Add Indexed Column To Dataframe With Pandas

I'm a beginning pandas user, and after studying the documentation I still can't find a straightforward way to do the following. I have a DataFrame with a pandas.DateRange index, a

Solution 1:

Do df.join(df2):

http://pandas.pydata.org/pandas-docs/stable/merging.html#joining-on-index

Solution 2:

df['C'] = df2['C'] should also work in this case.