title: Trying to drop NA rows when condition x is met in column 3
body: <p>Trying to Drop NA Rows which fulfill condition <code>df$e == 'a'</code> </p>
  
  <pre><code>c &lt;- c(1,2,3,4)
  b &lt;- c(NA,NA,NA,'f')
  e &lt;- c('e','a','e','a')
  df &lt;- tibble(c,b,e)
  
  
  # A tibble: 4 x 3
        c b     e    
    &lt;dbl&gt; &lt;lgl&gt; &lt;chr&gt;
  1     1 NA    e    
  2     2 NA    a    
  3     3 NA    e    
  4     4 f     a  
  </code></pre>
  
  <p>trying to get: </p>
  
  <pre><code># A tibble: 4 x 3
        c b     e    
    &lt;dbl&gt; &lt;lgl&gt; &lt;chr&gt;
  1     1 NA    e       
  3     3 NA    e  
  4     4 f     a 
  </code></pre>
  
  <p>I have tried variations of <code>df &lt;- which(df$e == 'a' &amp; is.na(df$b) == TRUE)</code> and <code>df %&gt;% ifelse(df$e == 'a', drop_na('b'))</code> with no success.</p>