title: How to add new column to an dataframe (to the front not end)?
body: <p>How to add a new variable to an existing data frame, but I want to add to the front not end.
  eg. my dataframe is</p>
  
  <pre><code>b c d
  1 2 3
  1 2 3
  1 2 3
  </code></pre>
  
  <p>I want to add a new variable a, so the dataframe will looks like</p>
  
  <pre><code>a b c d
  0 1 2 3
  0 1 2 3
  0 1 2 3
  </code></pre>