[R] Coding a new variable based on criteria in a dataset

[RaoulD]

Hi,

I'm a bit stuck and need some help with R code to code a variable F_R based
on a combination of conditions. 

The first condition would code F_R as "F" and would be based on the
min(Date) and Min(Time) for each combination of UniqueID & Reason. The
second condition would code the variable as "R" as it would be the rest of
the data that dont meet the first condition. 

For example: for "UID 1" & "Reason 1" the first record would be coded "F"
and the 4th record would be coded "R". 

   UniqueID   Reason       Date  Time
1     UID 1   Reason 1 19/12/2010 15:00
2     UID 1   Reason 2 19/12/2010 16:00
3     UID 1   Reason 3 19/12/2010 16:30
4     UID 1   Reason 1 20/12/2010 08:00
5     UID 1   Reason 2 20/12/2010 10:01
6     UID 1   Reason 3 20/12/2010 11:30
7     UID 1   Reason 1 21/12/2010 12:45
8     UID 1   Reason 2 21/12/2010 18:44
9     UID 1   Reason 3 21/12/2010 19:29
10    UID 2  Reason 1 19/12/2010 17:00
11    UID 2  Reason 2 19/12/2010 18:00
12    UID 2  Reason 3 19/12/2010 18:10
13    UID 2  Reason 1 20/12/2010 13:00
14    UID 2  Reason 2 20/12/2010 13:30
15    UID 2  Reason 3 20/12/2010 16:15 

Is a loop the most efficient way to do this or is there some pre-existing
function that can help me with this? The sample dataset is what is given
below.

Thanks in advance,
Raoul
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