How to calculate percent change compared to the beginning value using pandas?
<p>I have a <code>DataFrame</code> and need to calculate percent change compared to the beginning of the year by companies. Is there any way to use <code>pct_change()</code> or other method to perform this task? Thanks!</p>

<p><code>df</code> looks like </p>

<pre><code>security    date        price
IBM         1/1/2016    100
IBM         1/2/2016    102
IBM         1/3/2016    108
AAPL        1/1/2016    1000
AAPL        1/2/2016    980
AAPL        1/3/2016    1050
AAPL        1/4/2016    1070
</code></pre>

<p>results I want </p>

<pre><code>security    date        price   change
IBM         1/1/2016    100     NA
IBM         1/2/2016    102     2%
IBM         1/3/2016    108     8%
AAPL        1/1/2016    1000    NA
AAPL        1/2/2016    980     -2%
AAPL        1/3/2016    1050    5%
AAPL        1/4/2016    1070    7%
</code></pre>