How to split a data frame by rows, and then process the blocks?

Question

I have a data frame with several columns, one of which is a factor called \"site\". How can I split the data frame into blocks of rows each with a unique value of \"site\",

Answer 1

You can use isplit (from the "iterators" package) to create an iterator object that loops over the blocks defined by the site column:

require(iterators)
site.data <- read.table("isplit-data.txt",header=T) 
sites <- isplit(site.data,site.data$site)

Then you can use foreach (from the "foreach" package) to create a plot within each block:

require(foreach)
foreach(site=sites) %dopar% {
 pdf(paste(site$key[[1]],".pdf",sep=""))
 plot(site$value$year,site$value$peak,main=site$key[[1]])
 dev.off()
}

As a bonus, if you have a multiprocessor machine and call registerDoMC() first (from the "doMC" package), the loops will run in parallel, speeding things up. More details in this Revolutions blog post: Block-processing a data frame with isplit