Add default values to dataframe
<p>I have a dataframe that has hours of day as index, and a value for each hour. like so:</p>

<pre><code>    val
09   1
10   2
12   3
13   3
14   5
17   8
</code></pre>

<p>I want to fill the df, with values for all the hours of the day, so it will begin in 00 and go all the way to 23 and fill the value of it with 0.</p>

<p>I guess I can loop on the indexes and check if the value exits or the index and if not to add it, but I don't know how to loop DataFrame and I guess there is an easy way to do the task I want.</p>