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cppdraft/reduce.md

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+[reduce]
+
+# 26 Algorithms library [[algorithms]](./#algorithms)
+
+## 26.10 Generalized numeric operations [[numeric.ops]](numeric.ops#reduce)
+
+### 26.10.4 Reduce [reduce]
+
+[🔗](#lib:reduce)
+
+`template<class InputIterator>
+  constexpr typename iterator_traits<InputIterator>::value_type
+    reduce(InputIterator first, InputIterator last);
+`
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L12340)
+
+*Effects*: Equivalent to:return reduce(first, last, typename iterator_traits<InputIterator>::value_type{});
+
+[🔗](#lib:reduce_)
+
+`template<class ExecutionPolicy, class ForwardIterator>
+  typename iterator_traits<ForwardIterator>::value_type
+    reduce(ExecutionPolicy&& exec,
+           ForwardIterator first, ForwardIterator last);
+`
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L12358)
+
+*Effects*: Equivalent to:return reduce(std::forward<ExecutionPolicy>(exec), first, last, typename iterator_traits<ForwardIterator>::value_type{});
+
+[🔗](#lib:reduce__)
+
+`template<class InputIterator, class T>
+  constexpr T reduce(InputIterator first, InputIterator last, T init);
+`
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L12375)
+
+*Effects*: Equivalent to:return reduce(first, last, init, plus<>());
+
+[🔗](#lib:reduce___)
+
+`template<class ExecutionPolicy, class ForwardIterator, class T>
+  T reduce(ExecutionPolicy&& exec,
+           ForwardIterator first, ForwardIterator last, T init);
+`
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L12391)
+
+*Effects*: Equivalent to:return reduce(std::forward<ExecutionPolicy>(exec), first, last, init, plus<>());
+
+[🔗](#lib:reduce____)
+
+`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);
+`
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L12413)
+
+*Mandates*: All of
+
+- [(5.1)](#5.1)
+
+binary_op(init, *first),
+
+- [(5.2)](#5.2)
+
+binary_op(*first, init),
+
+- [(5.3)](#5.3)
+
+binary_op(init, init), and
+
+- [(5.4)](#5.4)
+
+binary_op(*first, *first)
+
+are convertible to T[.](#5.sentence-1)
+
+[6](#6)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L12424)
+
+*Preconditions*: 
+
+- [(6.1)](#6.1)
+
+  T meets the [*Cpp17MoveConstructible*](utility.arg.requirements#:Cpp17MoveConstructible "16.4.4.2 Template argument requirements [utility.arg.requirements]") (Table [31](utility.arg.requirements#tab:cpp17.moveconstructible "Table 31: Cpp17MoveConstructible requirements")) requirements[.](#6.1.sentence-1)
+
+- [(6.2)](#6.2)
+
+  binary_op neither invalidates iterators or subranges,
+ nor modifies elements in the range [first, last][.](#6.2.sentence-1)
+
+[7](#7)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L12434)
+
+*Returns*: *GENERALIZED_SUM*(binary_op, init, *i, …) for every i in [first, last)[.](#7.sentence-1)
+
+[8](#8)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L12439)
+
+*Complexity*: O(last - first) applications of binary_op[.](#8.sentence-1)
+
+[9](#9)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L12443)
+
+[*Note [1](#note-1)*:
+
+The difference between reduce and accumulate is thatreduce 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[.](#9.sentence-1)
+
+— *end note*]