title: How to set the whole column to the index of the row
body: <p>How to set the whole column to the index of the row?</p>
  
  <pre><code>  a b c d
  1 1 3 3 1
  2 2 3 4 5
  3 4 5 6 7
  4 6 5 7 8
  </code></pre>
  
  <p>becomes</p>
  
  <pre><code>  b c d
  1 3 3 1
  2 3 4 5
  4 5 6 7
  6 5 7 8
  </code></pre>
  
  <p>I have tried to use xts(), but the column type changed to character but not numeric</p>