std::inner_product(3)

NAME
std::inner_product - std::inner_product

Synopsis
   Defined in header <numeric>
   template< class InputIt1, class InputIt2, class T >
   T inner_product( InputIt1 first1, InputIt1 last1, (1) (constexpr since C++20)
                    InputIt2 first2, T init );
   template< class InputIt1, class InputIt2, class T,
             class BinaryOp1, class BinaryOp2 >
   T inner_product( InputIt1 first1, InputIt1 last1, (2) (constexpr since C++20)
                    InputIt2 first2, T init,
                    BinaryOp1 op1, BinaryOp2 op2 );
   Computes inner product (i.e. sum of products) or performs ordered map/reduce
   operation on the range [first1, last1) and the range of std::distance(first1,
   first2) elements beginning at first2.
   1) Initializes the accumulator acc (of type T) with the initial value init and then
   modifies it with the expression
   acc = acc + (*i1) * (*i2)
   (until C++11)
   acc = std::move(acc) + (*i1) * (*i2)
   (since C++11) for every iterator i1 in the range [first1, last1) in order and its
   corresponding iterator i2 in the range beginning at first2. For built-in meaning of
   + and *, this computes inner product of the two ranges.
   2) Initializes the accumulator acc (of type T) with the initial value init and then
   modifies it with the expression
   acc = op1(acc, op2(*i1, *i2))
   (until C++11)
   acc = op1(std::move(acc), op2(*i1, *i2))
   (since C++11) for every iterator i1 in the range [first1, last1) in order and its
   corresponding iterator i2 in the range beginning at first2.
   Given last2 as the std::distance(first1, last1)
   th next iterator of first2, if any of the following conditions is satisfied, the
   behavior is undefined:
     * T is not CopyConstructible.
     * T is not CopyAssignable.
     * op1 or op2 modifies any element of [first1, last1) or [first2, last2).
     * op1 or op2 invalidates any iterator or subrange in [first1, last1] or
       [first2, last2].

Parameters
   first1, last1 - the first range of elements
   first2 - the beginning of the second range of elements
   init - initial value of the sum of the products
                   binary operation function object that will be applied. This "sum"
                   function takes a value returned by op2 and the current value of the
                   accumulator and produces a new value to be stored in the
                   accumulator.
                   The signature of the function should be equivalent to the following:
   op1 -
                    Ret fun(const Type1 &a, const Type2 &b);
                   The signature does not need to have const &.
                   The types Type1 and Type2 must be such that objects of types T and
                   Type3 can be implicitly converted to Type1 and Type2 respectively.
                   The type Ret must be such that an object of type T can be assigned a
                   value of type Ret.
                   binary operation function object that will be applied. This
                   "product" function takes one value from each range and produces a
                   new value.
                   The signature of the function should be equivalent to the following:
   op2 - Ret fun(const Type1 &a, const Type2 &b);
                   The signature does not need to have const &.
                   The types Type1 and Type2 must be such that objects of types
                   InputIt1 and InputIt2 can be dereferenced and then implicitly
                   converted to Type1 and Type2 respectively. The type Ret must be
                   such that an object of type Type3 can be assigned a value of type
                   Ret.

Type requirements
   -
   InputIt1, InputIt2 must meet the requirements of LegacyInputIterator.
   -
   ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.

Return value
   acc after all modifications.

Possible implementation
                        inner_product (1)
   template<class InputIt1, class InputIt2, class T>
   constexpr // since C++20
   T inner_product(InputIt1 first1, InputIt1 last1, InputIt2 first2, T init)
   {
       while (first1 != last1)
       {
           init = std::move(init) + (*first1) * (*first2); // std::move since C++11
           ++first1;
           ++first2;
       }
       return init;
   }
                        inner_product (2)
   template<class InputIt1, class InputIt2, class T,
            class BinaryOp1, class BinaryOp2>
   constexpr // since C++20
   T inner_product(InputIt1 first1, InputIt1 last1, InputIt2 first2, T init,
                   BinaryOp1 op1, BinaryOp2 op2)
   {
       while (first1 != last1)
       {
           init = op1(std::move(init), op2(*first1, *first2)); // std::move since C++11
           ++first1;
           ++first2;
       }
       return init;
   }

Notes
   The parallelizable version of this algorithm, std::transform_reduce, requires op1
   and op2 to be commutative and associative, but std::inner_product makes no such
   requirement, and always performs the operations in the order given.

Example
// Run this code
 #include <functional>
 #include <iostream>
 #include <numeric>
 #include <vector>
 int main()
 {
     std::vector<int> a{0, 1, 2, 3, 4};
     std::vector<int> b{5, 4, 2, 3, 1};
     int r1 = std::inner_product(a.begin(), a.end(), b.begin(), 0);
     std::cout << "Inner product of a and b: " << r1 << '\n';
     int r2 = std::inner_product(a.begin(), a.end(), b.begin(), 0,
                        std::plus<>(), std::equal_to<>());
     std::cout << "Number of pairwise matches between a and b: " << r2 << '\n';
 }

Output:
 Inner product of a and b: 21
 Number of pairwise matches between a and b: 2
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 242 C++98 op1 and op2 could not have side effects they cannot modify the
                        ranges involved
   LWG 2055 C++11 acc was not moved while being it is moved
   (P0616R0) accumulated

See also
   transform_reduce applies an invocable, then reduces out of order
   (C++17) (function template)
   accumulate sums up or folds a range of elements
                    (function template)
   partial_sum computes the partial sum of a range of elements
                    (function template)