title: copy some rows from existing pandas dataframe to a new one
body: <p>And the copy has to be done for 'City' column starting with 'BH'.
  The copied df.index shouls be same as the original
  Eg -  </p>
  
  <pre><code>              STATE            CITY
  315           KA               BLR
  423           WB               CCU
  554           KA               BHU
  557           TN               BHY
  
  # state_df is new dataframe, df is existing
  state_df = pd.DataFrame(columns=['STATE', 'CITY'])      
  for index, row in df.iterrows():
      city = row['CITY']
  
      if(city.startswith('BH')):
          append row from df to state_df # pseudocode
  </code></pre>
  
  <p>Being new to pandas and Python, I need help in the pseudocode for the most efficient way.</p>