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std::inner_product - cppreference.com

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, last1) 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++20)acc = std::move(acc) + (*i1) * (*i2)(since C++20) 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++20)acc = op1(std::move(acc), op2(*i1, *i2))(since C++20) 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].

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  Type1 and  Type2 respectively. The type Ret must be such that an object of type T can be assigned a value of type Ret. ​

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  Type1 and  Type2 respectively. The type Ret must be such that an object of type Type3 can be assigned a value of type Ret. ​

acc after all modifications.

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++20
        ++first1;
        ++first2;
    }
 
    return init;
}

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++20
        ++first1;
        ++first2;
    }
 
    return init;
}

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.

#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

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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