Counting number of rows with two criteria in dataframe

Hi R-users,

I'm trying to find an elegant way to count the number of rows in a  
dataframe
with a unique combination of 2 values in the dataframe. My data is
specifically one column with a year, one with a month, and one with  
a day.
I'm trying to count the number of days in each year/month  
combination. But
for simplicity's sake, the following dataset will do:

x<-c(1,1,1,1,2,2,2,2,3,3,3,3)
y<-c(1,1,2,2,3,3,4,4,5,5,6,6)
z<-c(1,2,3,4,5,6,7,8,9,10,11,12)
X<-data.frame(x y z)

So with dataset X, how would I count the number of z values (3rd  
column in
X) with unique combinations of the first two columns (x and y)? (for
instance, in the above example, there are 2 instances per unique  
combination
of the first two columns). I can do this in Matlab and it's easy,  
but since
I'm new to R this is royally stumping me.

Thanks,
Ryan

-- 
Ryan Utz
Postdoctoral research scholar
University of California, Santa Barbara
(724) 272 7769

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