title: R dplyr filter function remove every other row
body: <p>I am applying the <code>R dplyr::filter</code> function to the <code>nycflights13</code> flights dataset.
  I am trying to select only flights in November and December, using two alternatives scripts.</p>
  <pre><code>library (tidyverse) 
  library (nycflights13)
  </code></pre>
  <p><strong>1</strong>
  <code>df1 &lt;- filter (flights, month == 11 | month == 12)</code></p>
  <p><strong>2</strong>
  <code>df2 &lt;- filter (flights, month == c(11,12))</code></p>
  <p><strong>df1</strong> yields the expected result (with 55403 total observations) and <strong>df2</strong> does too but with the dataset only contains half of the observations (27702 in total). <strong>df2</strong> has every other row removed.</p>
  <p>My question is: does anyone know why? I know the <strong>df2</strong> syntax is incorrect, but I am trying to understand why it yields the outcome that it does.</p>
  <p>Thanks</p>