Merging two dataframes with different indexes

Question

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?

Answer 1

Adding values and using at

df1.at[:len(df2),'d']=df2.c.values
df1
Out[1200]: 
            a  b    d
1900-01-01  1  2  3.0
1900-01-02  1  3  1.0
1900-01-03  3  3  NaN