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# iterator for splitting data using a factor
library(foreach)
# let's use isplit on a data frame
a <- foreach(i=isplit(airquality, airquality$Month), .combine=rbind) %do%
quantile(i$value, na.rm=TRUE)
# make it pretty and print it
rownames(a) <- levels(as.factor(airquality$Month))
print(a)
# use a list of factors to do an aggregated operation
it <- isplit(as.data.frame(state.x77),
list(Region=state.region, Cold=state.x77[,'Frost'] > 130),
drop=TRUE)
a <- foreach(i=it, .combine=rbind) %do% {
x <- mean(i$value)
dim(x) <- c(1, length(x))
colnames(x) <- names(i$value)
cbind(i$key, as.data.frame(x))
}
print(a)
# compare with the standard aggregate function
b <- aggregate(state.x77,
list(Region=state.region, Cold=state.x77[,'Frost'] > 130),
mean)
print(b)
cat('results identical:\n')
print(identical(a, b))
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