Fill the NA value in one column according to values of similar columns [duplicate]

Answer 1

try sort_values first to make Nan in last and then use group by with ffill()

df.sort_values(by=['C'],inplace=True)
df = df.groupby(['A','B']).ffill()

    A   B   C
0   aa  xx  2
1   bb  yy  3
2   cc  zz  8
3   aa  xx  2