std::accumulate() algorithm

since C++20

// (1)
template< class InputIt, class T >
constexpr T accumulate( InputIt first, InputIt last, T init );

// (2)
template< class InputIt, class T, class BinaryOperation >
constexpr T accumulate( InputIt first, InputIt last, T init, BinaryOperation op );

until C++20

// (1)
template< class InputIt, class T >
T accumulate( InputIt first, InputIt last, T init );

// (2)
template< class InputIt, class T, class BinaryOperation >
T accumulate( InputIt first, InputIt last, T init, BinaryOperation op );

Computes the sum of the given value init and the elements in the ange [first; last).

Initializes the accumulator acc (of type T) with the initial value init and then modifies it with:

• (1) acc = acc + *i (until C++20)acc = std::move(acc) + *i (since C++20)
• (2) 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 order.

Undefined Behaviour

If op invalidates any iterators (including the end iterators) or modifies any elements of the range involved, the behavior is undefined

.

Parameters

first last	The range of elements to fold.
init	Initial value of the fold.
op	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); 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 it. The type Type2 must be such that an object of type InputIt can be dereferenced and then implicitly converted to it. The type Ret must be such that an object of type T can be assigned a value of it's type.

Type requirements

InputIt	LegacyInputIterator
T	CopyAssignable CopyConstructible

Return value

acc after all modifications.

Complexity

Exactly last - first increments and assignments.

Exceptions

(none)

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

Examples

Main.cpp

#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';
}

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