[R] counting row repetitions without loop

[Doran, Harold]

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: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Waterman, 
> DG (David)
> Sent: Wednesday, February 06, 2008 9:08 AM
> To: r-help at 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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