title: Pandas: How to access the value of the index
body: <p>I have a dataframe and would like to use the values in the index to create another column.<br>
  For instance:</p>
  
  <pre><code>df=pd.DataFrame({'idx1':range(0,5), 'idx2':range(10000,10005), 'value':np.random.randn(5)})
  df.set_index(keys=['idx1','idx2'], inplace=True)
  print df
  
                 value
  idx1 idx2           
  0    10000 -1.470367
  1    10001  0.260693
  2    10002 -0.732319
  3    10003 -0.116977
  4    10004  1.106644
  </code></pre>
  
  <p>I'd like to do something like this:</p>
  
  <pre><code>df['idx1_mod']= df['idx1'] + 100
  </code></pre>
  
  <p>(Actually, I want to do more complicated things, but basically I need the value of the index.)</p>
  
  <p>Right now I'm resorting to reseting the index (to get the index fields as columns), doing my calcs with access to the columns, and then re-creating the index. I'm sure I'm missing something obvious, but I've looked a ton and keep missing it!</p>
  
  <p>Note - I also tried df.iterrows(), but it seems that gives a copy of the row and doesn't let me update the original dataframe. </p>