title: Test for NA and select values based on result
body: <p>My question is rather simple. What I want is if <code>A[i]!=NA</code>, then <code>C[i]=A[i]</code>, if <code>A[i]=NA</code>, then <code>C[i]=B[i]</code>, however, I always get some error messages. Can somebody help me out?</p>
  
  <pre><code>A   B   C
  NA  82.6    .
  NA  127.2   .
  NA  93.6    .
  NA  105 .
  NA  104 .
  NA  90.6    .
  NA  95.8    .
  NA  103 .
  NA  85.4    .
  NA  81.5    .
  NA  142.8   .
  NA  102.3   .
  NA  104 .
  NA  103 .
  NA  94.6    .
  NA  113.8   .
  NA  113.5   .
  NA  74.5    .
  NA  123.8   .
  NA  94  .
  NA  89.8    .
  NA  74  .
  NA  104 .
  NA  100.5   .
  NA  102.9   .
  NA  132.5   .
  NA  91  .
  NA  92.5    .
  NA  97  .
  NA  90  .
  54.6    51.7    .
  NA  61  .
  NA  80  .
  NA  77.5    .
  NA  NA  .
  NA  80.6    .
  NA  44.6    .
  NA  37.6    .
  NA  27  .
  NA  NA  .
  NA  NA  .
  NA  NA  .
  </code></pre>