Why reindex converts integer to decimal?

[new_to_python]

Hi, I have the following code:

frame = pd.DataFrame(np.arange(9).reshape((3,3)),
                     index=['a', 'c', 'd'],
                     columns=['Ohio', 'Texas', 'California'])

So the elements in the DataFrame are integers from 0 to 8 as indicated below:

Ohio Texas California
a 0 1 2
c 3 4 5
d 6 7 8

However, when I executed:

 frame2 = frame.reindex(['a', 'b', 'c', 'd']) 

, I got:

Ohio Texas California
a 0.0 1.0 2.0
b NaN NaN NaN
c 3.0 4.0 5.0
d 6.0 7.0 8.0


Q1: Why the elements became decimals?

Strangely, when I typed the following:

states = ['Texas', 'Utah', 'California']                                                                                                 
frame.reindex(columns=states)                                                                                                            

I got the following with the elements back to integer.

Texas Utah California
a 1 NaN 2
c 4 NaN 5
d 7 NaN 8


When I executed:

 frame.loc[['a', 'b', 'c', 'd'], states]      

Texas Utah California
a 1.0 NaN 2.0
b NaN NaN NaN
c 4.0 NaN 5.0
d 7.0 NaN 8.0

As shown above, the elements became decimals again.

How come? Could you please let me know what is going on?