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std::accumulate

Defined in header `<numeric>`
	(1)
template< class InputIt, class T > T accumulate( InputIt first, InputIt last, T init );		(until C++20)
template< class InputIt, class T > constexpr T accumulate( InputIt first, InputIt last, T init );		(since C++20)
	(2)
template< class InputIt, class T, class BinaryOperation > T accumulate( InputIt first, InputIt last, T init, BinaryOperation op );		(until C++20)
template< class InputIt, class T, class BinaryOperation > constexpr T accumulate( InputIt first, InputIt last, T init, BinaryOperation op );		(since C++20)

Computes the sum of the given value init and the elements in the range [first, last). The first version uses operator+ to sum up the elements, the second version uses the given binary function op, both applying std::move to their operands on the left hand side (since C++20).

`op` must not have side effects.	(until C++11)
`op` must not invalidate any iterators, including the end iterators, nor modify any elements of the range involved, nor *last.	(since C++11)

Parameters

first, last	-	the range of elements to sum
init	-	initial value of the sum
op	-	binary operation function object that will be applied. The binary operator takes the current accumulation value `a` (initialized to `init`) and the value of the current element `b`. 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 `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.
-`T` must meet the requirements of CopyAssignable and CopyConstructible.

Return value

1) The sum of the given value and elements in the given range.

2) The result of left fold of the given range over op

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.

Common mistakes

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 std::vector<double>.

Possible implementation

First version
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; }
Second version
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; }

First version

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;
}

Second version

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 <iostream>
#include <vector>
#include <numeric>
#include <string>
#include <functional>
 
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

adjacent_difference	computes the differences between adjacent elements in a range (function template)
inner_product	computes the inner product of two ranges of elements (function template)
partial_sum	computes the partial sum of a range of elements (function template)
reduce (C++17)	similar to `std::accumulate`, except out of order (function template)