11.10. Computing a Dot Product
Problem
You have two containers of numbers that are the same length and you want to compute their dot product.
Solution
Example 11-19 shows how you can
compute a dot product using the inner_product
function from the <numeric>
header.
Example 11-19. Computing the dot product
#include <numeric>
#include <iostream>
#include <vector>
using namespace std;
int main() {
int v1[] = { 1, 2, 3 };
int v2[] = { 4, 6, 8 };
cout << "the dot product of (1,2,3) and (4,6,8) is ";
cout << inner_product(v1, v1 + 3, v2, 0) << endl;
}The program in Example 11-19 produces the following output:
the dot product of (1,2,3) and (4,6,8) is 40
Discussion
The dot product is a form of inner product known as the Euclidean Inner Product. The inner_product
function is declared as follows:
template<class In, class In2, class T> T inner_product(In first, In last, In2 first2, T init); template<class In, class In2, class T, class BinOp, class BinOp2> T inner_product(In first, In last, In2 first2, T init, BinOp op, Binop2 op2);
The first form of inner_product sums the result of
multiplying corresponding elements from two containers. The second form of the inner_product function allows you to supply your own pairwise
operation and accumulation function. See Example 11-20 to see a sample implementation demonstrating how inner_product works.
Example 11-20. Sample implementation of inner_product()
template<class In, class In2, class T, class BinOp, class BinOp2> T inner_product(In first, In last, In2 ...