Computes the sum of the given valueinit and the elements in the range[first, last).

If any of the following conditions is satisfied, the behavior is undefined:

• T is notCopyConstructible.
• T is notCopyAssignable.
• op modifies any element of[first, last).
• op invalidates any iterator or subrange in[first, last].

[edit]Parameters

[edit]Return value

acc after all modifications.

[edit]Possible implementation

template<class InputIt,class T>constexpr// since C++20T accumulate(InputIt first, InputIt last, T init){for(; first!= last;++first)        init= std::move(init)+*first;// std::move since C++20 return init;}

template<class InputIt,class T,class BinaryOperation>constexpr// since C++20T 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;}

[edit]Notes

std::accumulate performs a leftfold. 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 asinit which can result in unwanted casting of the iterator elements. For example,std::accumulate(v.begin(), v.end(),0) likely does not give the result one wishes for whenv is of typestd::vector<double>.

[edit]Example

#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::multiplies<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 iteratorsstd::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';}

sum: 55product: 3628800dash-separated string: 1-2-3-4-5-6-7-8-9-10dash-separated string (right-folded): 10-9-8-7-6-5-4-3-2-1

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

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