R and data.table for loop bug/nuance. Code extract for embedding in a blog post

bad.R

#Extract of R code
#This doesn't work properly

#Need to know how many levels
nl <- length(levels(datatable$Grouping))
#then how many per graph
ng = 8
#Can then loop

for (i in 1:ceiling(nl/ng)) {
    print(
        xyplot(Y ~ Date | Grouping, datatable[levels(datatable$Grouping)[(i-1)*ng+1:(i*ng)]], layout=c(1,ng), ylim=c(-100,100), xlim=as.Date(c("2011-01-01", "2011-07-01")) )
	)
}

#Instead of using the start and end variables within the loop, the values are calculated directly
#as applied to the datatable. For some reason, from the 2nd loop onwards, `i*ng` seems to be 
#evaluated as `(i+1)*ng` which means Lattice automatically produces two pages of graphs, instead
#of just the one page per loop that it should. 

#This is probably just a data.table nuance actually, not a bug.

good.R

#Extract of R code
#This works as expected

#Need to know how many levels
nl <- length(levels(datatable$Grouping))
#then how many per graph
ng = 8
#Can then loop

for (i in 1:ceiling(nl/ng)) {
    start=(i-1)*ng+1
    end=i*ng
    print(
        xyplot(Y ~ Date | Grouping, datatable[levels(datatable$Grouping)[start:end]], layout=c(1,ng), ylim=c(-100,100), xlim=as.Date(c("2011-01-01", "2011-07-01")) )
	)
}