title: Selecting only numeric columns from a data frame
body: <p>Suppose, you have a data.frame like this: </p>
  
  <pre><code>x &lt;- data.frame(v1=1:20,v2=1:20,v3=1:20,v4=letters[1:20])
  </code></pre>
  
  <p>How would you select only those columns in x that are numeric? </p>