I have two dataframes:
df1 (index is date):
a b
1900-01-01 1 2
1900-01-02 1 3
1900-01-03 3 3
df2 (index is int):
c
0 3
1 1
after merge:
a b d
1900-01-01 1 2 3
1900-01-02 1 3 1
1900-01-03 3 3
Which function should I use?