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std::accumulate

Defined in header `<numeric>`
template< class InputIt, class T > T accumulate( InputIt first, InputIt last, T init );	(1)
template< class InputIt, class T, class BinaryOperation > T accumulate( InputIt first, InputIt last, T init, BinaryOperation op );	(2)

Defined in header <numeric>

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

(1)

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

(2)

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.

`op` must not have side effects.	(until C++11)
`op` must not invalidate any iterators, including the end iterators, or modify any elements of the range involved.	(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 `InputIterator`.
- `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

Although std::accumulate performs left fold by default, right fold may be achieved by using reverse iterators, e.g. std::accumulate(v.rbegin(), v.rend(), init, binop).

Possible implementation

First version
template<class InputIt, class T> T accumulate(InputIt first, InputIt last, T init) { for (; first != last; ++first) { init = init + *first; } return init; }
Second version
template<class InputIt, class T, class BinaryOperation> T accumulate(InputIt first, InputIt last, T init, BinaryOperation op) { for (; first != last; ++first) { init = op(init, *first); } return init; }

First version

template<class InputIt, class T>
T accumulate(InputIt first, InputIt last, T init)
{
    for (; first != last; ++first) {
        init = init + *first;
    }
    return init;
}

Second version

template<class InputIt, class T, class BinaryOperation>
T accumulate(InputIt first, InputIt last, T init, 
             BinaryOperation op)
{
    for (; first != last; ++first) {
        init = op(init, *first);
    }
    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>());
 
    std::string s = std::accumulate(std::next(v.begin()), v.end(),
                                    std::to_string(v[0]), // start with first element
                                    [](std::string a, int b) {
                                        return a + '-' + std::to_string(b);
                                    });
 
    std::cout << "sum: " << sum << '\n'
              << "product: " << product << '\n'
              << "dash-separated string: " << s << '\n';
}

Output:

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