std::inner_product() algorithm

since C++20

// (1)
template< class InputIt1, class InputIt2, class T >
constexpr T inner_product( InputIt1 first1, InputIt1 last1, InputIt2 first2, T init );

// (2)
template< class InputIt1, class InputIt2, class T, class BinaryOperation1, class BinaryOperation2 >
constexpr T inner_product( InputIt1 first1, InputIt1 last1, InputIt2 first2,
                           T init, BinaryOperation1 op1, BinaryOperation2 op2 );

until C++20

// (1)
template< class InputIt1, class InputIt2, class T >
T inner_product( InputIt1 first1, InputIt1 last1, InputIt2 first2, T init );

// (2)
template< class InputIt1, class InputIt2, class T, class BinaryOperation1, class BinaryOperation2 >
T inner_product( InputIt1 first1, InputIt1 last1, InputIt2 first2,
                 T init, BinaryOperation1 op1, BinaryOperation2 op2 );

Computes inner product (i.e. sum of products) or performs ordered map/reduce operation on the range [first1; last1) and the range beginning at first2.

Initializes the accumulator acc (of type T) with the initial value init and then modifies it with the expression =

• (1) acc = acc + (*i1) * (*i2) (until C++11)acc = std::move(acc) + (*i1) * (*i2) (since C++11)
• (2) 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.

For built-in meaning of + and *, this computes inner product of the two ranges.

Undefined Behaviour

If op1 or op2 invalidates any iterators (including the end iterators) or modifies any elements of the range involved, the behavior is undefined.

Parameters

first1 last1	The firstrange of elements.
first2	The beginning of the second range of elements.
init	Initial value of the products.
op1	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: 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 them, respectively. The type Ret must be such that an object of type T can be assigned a value of it's type.
op2	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: 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 them, respectively. The type Ret must be such that an object of type Type3 can be assigned a value of it's type.

Type requirements

InputIt1 InputIt2	LegacyInputIterator
ForwardIt1 ForwardIt2	LegacyForwardIterator
T	CopyAssignable CopyConstructible

Return value

acc after all modifications.

Complexity

Exactly last - first increments, assignments and applications of both op1 and op2.

Exceptions

(none)

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 BinaryOperation1, class BinaryOperation2>
constexpr // since C++20
T inner_product(InputIt1 first1, InputIt1 last1,
                InputIt2 first2, T init,
                BinaryOperation1 op1
                BinaryOperation2 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.

Examples

Main.cpp

#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