Hi,

I'm sure there's an easy approach to this issue, I'm just not seeing it.

I have a data frame of the following form:

  Date     class    subclass   count
8/1/2009    A        X          1
8/1/2009    B        X          2
8/1/2009    A        Y          9
8/1/2009    B        Y          3
8/2/2009    A        X          1
8/2/2009    B        X          5
8/2/2009    A        Y          4
8/2/2009    B        Y          2
8/3/2009    A        X          6
8/3/2009    B        X          4
8/3/2009    A        Y          3
8/3/2009    B        Y          4
8/4/2009    A        X          1
8/4/2009    B        X          9
8/4/2009    A        Y          3
8/4/2009    B        Y          5
8/5/2009    A        X          3
8/5/2009    B        X          7
8/5/2009    A        Y          2
8/5/2009    B        Y          1

I would like to create a data frame of the sum the daily counts for,  
say, class 'A', like so:


  Date     sum_of_counts
8/1/2009       10
8/2/2009        5
8/3/2009        9
8/4/2009        4
8/5/2009        5

I ultimately would like to do sum of counts on all classes and  
subclasses.  It seems that this is equivalent to a GROUP BY query in  
SQL.

I'm sure this is possible in R. Any suggestions?

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