title: Perform mathematical operation in two Pandas dataframes using their index value
body: <p>I would like to perform mathmatical operations by index in two dataframes and store into one dataframe. Normally I use numpy and it was fine but with pd dataframe I am getting this error 'Must have equal len keys and value when setting with an iterable'. I have checked shape and len to verify but they they are same for all. The command I have used,</p>
  <pre><code>df_output.iloc[6,2:] = df_output.iloc[2,2:]*df_pad_Datetime.iloc[3,2:]/1000+ df_output.iloc[6,1:-1]
  
  </code></pre>
  <p>df_output row 6, column starting from 2 to end will be modified with the given condition.<br />
  Shape of df_output is (16,542) and<br />
  the shape of df_pad_Datetime is (4,542).</p>
  <p>df_output.iloc[6,1:-1] suppose to take all the column value starting from 1 to end except the last column for the give row.</p>
  <p>Am I missing something?</p>