[R] Subset by Factor by date

[T.D.Rudolph]

I have a dataframe, x, with over 60,000 rows that contains one Factor, "id",
with 27 levels.  
The dataframe contains numerous continuous values (along column "diff") per
day (column "date") for every level of id.  I would like to select only one
row per animal per day, i.e. that containing the minimum value of "diff",
along the full length of 1:nrow(x).  I am not yet able to conduct anything
beyond the simplest of functions and I was hoping someone could suggest an
effective way of producing this output.

e.g. given this input:

id  day         diff
1  01-01-09  0.5
1  01-01-09  0.7
2  01-01-09  0.2
2  01-01-09  0.4
1  01-02-09  0.1
1  01-02-09  0.3
2  01-02-09  0.3
2  01-02-09  0.4

I would like to produce this output:
id day          diff
1  01-01-09  0.5
2  01-01-09  0.2
1  01-02-09  0.1
2  01-02-09  0.3

It doesn't seem extremely difficult but I'm sure there are easier ways than
how I am currently approaching it!
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