template) [edit]
applies a functor, then reduces out of order
(function template) [edit]