Computing the scalar product of two vectors in C++

Question

I am trying to write a program with a function double_product(vector a, vector b) that computes the scalar product of two vectors. T

Answer 1

Unless you need to do this on your own (e.g., writing it is homework), you should really use the standard algorithm that's already written to do exactly what you want:

#include 
#include 
#include 

int main() {
    std::vector a {1, 2, 3};
    std::vector b {4, 5, 6};

    std::cout << "The scalar product is: "
              << std::inner_product(std::begin(a), std::end(a), std::begin(b), 0.0);
    return 0;
}

Note that while begin(a) and end(a) are new in C++11, std::inner_product has been available since C++98. If you are using C++ 98 (or 03), it's pretty easy to write your own equivalent of begin and end to work with arrays though:

template 
T *begin(T (&array)[N]) {
    return array;
}

template 
T *end(T (&array)[N]) {
    return array + N;
}

Using these, a C++ 98 version of the previous code could look something like this:

int main() {
    double a[] = {1, 2, 3};
    double b[] = {4, 5, 6};

    std::cout << "The scalar product is: "
              << std::inner_product(begin(a), end(a), begin(b), 0.0);
    return 0;
}

Note that the begin and end above will only work for arrays, where the begin and end in C++11 (and later) will also work for normal collection types that define a .begin() and .end() (though it's trivial to add overloads to handle those as well, of course):

template 
typename Coll::iterator begin(Coll const& c) { return c.begin(); }

template 
typename Coll::iterator end(Coll const& c) { return c.end(); }