title: Computing the scalar product of two vectors in C++
body: <p>I am trying to write a program with a function <code>double_product(vector&lt;double&gt; a, vector&lt;double&gt; b)</code> that computes the scalar product of two vectors. The scalar product is</p>
  
  <pre><code>$a_{0}b_{0}+a_{1}b_{1}+...+a_{n-1}b_{n-1}$.
  </code></pre>
  
  <p>Here is what I have. It is a mess, but I am trying!</p>
  
  <pre><code>#include &lt;iostream&gt;
  #include &lt;vector&gt;
  
  using namespace std;
  
  class Scalar_product
  {
      public:
      Scalar_product(vector&lt;double&gt; a, vector&lt;double&gt; b);
  };
  double scalar_product(vector&lt;double&gt; a, vector&lt;double&gt; b)
  {
      double product = 0;
      for (int i = 0; i &lt;= a.size()-1; i++)
          for (int i = 0; i &lt;= b.size()-1; i++)
              product = product + (a[i])*(b[i]);
      return product;
  }
  
  int main() {
      cout &lt;&lt; product &lt;&lt; endl;
      return 0;
  }
  </code></pre>