Fill a new pandas column with row numbers : <p>I have the following DataFrame <code>data</code> with random index values:</p>

<pre><code>      A   B
100   0   7
203   5   4
5992  0  10
2003  9   8
20   10   5
12    6   2
</code></pre>

<p>I would like to add a new column 'C' with row numbers. For example:</p>

<pre><code>      A   B   C
100   0   7   0
203   5   4   1
5992  0  10   2
2003  9   8   3
20   10   5   4
12    6   2   5
</code></pre>

<p>Thank you for your help!</p>
