title: Mutating multiple columns in a data frame using dplyr
body: <p>I have the following data frame <code>df</code>:</p>
  
  <pre><code>  v1 v2 v3 v4
  1  1  5  7  4
  2  2  6 10  3
  </code></pre>
  
  <p>And I want to obtain the following data frame <code>df2</code> multiplying columns v1*v3 and v2*v4:</p>
  
  <pre><code>  v1 v2 v3 v4 v1v3 v2v4
  1  1  5  7  4    7   20
  2  2  6 10  3   20   18
  </code></pre>
  
  <p>How can I do that using <code>dplyr</code>? Using <code>mutate_each</code>?</p>
  
  <p>I need a solution that can be <strong>generalized to a large number of variables</strong> and not only 4 (v1 to v4).
  This is the code to generate the example:</p>
  
  <pre><code>v1 &lt;- c(1, 2)
  v2 &lt;- c(5,6)
  v3 &lt;- c(7, 10)
  v4 &lt;- c(4, 3)
  df &lt;- data.frame(v1, v2, v3, v4)
  v1v3 &lt;- c(v1 * v3)
  v2v4 &lt;- c(v2 * v4)
  df2 &lt;- cbind(df, v1v3, v2v4)
  </code></pre>