title: Undefined columns selected when subsetting data frame
body: <p>I have a data frame, <code>str(data)</code> to show more about my data frame the result is the following: </p>
  
  <pre><code>&gt; str(data)
  'data.frame':   153 obs. of  6 variables:
  $ Ozone  : int  41 36 12 18 NA 28 23 19 8 NA ...
  $ Solar.R: int  190 118 149 313 NA NA 299 99 19 194 ...
  $ Wind   : num  7.4 8 12.6 11.5 14.3 14.9 8.6 13.8 20.1 8.6 ...
  $ Temp   : int  67 72 74 62 56 66 65 59 61 69 ...
  $ Month  : int  5 5 5 5 5 5 5 5 5 5 ...
  $ Day    : int  1 2 3 4 5 6 7 8 9 10 ...
  </code></pre>
  
  <p>However, for example, when I want to subset the amounts of <code>Ozone</code> above 14 I use the following code which gives me an error:</p>
  
  <pre><code>&gt;  data[data$Ozone &gt; 14 ]
  </code></pre>
  
  <blockquote>
    <p>Error in <code>[.data.frame</code>(data, data$Ozone > 14) : undefined columns selected</p>
  </blockquote>