# 27 Algorithms library [algorithms]

## 27.10 Generalized numeric operations [numeric.ops]

### 27.10.4 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{});
```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{});
```template<class InputIterator, class T> constexpr T reduce(InputIterator first, InputIterator last, T init); ```
Effects: Equivalent to: return reduce(first, last, init, plus<>());
```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<>());
```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
• binary_op(init, *first),
• binary_op(*first, init),
• binary_op(init, init), and
• binary_op(*first, *first)
are convertible to T.
Preconditions:
• T meets the Cpp17MoveConstructible (Table 31) requirements.
• 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: 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.
â€” end note]