I think this does what you want, but there may be a more efficient way
x y
4 5
6 7
6 6
7 5
5 7
6 7
4 5
6 7
7 6
5 6
dat <- read.table('clipboard', header=TRUE) # copy sample data above
dat$patt <- paste(dat$x,dat$y, sep='')
mm <- as.data.frame(with(dat, table(patt)))
dat <- merge(dat, mm, by='patt')
mat <- matrix(0, ncol=10, nrow=10)
gg <- matrix(c(dat$x, dat$y), ncol=2)
mat[gg] <- dat$Freq
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] On Behalf Of Waterman,
> DG (David)
> Sent: Wednesday, February 06, 2008 9:08 AM
> To: r-help@r-project.org
> Subject: [R] counting row repetitions without loop
>
> Hi,
>
> I have a data frame consisting of coordinates on a 10*10 grid, i.e.
>
> > example
> x y
> 1 4 5
> 2 6 7
> 3 6 6
> 4 7 5
> 5 5 7
> 6 6 7
> 7 4 5
> 8 6 7
> 9 7 6
> 10 5 6
>
> What I would like to do is return an 10*10 matrix consisting
> of counts at each position, so in the above example I would
> have a matrix where, for example, cell [4,5] contains 2 and
> [6,7] contains 3. At the moment I have implemented this using
> a for loop over the rows of the data frame, however the data
> frames I want to process are very long so the loop takes many
> minutes to complete. Can I do this in a more efficient way?
>
> Cheers,
> David
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