title: Filling the missing index and filling its value with 0
body: <p>I have a pandas dataframe with a column value as index number </p>
  
  <pre><code>      Sales
  140   100
  142   200
  145   300
  </code></pre>
  
  <p>I want to fill the missing index and also want to fill the value of missing index with 0</p>
  
  <pre><code>      Sales
  140   100
  141   0
  142   200
  143   0
  144   0
  145   300
  </code></pre>
  
  <p>I also want to fill missing values as the missing index number like </p>
  
  <pre><code>     Week_num
  140   140
  142   142
  145   145
  
        Week_Num
  140   140
  141   141
  142   142
  143   143
  144   144
  145   145
  </code></pre>
  
  <p>I request you to help me how to code this out?</p>