std::inner_product

From Cppreference

Defined in header `<numeric>`

template< class InputIterator1, class InputIterator2 class T > T inner_product( InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, T value );	(1)

template< class InputIterator1, class InputIterator2, class T, class BinaryOperation1 class BinaryOperation2 > T inner_product( InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, T value, BinaryOperation1 op1 BinaryOperation2 op2 );	(2)

Computes inner product (i.e. sum of products) of the range [first1, last1) and another range beginning at first2. The first version uses operator* to compute product of the element pairs and operator+ to sum up the products, the second version uses op2 and op1 for these tasks respectively.

The signature does not need to have const &.
The types Type1 and Type2 must be such that objects of types InputIterator1 and InputIterator2 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.

[edit] Return value

The inner product of two ranges.

[edit] Equivalent function

First version

template<class InputIterator1, class InputIterator2, class T>
T inner_product(InputIterator1 first1, InputIterator1 last1,
                InputIterator2 first2, T value)
{
    while (first1 != last1) {
         value = value + *first1 * *first2;
         ++first1;
         ++first2;
    }
    return value;
}

Second version

template<class InputIterator1, class InputIterator2,
         class T,
         class BinaryOperation1, class BinaryOperation2>
T inner_product(InputIterator1 first1, InputIterator1 last1,
                InputIterator2 first2, T value,
                BinaryOperation1 op1
                BinaryOperation2 op2)
{
    while (first1 != last1) {
         value = op1(value, op2(*first1, *first2));
         ++first1;
         ++first2;
    }
    return value;
}

[edit] Example

#include <numeric>
#include <iostream>
#include <vector>
#include <functional>
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<int>(), std::equal_to<int>());
    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

[edit] See also

accumulate

sums up a range of elements
(function template)

partial_sum

computes the partial sum of a range of elements
(function template)