[R] Counting number of rows with two criteria in dataframe

Ista Zahn izahn at psych.rochester.edu
Tue Jan 25 20:51:02 CET 2011

Hi Ryan,
One option would be

X$a <- paste(X$x, X$y, sep=".")


On Tue, Jan 25, 2011 at 2:25 PM, Ryan Utz <utz.ryan at gmail.com> 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.
> Thanks,
> Ryan
> --
> Ryan Utz
> Postdoctoral research scholar
> University of California, Santa Barbara
> (724) 272 7769
>        [[alternative HTML version deleted]]
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Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology

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