R help - Apr 2016 - multiplication by groups

Hi 


I have two data frames as shown below (second one is obtained by aggregating
rows of similar IDs in df1.). They both have similar number of columns but rows
of df2 are lesser than rows of df1.


df1:
ID    A            B 
1      1            2 
1      0            3 
2     5            NA 
2      1            3 
3      1            4 
4      NA           NA 
4      0            1 
4      3            0 
5      2            5 
5      7            NA 


df2:
ID    A        B
1      1        5 
2      6        3 
3      1        4
4      3        1 

5     9        5

Now, to obtain weight of each value of df1, I want to divide each row of df1 by
the row of df2 having similar ID. What I want is as below:

ID        A                B 
1         1      0.4 
1      0      0.6 
2      0.83    NA         
2      0.17     1
3         1        4
4         NA               NA 
4         0        1 
4      1        0
5      0.22     1
5      0.78    NA

 
Kindly guide me in this regard. 

Thanks 
Saba