title: Renaming columns in csv by column index
body: <p>I have a csv file:</p>
  
  <pre><code>visitIp        userId   idSite
  128.227.50.161   a        35
  24.222.206.154   a        35
  10.12.0.1        a        35
  10.12.0.1        a        35
  10.12.0.1        a        35
  24.222.206.154   a        35
  </code></pre>
  
  <p>I want to rename the column in the third index that is 'idSite' to just Id. The reason I want to do it by index is that there might be other csvs that have different column names in the 3rd column. So i need to rename by index as not by name. This is what i tried. But it does not seem to be working:</p>
  
  <pre><code>import pandas as pd
  df = pd.read_csv('Book1.csv',dtype='unicode')
  df1 = df.rename(columns = {'df.ix[:,2:3]':'id'})
  print df1
  </code></pre>