title: extracting dataframe with nan values
body: <p>I have a data frame which consists of more than million values and there is one specific column which consists of some values and some nans. Is there a way I can extract a subset of dataframe which can extract values corresponding the nans?</p>
  
  <p>for example: </p>
  
  <p>I have something in this format </p>
  
  <pre><code>+---+----+-----+
  | A | B  |  C  |
  +---+----+-----+
  | 1 | b1 | NaN |
  | 2 | b2 | c2  |
  | 3 | b3 | NaN |
  | 4 | b4 | c4  |
  | 5 | b5 | NaN |
  +---+----+-----+
  </code></pre>
  
  <p>and I want to extract the data which has a nan in column C: </p>
  
  <pre><code>+---+----+-----+
  | A | B  |  C  |
  +---+----+-----+
  | 1 | b1 | NaN |
  | 3 | b3 | NaN |
  | 5 | b5 | NaN |
  +---+----+-----+
  </code></pre>