pandas 将索引值相加

    s1 = pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd'])
    s2 = pd.Series([10, 20, 30, 40], index=['a', 'b', 'c', 'd'])
    print s1 + s2

a    11
b    22
c    33
d    44
dtype: int64

    s1 = pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd'])
    s2 = pd.Series([10, 20, 30, 40], index=['b', 'd', 'a', 'c'])
    print s1 + s2

a    31
b    12
c    43
d    24
dtype: int64

    s1 = pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd'])
    s2 = pd.Series([10, 20, 30, 40], index=['c', 'd', 'e', 'f'])
    print s1 + s2

a     NaN
b     NaN
c    13.0
d    24.0
e     NaN
f     NaN
dtype: float64

    s1 = pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd'])
    s2 =