title: Select x>0 in x when NA in x
body: <pre><code>&gt; x
   [1] -0.0006993769            NA  0.0605337483  0.0112900315  0.0166747653  0.0498045070 -0.0120633548  0.0034724936
   [9] -0.0107294362 -0.0021350540 -0.0092103916 -0.0369649485 -0.0136530145 -0.0127135534  0.0159644319 -0.0075359604
  [17]  0.0138871055  0.0226605719 -0.0118107427  0.0168362283 -0.0175228517
  
  &gt; x&gt;0
   [1] FALSE    NA  TRUE  TRUE  TRUE  TRUE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE  TRUE  TRUE FALSE  TRUE
  [21] FALSE
  
  &gt; x[x&gt;0]
   [1]          NA 0.060533748 0.011290032 0.016674765 0.049804507 0.003472494 0.015964432 0.013887105 0.022660572 0.016836228
  </code></pre>
  
  <p>do not want to get NA in <code>x[x&gt;0]</code>, why NA were selected in this sequence</p>