# std::accumulate

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< cpp‎ | algorithm

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;
    }

### Example

Run this code

    #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

### See also

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)

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