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std::accumulate(3) C++ Standard Libary std::accumulate(3) NAME std::accumulate - std::accumulate Synopsis Defined in header <numeric> template< class InputIt, class T > (1) (const- expr since C++20) T accumulate( InputIt first, InputIt last, T init ); template< class InputIt, class T, class BinaryOp > T accumulate( InputIt first, InputIt last, T init, (2) (const- expr since C++20) BinaryOp op ); Computes the sum of the given value init and the elements in the range [first, last). 1) Initializes the accumulator acc (of type T) with the initial value init and then modifies it with acc = acc + *i (until C++20) acc = std::move(acc) + *i (since C++20) for every iterator i in the range [first, last) in or- der. 2) Initializes the accumulator acc (of type T) with the initial value init and then modifies it with acc = op(acc, *i) (until C++20) acc = op(std::move(acc), *i) (since C++20) for every iterator i in the range [first, last) in or- der. If any of the following conditions is satisfied, the behavior is un- defined: * T is not CopyConstructible. * T is not CopyAssignable. * op modifies any element of [first, last). * op invalidates any iterator or subrange in [first, last]. Parameters first, last - the range of elements to sum init - initial value of the sum binary operation function object that will be applied. The signature of the function should be equivalent to the following: Ret fun(const Type1 &a, const Type2 &b); op - The signature does not need to have const &. The type Type1 must be such that an object of type T can be implicitly converted to Type1. The type Type2 must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type2. The type Ret must be such that an object of type T can be assigned a value of type Ret. Type requirements - InputIt must meet the requirements of LegacyInputIterator. Return value acc after all modifications. Possible implementation accumulate (1) template<class InputIt, class T> constexpr // since C++20 T accumulate(InputIt first, InputIt last, T init) { for (; first != last; ++first) init = std::move(init) + *first; // std::move since C++20 return init; } accumulate (2) template<class InputIt, class T, class BinaryOperation> constexpr // since C++20 T accumulate(InputIt first, InputIt last, T init, BinaryOperation op) { for (; first != last; ++first) init = op(std::move(init), *first); // std::move since C++20 return init; } Notes std::accumulate performs a left fold. In order to perform a right fold, one must reverse the order of the arguments to the binary operator, and use reverse iterators. If left to type inference, op operates on values of the same type as init which can result in unwanted casting of the iterator elements. For example, std::accumulate(v.begin(), v.end(), 0) likely does not give the re- sult one wishes for when v is of type std::vector<double>. Example // Run this code #include <functional> #include <iostream> #include <numeric> #include <string> #include <vector> int main() { std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; int sum = std::accumulate(v.begin(), v.end(), 0); int product = std::accumulate(v.begin(), v.end(), 1, std::multi- plies<int>()); auto dash_fold = [](std::string a, int b) { return std::move(a) + '-' + std::to_string(b); }; std::string s = std::accumulate(std::next(v.begin()), v.end(), std::to_string(v[0]), // start with first element dash_fold); // Right fold using reverse iterators std::string rs = std::accumulate(std::next(v.rbegin()), v.rend(), std::to_string(v.back()), // start with last element dash_fold); std::cout << "sum: " << sum << '\n' << "product: " << product << '\n' << "dash-separated string: " << s << '\n' << "dash-separated string (right-folded): " << rs << '\n'; } Output: sum: 55 product: 3628800 dash-separated string: 1-2-3-4-5-6-7-8-9-10 dash-separated string (right-folded): 10-9-8-7-6-5-4-3-2-1 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior LWG 242 C++98 op could not have side effects it cannot modify the ranges involved LWG 2055 C++20 acc was not moved while being it is moved (P0616R0) accumulated See also adjacent_difference computes the differences between adjacent ele- ments in a range (function template) inner_product computes the inner product of two ranges of ele- ments (function template) partial_sum computes the partial sum of a range of elements (function template) reduce similar to std::accumulate, except out of order (C++17) (function template) ranges::fold_left left-folds a range of elements (C++23) (niebloid) http://cppreference.com 2024.06.10 std::accumulate(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Possible implementation | Notes | Example | Output: | See also
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