[R] Advanced Filtering problem

[T.D.Rudolph]

http://www.nabble.com/file/p18018170/subdata.csv subdata.csv 

I've attached 100 rows of a data frame I am working with.  
I have one factor, id, with 27 levels.  There are two columns of reference
data, x and y (UTM coordinates), one column "date" in POSIXct format, and
one column "diff" in times format (chron package).

What I am trying to do is as follows:
For each day of the year (date, irrespective of time), select that row for
each id which contains the smallest "diff" value, resulting in an output
containing in general one value per id per day.

"aggregate" has been suggested but it only produces the columns considered
in the function and I need all columns intact.  My data frame contains
almost 70,000 entries so manual sorting is not an option.  I know R is
robust but my programming skills are elementary.  The only way I know to
approach it is to first separate every id, then filter, then recombine
somehow.  Is there not a more efficient way for this relatively
straight-forward filtering exercise?

Tyler
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