title: unique value of a column and converting it to a list
body: <p>I have a R dataFrame</p>
  
  <pre><code>Serial N       voltage       current
     B              10            nan
     B              10            nan
     C              12            nan
     D              40            nan
     .               .             .
     AB             12            nan
  </code></pre>
  
  <p>I would like to get a list of serial N as a list and not a Factor.</p>
  
  <p>I did something like this</p>
  
  <pre><code>temp=unique(df$Serial_N)
  </code></pre>
  
  <p>but I get a factor. How can I convert it to a list such that I can loop though it or access an serial N I want from it like</p>
  
  <pre><code>temp[[1]]
  </code></pre>