title: Extract names of the levels of a factor
body: <p>I'm trying to read a huge matrix (2.8gb) in R, thus, so far, the best I have found is</p>
  
  <pre><code> require(data.table)
  
   DT&lt;-fread("bigmatrix.csv")
  </code></pre>
  
  <p>of which I know almost nothing!</p>
  
  <p>After this I'm able to tell you that this matrix has 3 columns and 50 milion rows.</p>
  
  <p>Each row is of the type</p>
  
  <pre><code>             object1                       object 2           distance
   1: Kho.CENTRAL_KHOISAN.GWI           Kho.CENTRAL_KHOISAN.GWI 0.0000000
   2: Kho.CENTRAL_KHOISAN.GWI         Kho.CENTRAL_KHOISAN.GXANA 0.2195843  
   3: Kho.CENTRAL_KHOISAN.GWI Kho.CENTRAL_KHOISAN.KHOEKHOEGOWAB 0.6749363
   4: Kho.CENTRAL_KHOISAN.GWI          Kho.CENTRAL_KHOISAN.KHWE 0.6089206
   5: Kho.CENTRAL_KHOISAN.GWI        Kho.CENTRAL_KHOISAN.KORANA 0.7163111
   6: Kho.CENTRAL_KHOISAN.GWI         Kho.CENTRAL_KHOISAN.KWADI 0.8017179
  </code></pre>
  
  <p>So it's comparing distances of 2 objects pairwise for approximately 6900 objects</p>
  
  <p>Now comes my problem:</p>
  
  <p>I want to excract the pairwise comparison of just 41 objects. However I don't know how the guy who gave me this dataset has called these 41 objects!! </p>
  
  <p>So my solution would be to find the levels of DT$object1, write them in a file and then scan them to find the 41 I need, how can I do it?</p>
  
  <p>I tried</p>
  
  <pre><code>foo&lt;-factor(DT$object1)
  </code></pre>
  
  <p>So when I call</p>
  
  <pre><code>foo
  
  ....
  
  6895 Levels: AA.BEJA.BEJA AA.BEJA.BEJA_2 AA.BERBER.AWJILAH ... Zun.ZUNI.ZUNI
  </code></pre>
  
  <p>but </p>
  
  <pre><code>foo$Levels
  </code></pre>
  
  <p>gives me an error!</p>
  
  <p>I'm sure there is a smarter way than what I would do in C++ (i.e. loop over each row, insert the name of Object 1 in a vector of strings only if it's not present yet), but how do I do it?</p>
  
  <hr>
  
  <p><strong>EDIT:</strong> Now another question arose:</p>
  
  <p>I have identified the 41 objects that I need, how do I exctract from the data.table the rows that are relevant to me?</p>
  
  <p>I can store the names of the objects in a data frame or a vector</p>