title: Pandas column addition/subtraction
body: <p>I am using a pandas/python dataframe.  I am trying to do a lag subtraction. </p>
  
  <p>I am currently using:</p>
  
  <pre><code>newCol = df.col - df.col.shift()
  </code></pre>
  
  <p>This leads to a NaN in the first spot:</p>
  
  <pre><code>NaN
  45
  63
  23
  ...
  </code></pre>
  
  <p>First question:  Is this the best way to do a subtraction like this?</p>
  
  <p>Second:  If I want to add a column (same number of rows) to this new column.  Is there a way that I can make all the NaN's 0's for the calculation?</p>
  
  <p>Ex:</p>
  
  <pre><code>col_1 = 
  Nan
  45
  63
  23
  
  col_2 = 
  10
  10
  10
  10
  
  new_col = 
  10
  55
  73
  33
  </code></pre>
  
  <p>and NOT</p>
  
  <pre><code>NaN
  55
  73
  33
  </code></pre>
  
  <p>Thank you.</p>