How to calculate differences between consecutive rows in pandas data frame?
<p>I've got a data frame, <code>df</code>, with three columns: <code>count_a</code>, <code>count_b</code> and <code>date</code>; the counts are floats, and the dates are consecutive days in 2015.</p>

<p>I'm trying to figure out the difference between each day's counts in both the <code>count_a</code> and <code>count_b</code> columns — meaning, I'm trying to calculate the difference between each row and the preceding row for both of those columns. I've set the date as the index, but am having trouble figuring out how to do this; there were a couple of hints about using <code>pd.Series</code> and <code>pd.DataFrame.diff</code> but I haven't had any luck finding an applicable answer or set of instructions. </p>

<p>I'm a bit stuck, and would appreciate some guidance here. </p>

<p>Here's what my data frame looks like: </p>

<pre><code>df=pd.Dataframe({'count_a': {Timestamp('2015-01-01 00:00:00'): 34175.0,
  Timestamp('2015-01-02 00:00:00'): 72640.0,
  Timestamp('2015-01-03 00:00:00'): 109354.0,
  Timestamp('2015-01-04 00:00:00'): 144491.0,
  Timestamp('2015-01-05 00:00:00'): 180355.0,
  Timestamp('2015-01-06 00:00:00'): 214615.0,
  Timestamp('2015-01-07 00:00:00'): 250096.0,
  Timestamp('2015-01-08 00:00:00'): 287880.0,
  Timestamp('2015-01-09 00:00:00'): 332528.0,
  Timestamp('2015-01-10 00:00:00'): 381460.0,
  Timestamp('2015-01-11 00:00:00'): 422981.0,
  Timestamp('2015-01-12 00:00:00'): 463539.0,
  Timestamp('2015-01-13 00:00:00'): 505395.0,
  Timestamp('2015-01-14 00:00:00'): 549027.0,
  Timestamp('2015-01-15 00:00:00'): 595377.0,
  Timestamp('2015-01-16 00:00:00'): 649043.0,
  Timestamp('2015-01-17 00:00:00'): 707727.0,
  Timestamp('2015-01-18 00:00:00'): 761287.0,
  Timestamp('2015-01-19 00:00:00'): 814372.0,
  Timestamp('2015-01-20 00:00:00'): 867096.0,
  Timestamp('2015-01-21 00:00:00'): 920838.0,
  Timestamp('2015-01-22 00:00:00'): 983405.0,
  Timestamp('2015-01-23 00:00:00'): 1067243.0,
  Timestamp('2015-01-24 00:00:00'): 1164421.0,
  Timestamp('2015-01-25 00:00:00'): 1252178.0,
  Timestamp('2015-01-26 00:00:00'): 1341484.0,
  Timestamp('2015-01-27 00:00:00'): 1427600.0,
  Timestamp('2015-01-28 00:00:00'): 1511549.0,
  Timestamp('2015-01-29 00:00:00'): 1594846.0,
  Timestamp('2015-01-30 00:00:00'): 1694226.0,
  Timestamp('2015-01-31 00:00:00'): 1806727.0,
  Timestamp('2015-02-01 00:00:00'): 1899880.0,
  Timestamp('2015-02-02 00:00:00'): 1987978.0,
  Timestamp('2015-02-03 00:00:00'): 2080338.0,
  Timestamp('2015-02-04 00:00:00'): 2175775.0,
  Timestamp('2015-02-05 00:00:00'): 2279525.0,
  Timestamp('2015-02-06 00:00:00'): 2403306.0,
  Timestamp('2015-02-07 00:00:00'): 2545696.0,
  Timestamp('2015-02-08 00:00:00'): 2672464.0,
  Timestamp('2015-02-09 00:00:00'): 2794788.0},
 'count_b': {Timestamp('2015-01-01 00:00:00'): nan,
  Timestamp('2015-01-02 00:00:00'): nan,
  Timestamp('2015-01-03 00:00:00'): nan,
  Timestamp('2015-01-04 00:00:00'): nan,
  Timestamp('2015-01-05 00:00:00'): nan,
  Timestamp('2015-01-06 00:00:00'): nan,
  Timestamp('2015-01-07 00:00:00'): nan,
  Timestamp('2015-01-08 00:00:00'): nan,
  Timestamp('2015-01-09 00:00:00'): nan,
  Timestamp('2015-01-10 00:00:00'): nan,
  Timestamp('2015-01-11 00:00:00'): nan,
  Timestamp('2015-01-12 00:00:00'): nan,
  Timestamp('2015-01-13 00:00:00'): nan,
  Timestamp('2015-01-14 00:00:00'): nan,
  Timestamp('2015-01-15 00:00:00'): nan,
  Timestamp('2015-01-16 00:00:00'): nan,
  Timestamp('2015-01-17 00:00:00'): nan,
  Timestamp('2015-01-18 00:00:00'): nan,
  Timestamp('2015-01-19 00:00:00'): nan,
  Timestamp('2015-01-20 00:00:00'): nan,
  Timestamp('2015-01-21 00:00:00'): nan,
  Timestamp('2015-01-22 00:00:00'): nan,
  Timestamp('2015-01-23 00:00:00'): nan,
  Timestamp('2015-01-24 00:00:00'): 71.0,
  Timestamp('2015-01-25 00:00:00'): 150.0,
  Timestamp('2015-01-26 00:00:00'): 236.0,
  Timestamp('2015-01-27 00:00:00'): 345.0,
  Timestamp('2015-01-28 00:00:00'): 1239.0,
  Timestamp('2015-01-29 00:00:00'): 2228.0,
  Timestamp('2015-01-30 00:00:00'): 7094.0,
  Timestamp('2015-01-31 00:00:00'): 16593.0,
  Timestamp('2015-02-01 00:00:00'): 27190.0,
  Timestamp('2015-02-02 00:00:00'): 37519.0,
  Timestamp('2015-02-03 00:00:00'): 49003.0,
  Timestamp('2015-02-04 00:00:00'): 63323.0,
  Timestamp('2015-02-05 00:00:00'): 79846.0,
  Timestamp('2015-02-06 00:00:00'): 101568.0,
  Timestamp('2015-02-07 00:00:00'): 127120.0,
  Timestamp('2015-02-08 00:00:00'): 149955.0,
  Timestamp('2015-02-09 00:00:00'): 171440.0}})
</code></pre>