title: Add columns in pandas dataframe dynamically
body: <p>I have following code to load dataframe</p>
  <pre><code>import pandas as pd
  
  ufo = pd.read_csv('csv_path')
  print(ufo.loc[[0,1,2] , :])
  </code></pre>
  <p>which gives following output, see the structure of the csv</p>
  <pre><code>          City Colors Reported Shape Reported State             Time
  0       Ithaca             NaN       TRIANGLE    NY   6/1/1930 22:00
  1  Willingboro             NaN          OTHER    NJ  6/30/1930 20:00
  2      Holyoke             NaN           OVAL    CO  2/15/1931 14:00
  </code></pre>
  <p>Now, I want to add an extra column based on existing column.
  I have a list which consist of indices of participating columns.
  It can be <strong>0,1</strong> or <strong>0,2,3</strong> or <strong>1,2,3</strong> anything.</p>
  <p>I need to create it dynamically.
  I could come up with following</p>
  <pre><code>df1['combined'] = df1['City']+','+df1['State']
  </code></pre>
  <p>Putting index doesn't seem to work.
  I want to join those columns. using <code>','.join()</code></p>