title: How to convert an entire data.frame to numeric
body: <p>I want to convert an entire <code>data.frame</code> containing more than 130 columns to numeric.</p>
  
  <p>I know that I need to use <code>as.numeric</code>, but the problem is that I have to apply this function separately to each one of the 130 columns. I tried to apply it to the entire <code>data.frame</code>, but I got the following error message:</p>
  
  <pre><code>Error: (list) object cannot be coerced to type 'double'
  </code></pre>
  
  <p>How can I do that by a relatively short code?</p>