[R] replacing values of rows with identical row names in two dataframes

[Saba Sehrish]

Hi 

I have two dataframes(df1, df2) with equal number of columns (1566) but lesser rows in df2 (2772 in df1 and 40 in df2). Row names are 
identical in both dataframes (date). I want to replace NAs of df1 with the values of df2 for all those rows having identical row names (date) but 
without affecting already existing values in those rows of df1. 

Please see below: 

df1: 
date     11A  11A   21B   3CC   3CC 
20040101  100   150   NA   NA   140 
20040115   200   NA   200   NA   NA 
20040131   NA   165   180   190   190 
20040205   NA   NA   NA   NA   NA 
20040228   NA   NA   NA   NA   NA 
20040301  150   155   170   150   160 
20040315   NA   NA   180   190   200 
20040331   NA   NA   NA   175   180 

df2: 
date     11A  11A   21B   3CC   3CC 
20040131   170   NA   NA   NA   NA 
20040228   140   145   165   150   155 
20040331   NA   145   160   NA   NA 

I want the resulting dataframe to be: 

df3: 
date         11A  11A   21B   3CC   3CC 
20040101      100   150   NA   NA   140 
20040115      200   NA   200   NA   NA 
20040131      170   165   180   190   190 
20040205      NA   NA   NA   NA   NA 
20040228      140   145   165   150   155 
20040301      150   155   170   150   160 
20040315      NA   NA   180   190   200 
20040331      NA   145   160   175   180 

If it is possible, I would prefer to use "for loop" and "which" function to achieve the result. 

Please guide me in this regard. 

Thanks 
Saba