std:: inner_product

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 .

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

Return value

acc after all modifications.

Possible implementation

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;
}

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

#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';
}

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.

See also