std::accumulate(3)

NAME
std::accumulate - std::accumulate

Synopsis
   Defined in header <numeric>
   template< class InputIt, class T >
   T accumulate( InputIt first, InputIt last, T (until C++20)
   init );
   template< class InputIt, class T >
   constexpr T accumulate( InputIt first, InputIt (since C++20)
   last, T init );
   template< class InputIt, class T, class
   BinaryOperation >
   T accumulate( InputIt first, InputIt last, T (1) (until C++20)
   init,
   BinaryOperation op ); (2)
   template< class InputIt, class T, class
   BinaryOperation >
   constexpr T accumulate( InputIt first, InputIt (since C++20)
   last, T init,
   BinaryOperation op );
   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 (since C++11)
   modify any elements of the range involved, nor *last.

Parameters
   first, last - the range of elements to sum
   init - initial value of the sum
                 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:
   op - 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;
   }

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>());
     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

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 similar to std::accumulate, except out of order
   (C++17) (function template)