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.r...@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]]
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
[[alternative HTML version deleted]]
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
