title: Behavior of summing !is.na() results
body: <p>Why does the first line return TRUE, and the third line returns 1? I would expect both lines to return 1. What is the exact meaning of those extra two parentheses in the third line?</p>
  
  <pre><code>!is.na(5) + !is.na(NA)
  # TRUE
  (!is.na(5)) + (!is.na(NA))
  # 1
  </code></pre>
  
  <p>edit: should check these multiple times. The original problem was with <code>!is.na()</code>, thought it replicated for <code>is.na()</code>. But it didn't :)</p>