I have a data frame as shown below. How can I replace the NA? Looks like for the same device, source and timestamp the lat and long are same as well. So, the NA values should be: aaa, US, 33.23, -117.31 for the 2nd row. Am I thinking the right way? If not, then what is the best way to replace those missing values and in a correct way? 

```
city  country   lat    long        timestamp           source    device 
aaa     US    33.23  -117.31    2015-04-12 11:51:16     ad-f        w                  
<NA>   <NA>    NA      NA       2015-04-12 11:51:16     ad-f        w              
bbb     US    33.78  -117.96    2015-04-12 11:51:16     se-f        m                         
bbb     US    33.78  -117.96    2015-04-12 11:51:16     se-f        m                             
aaa     US    33.23  -117.31    2015-04-12 11:51:16     ad-f        w     
<NA>   <NA>    NA      NA       2015-04-04 17:38:26     se-f        m              
<NA>   <NA>    NA      NA       2015-04-04 17:38:26     ad-g        m              
ddd     US    33.99  -118.09    2015-04-04 17:38:26     ad-g        m              
eee     US    40.67  -73.94     2015-04-04 17:38:26     se-f        m              
<NA>   <NA>    NA      NA       2015-03-02 00:04:12     ad-g        w          

```

Thanks in advance.