title: R: Data Table cannot handle "?" as an na.strings() option
body: <p>I am trying to read in a very large data set using a data table in R (<code>library(data.table)</code>)</p>
  
  <p>The data set codes missing values with a question mark <code>?</code>.  I am attempting to read in the data while simultaneously converting one column to a Date format.  I suspect my code will work, but <code>fread</code> is coverting all of the columns to character classes because it cannot handle the <code>?</code> as an <code>na.strings()</code> option.</p>
  
  <p>I could resolve this through read.csv, but it's prohibitively slow to do so.  Does anybody know how to resolve this?</p>
  
  <p>Here is my code:</p>
  
  <pre><code>setClass("date")
  setAs("character","date", function(from) as.Date(from, format="%d/%m/%Y") )
  
  DT&lt;- fread("data.txt", na.strings="?", colClasses = c("date", "character", rep("numeric", 7)))
  </code></pre>