title: (R) indentifies just one column in my data frame
body: <p>I' m new to R. I have my data frame.</p>
  
  <pre><code>test&lt;-read.csv("test.csv", header=TRUE)
  
  sites   Jahr1   Jahr2   Difference1 Difference2
  site1   0   2.208.630   2.208.630   
  site2   10.531.793  9.439.612   1.092.181   10%
  site3   1.942.681   1.283.180   659.501 34%
  site4       456.168 456.168 
  site5       450.697 450.697 
  site6   6.022.843   5.644.573   378.270 6%
  </code></pre>
  
  <p>Now I want to look at my second coloumn...</p>
  
  <pre><code>names(test)
  [1] "sites.Jahr1.Jahr2.Difference1.Difference2....."
  
  test [,2]
  Fehler in `[.data.frame`(test, , 2) : undefined columns selected
  </code></pre>
  
  <p>I think for advanced users, this is a very low level problem :)
  Can you help me?</p>