On Jan 25, 2011, at 2:25 PM, Ryan Utz wrote:
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.
> tapply(X$z, list(X$x, X$y), function(xx) length(unique(xx)) )
1 2 3 4 5 6
1 2 2 NA NA NA NA
2 NA NA 2 2 NA NA
3 NA NA NA NA 2 2
Thanks,
Ryan
--
Ryan Utz
Postdoctoral research scholar
University of California, Santa Barbara
(724) 272 7769
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