title: groupby and for loop with pandas
body: <p>I'm newbie in python3, I was R user few month ago.<br/>
  Here is the problem.<br/>
  my dataframe(<strong>df1</strong>) somewhat looks like this:</p>
  
  <pre><code>   Date         quarter        number
  2015-10-09          1           250
  2015-10-09          2           400
  2015-10-09          3           310
  2015-10-10          1           300
  2015-10-10          2           410
  2015-10-10          3           280
      .               .            .
      .               .            .
  </code></pre>
  
  <p>Data are more than 5000. Above image is just few lines.<br/>
  What I want is that the absolute difference numbers among quarter for every day.<br/>
  The result dataframe which I want should looks like following frame:</p>
  
  <pre><code>   Date          absolute_interval
  2015-10-09            150
  2015-10-09            90
  2015-10-10            110
  2015-10-10            130
      .                 .            
      .                 .         
  </code></pre>
  
  <p>I tried to use <strong>df2=df1.groupby('Date')</strong>.<br/>
  And then I want to apply for loop.<br/>
  my pseudo code is </p>
  
  <pre><code>for k in df2:
      for u in df2[k]:
  </code></pre>
  
  <p>But I don't think this will work, so I'm stuck in here currently.<br/>
  Any Solution??<br/>
  Thanks and have a good day.</p>