title: Locate index of rows in a dataframe that have the value of NA
body: <p>Suppose that we have the following data frame:</p>
  
  <pre><code>&gt;  dataset1
        x
    1   1
    2   2
    3   3
    4   NA
    5   5
  </code></pre>
  
  <p>I want to come up with a R command that computes the row index of the 1-column data frame that contains the value of 'NA'. More specifically, in above dataset1 example, such command would return 4 - because the 'NA' appears in the 4th row of the data frame. How can I make this happen? thank you!</p>