26 Algorithms library [algorithms]

26.10 Generalized numeric operations [numeric.ops]

26.10.4 Reduce [reduce]

template<class InputIterator>
  constexpr typename iterator_traits<InputIterator>::value_type
    reduce(InputIterator first, InputIterator last);

Effects: Equivalent to: return reduce(first, last, typename iterator_traits<InputIterator>::value_type{});

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template<class ExecutionPolicy, class ForwardIterator>
  typename iterator_traits<ForwardIterator>::value_type
    reduce(ExecutionPolicy&& exec,
           ForwardIterator first, ForwardIterator last);

Effects: Equivalent to: return reduce(std::forward<ExecutionPolicy>(exec), first, last, typename iterator_traits<ForwardIterator>::value_type{});

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template<class InputIterator, class T>
  constexpr T reduce(InputIterator first, InputIterator last, T init);

Effects: Equivalent to: return reduce(first, last, init, plus<>());

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template<class ExecutionPolicy, class ForwardIterator, class T>
  T reduce(ExecutionPolicy&& exec,
           ForwardIterator first, ForwardIterator last, T init);

Effects: Equivalent to: return reduce(std::forward<ExecutionPolicy>(exec), first, last, init, plus<>());

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template<class InputIterator, class T, class BinaryOperation>
  constexpr T reduce(InputIterator first, InputIterator last, T init,
                     BinaryOperation binary_op);
template<class ExecutionPolicy, class ForwardIterator, class T, class BinaryOperation>
  T reduce(ExecutionPolicy&& exec,
           ForwardIterator first, ForwardIterator last, T init,
           BinaryOperation binary_op);

Mandates: All of

(5.1)
binary_op(init, *first),
(5.2)
binary_op(*first, init),
(5.3)
binary_op(init, init), and
(5.4)
binary_op(*first, *first)

are convertible to T.

Preconditions:

(6.1)
T meets the Cpp17MoveConstructible (Table 31) requirements.
(6.2)
binary_op neither invalidates iterators or subranges, nor modifies elements in the range [first, last].

Returns: GENERALIZED_SUM(binary_op, init, *i, ...) for every i in [first, last).

Complexity:

O (last - first)

applications of binary_op.

[Note 1:

The difference between reduce and accumulate is that reduce applies binary_op in an unspecified order, which yields a nondeterministic result for non-associative or non-commutative binary_op such as floating-point addition.

— end note]