title: Converting data frame's column string values to integers
body: <pre class="lang-r prettyprint-override"><code>cars &lt;- data.frame(engine_volume = c('1598 ccm','1605 ccm','1900 ccm','1598 ccm','1605 ccm','1900 ccm', '1598 ccm','1605 ccm','1900 ccm', '1598 ccm'))
  </code></pre>
  <p>How to convert string values from <code>engine.volume</code> to integers?</p>