title: Select rows of a data.frame that contain only numbers in a certain column
body: <p>How to select only the rows that contain a number in column <em>b</em>.</p>
  
  <pre><code>a &lt;- c(1,5,3,1,-8,6,-1)
  b &lt;- c(4,-2,1,0,"c",2,"DX")
  
  df &lt;- data.frame(a,b)
  df
  
  #    a  b
  # 1  1  4
  # 2  5 -2
  # 3  3  1
  # 4  1  0
  # 5 -8  c
  # 6  6  2
  # 7 -1  DX
  </code></pre>
  
  <p>The output should look like this:</p>
  
  <pre><code>#    a  b
  # 1  1  4
  # 2  5 -2
  # 3  3  1
  # 4  1  0
  # 5  6  2
  </code></pre>