std::accumulate

template< class InputIt, class T >
T accumulate( InputIt first, InputIt last, T init );

template< class InputIt, class T, class BinaryOp >
T accumulate( InputIt first, InputIt last, T init, BinaryOp op );

Computes the sum of the given value init and all elements of the target range [first, last).

Let add_op be the add operation, it is operator+(until C++20)an object of type std::plus<>(since C++20) for overload (1), or op for overload (2), performs the following operations in order:

1. Initializes the accumulator acc (of type T) with the initial value init.
2. For every iterator iter in the target range in order, acc is modified with acc = add_op(acc, *iter)(until C++20)acc = add_op(std::move(acc), *iter)(since C++20).
3. Returns acc after all modifications.

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

• T is not CopyConstructible.
• T is not CopyAssignable.
• add_op modifies any element of the target range.
• add_op invalidates any iterator or subrange in [first, last].

Parameters

Return value

As described above.

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, add_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 result one wishes for when v is of type std::vector<double>.

Possible implementation

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;
}

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;
}

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 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';
}

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 retroactively to previously published C++ standards.

See also