title: R making a list of factors in a dataframe column
body: <p>Suppose I have a column in a dataframe:</p>
  
  <p>A A A A B B B B B B B B B C C D D E E E E E E E E E F F F F F F F F F</p>
  
  <p>How can I make a list of the factors within that column?
   ie:</p>
  
  <p>A B C D E F</p>
  
  <p>Thank you for your help.</p>