[R] Repeated Aggregation with data.table

[Elliot Joel Bernstein]

I have been using ddply to do aggregation, and I frequently define a
single aggregation function that I use to aggregate over different
groups. For example,

require(plyr)

dat <- data.frame(x = sample(3, 100, replace=TRUE), y = sample(3, 100,
replace = TRUE), z = rnorm(100))

f <- function(x) { data.frame(mean.z = mean(x$z), sd.z = sd(x$z)) }

ddply(dat, "x", f)
ddply(dat, "y", f)
ddply(dat, c("x", "y"), f)

I recently discovered the data.table package, which dramatically
speeds up the aggregation:

require(data.table)
dat <- data.table(dat)

dat[, list(mean.z = mean(z), sd.z = sd(z)), list(x)]
dat[, list(mean.z = mean(z), sd.z = sd(z)), list(y)]
dat[, list(mean.z = mean(z), sd.z = sd(z)), list(x,y)]

But I can't figure out how to save the aggregation function
"list(mean.z = mean(z), sd.z = sd(z))" as a variable that I can reuse,
similar to the function "f" above. Can someone please explain how to
do that?

Thanks.

- Elliot

-- 
Elliot Joel Bernstein, Ph.D. | Research Associate | FDO Partners, LLC
134 Mount Auburn Street | Cambridge, MA | 02138
Phone: (617) 503-4619 | Email: elliot.bernstein at fdopartners.com