title: Get row index from DataFrame row
body: <p>Is it possible to get the row number (i.e. "the ordinal position of the index value") of a <code>DataFrame</code> row without adding an extra row that contains the row number (the index can be arbitrary, i.e. even a MultiIndex)?</p>
  
  <pre><code>&gt;&gt;&gt; import pandas as pd
  &gt;&gt;&gt; df = pd.DataFrame({'a': [2, 3, 4, 2, 4, 6]})
  &gt;&gt;&gt; result = df[df.a &gt; 3]
  &gt;&gt;&gt; result.iloc[0]
  a    4
  Name: 2, dtype: int64
  # but how can I get the original row index of iloc[0] in df?
  </code></pre>
  
  <p>I could have done <code>df['row_index'] = range(len(df))</code> which would maintain the original row number, but I am wondering if Pandas has a built-in way of doing this.</p>