title: Calculating summary stats for data frames containing mixed data types
body: <p>been scratching my head on this, but I have a data set from a CSV that's like</p>
  
  <pre><code>A   B   C   D
  1   2   3.1 1
  2   3   2   N/A
  3   N/A 3.5 3
  4   3   5   5
  5   4   6   2.5
  </code></pre>
  
  <p>I'm using <code>read.csv</code>, but the resultant data frame converts the columns that contain "N/A" into type "chr")</p>
  
  <p>I'm using <code>apply(result,2,mean)</code> to calculate column means but I get the error: "argument is not numeric or logical: returning NA"</p>
  
  <p>Is there a simple way to do what I'm trying to do, instead of looping through the data frame column by column, turning them into lists, converting to numeric, then cbinding them back together?</p>