[R] Replace missing values within a group with the non-missing value

[GRAHAM LEASK]

I have a large dataset organised in choice groups see below. Each choice 
represents a separate occasion with 1 product chosen out of the 6 offered.
 
 
 
     
+-------------------------------------------------------------------------------------------------+
     | dn   obs   choice      acid   br             
    date       cdate   situat~n   mth   year   set |
     
|-------------------------------------------------------------------------------------------------|
  1. |  4     1        0     LOSEC    1          
          .           .                .   
   .     1 |
  2. |  4     1        0    NEXIUM    2           
         .           .                .    
  .     1 |
  3. |  4     1        0    PARIET    3           
         .           .                .    
  .     1 |
  4. |  4     1        0   PROTIUM    4           
         .           .                .    
  .     1 |
  5. |  4     1        0    ZANTAC    5           
         .           .                .    
  .     1 |
     
|-------------------------------------------------------------------------------------------------|
  6. |  4     1        1     ZOTON    6   23aug2000 
01:00:00   23aug2000         NS   487   2000     1 |
  7. |  4     2        0     LOSEC    1          
          .           .                .   
   .     2 |
  8. |  4     2        0    NEXIUM    2           
         .           .                .    
  .     2 |
  9. |  4     2        1    PARIET    3   25sep2000 
01:00:00   25sep2000          L   488   2000     2 |
10. |  4     2        0   PROTIUM    4            
        .           .                .     
 .     2 |
     
|-------------------------------------------------------------------------------------------------|
11. |  4     2        0    ZANTAC    5            
        .           .                .     
 .     2 |
12. |  4     2        0     ZOTON    6           
         .           .                .    
  .     2 |
13. |  4     3        0     LOSEC    1           
         .           .                .    
  .     3 |
14. |  4     3        0    NEXIUM    2            
        .           .                .     
 .     3 |
15. |  4     3        0    PARIET    3            
        .           .                .     
 .     3 |
     
|-------------------------------------------------------------------------------------------------|
16. |  4     3        0   PROTIUM    4            
        .           .                .     
 .     3 |
17. |  4     3        0    ZANTAC    5            
        .           .                .     
 .     3 |
18. |  4     3        1     ZOTON    6   20sep2000 
00:00:00   20sep2000          R   488   2000     3 |
19. |  4     4        0     LOSEC    1           
         .           .                .    
  .     4 |
 
I wish to fill in the missing values in each choice set – delineated by dn 
(Doctor) obs (Observation number) and choices (1 to 6).
For each choice set one choice is chosen which contains full time information 
for that choice set ie in set 1 choice 6 was chosen and
shows the month 487. The other 5 choices show mth as missing. I want to fill 
these with the correct mth.
 
Clearly on different occasions the date will differ but each choice set has 
only one date.
 
Is there a simple elegant way to do this in R? 
Kind regards




Graham 
        [[alternative HTML version deleted]]


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