title: Creating an empty Pandas DataFrame column with a fixed first value then filling it with a formula
body: <p>I'd like to create an emtpy column in an existing DataFrame with the first value in only one column to = 100. After that I'd like to iterate and fill the rest of the column with a formula, like row[C][t-1] * (1 + row[B][t])</p>
  
  <p>very similar to:
  <a href="https://stackoverflow.com/questions/13784192/creating-an-empty-pandas-dataframe-then-filling-it">Creating an empty Pandas DataFrame, then filling it?</a></p>
  
  <p>But the difference is fixing the first value of column 'C' to 100 vs entirely formulas.</p>
  
  <pre><code>import datetime
  import pandas as pd
  import numpy as np
  
  todays_date = datetime.datetime.now().date()
  index = pd.date_range(todays_date-datetime.timedelta(10), periods=10, freq='D')
  
  columns = ['A','B','C']
  
  df_ = pd.DataFrame(index=index, columns=columns)
  df_ = df_.fillna(0)
  
  data = np.array([np.arange(10)]*3).T
  df = pd.DataFrame(data, index=index, columns=columns)
  
  df['B'] = df['A'].pct_change()
  df['C'] = df['C'].shift() * (1+df['B'])
  
  ## how do I set 2016-10-03 in Column 'C' to equal 100 and then calc consequtively from there?
  
  df
  </code></pre>