Hi:

Here are two more candidates, using the plyr and data.table packages:

library(plyr)
ddply(X, .(x, y), function(d) length(unique(d$z)))
  x y V1
1 1 1  2
2 1 2  2
3 2 3  2
4 2 4  2
5 3 5  2
6 3 6  2

The function counts the number of unique z values in each sub-data frame
with the same x and y values. The argument d in the anonymous function is a
data frame object.

# data.table version:

library(data.table)
dX <- data.table(X, key = 'x, y')
dX[, list(nz = length(unique(z))), by = 'x, y']
     x y nz
[1,] 1 1  2
[2,] 1 2  2
[3,] 2 3  2
[4,] 2 4  2
[5,] 3 5  2
[6,] 3 6  2

The key columns sort the data by x, y combinations and then find nz in each
data subset.

If you intend to do a lot of summarization/data manipulation in R, these
packages are worth learning.

HTH,
Dennis

On Tue, Jan 25, 2011 at 11:25 AM, Ryan Utz <utz.ryan@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
>
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>
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> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
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>

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